1========================
2ftrace - Function Tracer
3========================
4
5Copyright 2008 Red Hat Inc.
6
7:Author:   Steven Rostedt <srostedt@redhat.com>
8:License:  The GNU Free Documentation License, Version 1.2
9          (dual licensed under the GPL v2)
10:Original Reviewers:  Elias Oltmanns, Randy Dunlap, Andrew Morton,
11		      John Kacur, and David Teigland.
12
13- Written for: 2.6.28-rc2
14- Updated for: 3.10
15- Updated for: 4.13 - Copyright 2017 VMware Inc. Steven Rostedt
16- Converted to rst format - Changbin Du <changbin.du@intel.com>
17
18Introduction
19------------
20
21Ftrace is an internal tracer designed to help out developers and
22designers of systems to find what is going on inside the kernel.
23It can be used for debugging or analyzing latencies and
24performance issues that take place outside of user-space.
25
26Although ftrace is typically considered the function tracer, it
27is really a framework of several assorted tracing utilities.
28There's latency tracing to examine what occurs between interrupts
29disabled and enabled, as well as for preemption and from a time
30a task is woken to the task is actually scheduled in.
31
32One of the most common uses of ftrace is the event tracing.
33Throughout the kernel is hundreds of static event points that
34can be enabled via the tracefs file system to see what is
35going on in certain parts of the kernel.
36
37See events.rst for more information.
38
39
40Implementation Details
41----------------------
42
43See Documentation/trace/ftrace-design.rst for details for arch porters and such.
44
45
46The File System
47---------------
48
49Ftrace uses the tracefs file system to hold the control files as
50well as the files to display output.
51
52When tracefs is configured into the kernel (which selecting any ftrace
53option will do) the directory /sys/kernel/tracing will be created. To mount
54this directory, you can add to your /etc/fstab file::
55
56 tracefs       /sys/kernel/tracing       tracefs defaults        0       0
57
58Or you can mount it at run time with::
59
60 mount -t tracefs nodev /sys/kernel/tracing
61
62For quicker access to that directory you may want to make a soft link to
63it::
64
65 ln -s /sys/kernel/tracing /tracing
66
67.. attention::
68
69  Before 4.1, all ftrace tracing control files were within the debugfs
70  file system, which is typically located at /sys/kernel/debug/tracing.
71  For backward compatibility, when mounting the debugfs file system,
72  the tracefs file system will be automatically mounted at:
73
74  /sys/kernel/debug/tracing
75
76  All files located in the tracefs file system will be located in that
77  debugfs file system directory as well.
78
79.. attention::
80
81  Any selected ftrace option will also create the tracefs file system.
82  The rest of the document will assume that you are in the ftrace directory
83  (cd /sys/kernel/tracing) and will only concentrate on the files within that
84  directory and not distract from the content with the extended
85  "/sys/kernel/tracing" path name.
86
87That's it! (assuming that you have ftrace configured into your kernel)
88
89After mounting tracefs you will have access to the control and output files
90of ftrace. Here is a list of some of the key files:
91
92
93 Note: all time values are in microseconds.
94
95  current_tracer:
96
97	This is used to set or display the current tracer
98	that is configured. Changing the current tracer clears
99	the ring buffer content as well as the "snapshot" buffer.
100
101  available_tracers:
102
103	This holds the different types of tracers that
104	have been compiled into the kernel. The
105	tracers listed here can be configured by
106	echoing their name into current_tracer.
107
108  tracing_on:
109
110	This sets or displays whether writing to the trace
111	ring buffer is enabled. Echo 0 into this file to disable
112	the tracer or 1 to enable it. Note, this only disables
113	writing to the ring buffer, the tracing overhead may
114	still be occurring.
115
116	The kernel function tracing_off() can be used within the
117	kernel to disable writing to the ring buffer, which will
118	set this file to "0". User space can re-enable tracing by
119	echoing "1" into the file.
120
121	Note, the function and event trigger "traceoff" will also
122	set this file to zero and stop tracing. Which can also
123	be re-enabled by user space using this file.
124
125  trace:
126
127	This file holds the output of the trace in a human
128	readable format (described below). Opening this file for
129	writing with the O_TRUNC flag clears the ring buffer content.
130        Note, this file is not a consumer. If tracing is off
131        (no tracer running, or tracing_on is zero), it will produce
132        the same output each time it is read. When tracing is on,
133        it may produce inconsistent results as it tries to read
134        the entire buffer without consuming it.
135
136  trace_pipe:
137
138	The output is the same as the "trace" file but this
139	file is meant to be streamed with live tracing.
140	Reads from this file will block until new data is
141	retrieved.  Unlike the "trace" file, this file is a
142	consumer. This means reading from this file causes
143	sequential reads to display more current data. Once
144	data is read from this file, it is consumed, and
145	will not be read again with a sequential read. The
146	"trace" file is static, and if the tracer is not
147	adding more data, it will display the same
148	information every time it is read.
149
150  trace_options:
151
152	This file lets the user control the amount of data
153	that is displayed in one of the above output
154	files. Options also exist to modify how a tracer
155	or events work (stack traces, timestamps, etc).
156
157  options:
158
159	This is a directory that has a file for every available
160	trace option (also in trace_options). Options may also be set
161	or cleared by writing a "1" or "0" respectively into the
162	corresponding file with the option name.
163
164  tracing_max_latency:
165
166	Some of the tracers record the max latency.
167	For example, the maximum time that interrupts are disabled.
168	The maximum time is saved in this file. The max trace will also be
169	stored,	and displayed by "trace". A new max trace will only be
170	recorded if the latency is greater than the value in this file
171	(in microseconds).
172
173	By echoing in a time into this file, no latency will be recorded
174	unless it is greater than the time in this file.
175
176  tracing_thresh:
177
178	Some latency tracers will record a trace whenever the
179	latency is greater than the number in this file.
180	Only active when the file contains a number greater than 0.
181	(in microseconds)
182
183  buffer_percent:
184
185	This is the watermark for how much the ring buffer needs to be filled
186	before a waiter is woken up. That is, if an application calls a
187	blocking read syscall on one of the per_cpu trace_pipe_raw files, it
188	will block until the given amount of data specified by buffer_percent
189	is in the ring buffer before it wakes the reader up. This also
190	controls how the splice system calls are blocked on this file::
191
192	  0   - means to wake up as soon as there is any data in the ring buffer.
193	  50  - means to wake up when roughly half of the ring buffer sub-buffers
194	        are full.
195	  100 - means to block until the ring buffer is totally full and is
196	        about to start overwriting the older data.
197
198  buffer_size_kb:
199
200	This sets or displays the number of kilobytes each CPU
201	buffer holds. By default, the trace buffers are the same size
202	for each CPU. The displayed number is the size of the
203	CPU buffer and not total size of all buffers. The
204	trace buffers are allocated in pages (blocks of memory
205	that the kernel uses for allocation, usually 4 KB in size).
206	A few extra pages may be allocated to accommodate buffer management
207	meta-data. If the last page allocated has room for more bytes
208	than requested, the rest of the page will be used,
209	making the actual allocation bigger than requested or shown.
210	( Note, the size may not be a multiple of the page size
211	due to buffer management meta-data. )
212
213	Buffer sizes for individual CPUs may vary
214	(see "per_cpu/cpu0/buffer_size_kb" below), and if they do
215	this file will show "X".
216
217  buffer_total_size_kb:
218
219	This displays the total combined size of all the trace buffers.
220
221  buffer_subbuf_size_kb:
222
223	This sets or displays the sub buffer size. The ring buffer is broken up
224	into several same size "sub buffers". An event can not be bigger than
225	the size of the sub buffer. Normally, the sub buffer is the size of the
226	architecture's page (4K on x86). The sub buffer also contains meta data
227	at the start which also limits the size of an event.  That means when
228	the sub buffer is a page size, no event can be larger than the page
229	size minus the sub buffer meta data.
230
231	Note, the buffer_subbuf_size_kb is a way for the user to specify the
232	minimum size of the subbuffer. The kernel may make it bigger due to the
233	implementation details, or simply fail the operation if the kernel can
234	not handle the request.
235
236	Changing the sub buffer size allows for events to be larger than the
237	page size.
238
239	Note: When changing the sub-buffer size, tracing is stopped and any
240	data in the ring buffer and the snapshot buffer will be discarded.
241
242  free_buffer:
243
244	If a process is performing tracing, and the ring buffer	should be
245	shrunk "freed" when the process is finished, even if it were to be
246	killed by a signal, this file can be used for that purpose. On close
247	of this file, the ring buffer will be resized to its minimum size.
248	Having a process that is tracing also open this file, when the process
249	exits its file descriptor for this file will be closed, and in doing so,
250	the ring buffer will be "freed".
251
252	It may also stop tracing if disable_on_free option is set.
253
254  tracing_cpumask:
255
256	This is a mask that lets the user only trace on specified CPUs.
257	The format is a hex string representing the CPUs.
258
259  set_ftrace_filter:
260
261	When dynamic ftrace is configured in (see the
262	section below "dynamic ftrace"), the code is dynamically
263	modified (code text rewrite) to disable calling of the
264	function profiler (mcount). This lets tracing be configured
265	in with practically no overhead in performance.  This also
266	has a side effect of enabling or disabling specific functions
267	to be traced. Echoing names of functions into this file
268	will limit the trace to only those functions.
269	This influences the tracers "function" and "function_graph"
270	and thus also function profiling (see "function_profile_enabled").
271
272	The functions listed in "available_filter_functions" are what
273	can be written into this file.
274
275	This interface also allows for commands to be used. See the
276	"Filter commands" section for more details.
277
278	As a speed up, since processing strings can be quite expensive
279	and requires a check of all functions registered to tracing, instead
280	an index can be written into this file. A number (starting with "1")
281	written will instead select the same corresponding at the line position
282	of the "available_filter_functions" file.
283
284  set_ftrace_notrace:
285
286	This has an effect opposite to that of
287	set_ftrace_filter. Any function that is added here will not
288	be traced. If a function exists in both set_ftrace_filter
289	and set_ftrace_notrace,	the function will _not_ be traced.
290
291  set_ftrace_pid:
292
293	Have the function tracer only trace the threads whose PID are
294	listed in this file.
295
296	If the "function-fork" option is set, then when a task whose
297	PID is listed in this file forks, the child's PID will
298	automatically be added to this file, and the child will be
299	traced by the function tracer as well. This option will also
300	cause PIDs of tasks that exit to be removed from the file.
301
302  set_ftrace_notrace_pid:
303
304        Have the function tracer ignore threads whose PID are listed in
305        this file.
306
307        If the "function-fork" option is set, then when a task whose
308	PID is listed in this file forks, the child's PID will
309	automatically be added to this file, and the child will not be
310	traced by the function tracer as well. This option will also
311	cause PIDs of tasks that exit to be removed from the file.
312
313        If a PID is in both this file and "set_ftrace_pid", then this
314        file takes precedence, and the thread will not be traced.
315
316  set_event_pid:
317
318	Have the events only trace a task with a PID listed in this file.
319	Note, sched_switch and sched_wake_up will also trace events
320	listed in this file.
321
322	To have the PIDs of children of tasks with their PID in this file
323	added on fork, enable the "event-fork" option. That option will also
324	cause the PIDs of tasks to be removed from this file when the task
325	exits.
326
327  set_event_notrace_pid:
328
329	Have the events not trace a task with a PID listed in this file.
330	Note, sched_switch and sched_wakeup will trace threads not listed
331	in this file, even if a thread's PID is in the file if the
332        sched_switch or sched_wakeup events also trace a thread that should
333        be traced.
334
335	To have the PIDs of children of tasks with their PID in this file
336	added on fork, enable the "event-fork" option. That option will also
337	cause the PIDs of tasks to be removed from this file when the task
338	exits.
339
340  set_graph_function:
341
342	Functions listed in this file will cause the function graph
343	tracer to only trace these functions and the functions that
344	they call. (See the section "dynamic ftrace" for more details).
345	Note, set_ftrace_filter and set_ftrace_notrace still affects
346	what functions are being traced.
347
348  set_graph_notrace:
349
350	Similar to set_graph_function, but will disable function graph
351	tracing when the function is hit until it exits the function.
352	This makes it possible to ignore tracing functions that are called
353	by a specific function.
354
355  available_filter_functions:
356
357	This lists the functions that ftrace has processed and can trace.
358	These are the function names that you can pass to
359	"set_ftrace_filter", "set_ftrace_notrace",
360	"set_graph_function", or "set_graph_notrace".
361	(See the section "dynamic ftrace" below for more details.)
362
363  available_filter_functions_addrs:
364
365	Similar to available_filter_functions, but with address displayed
366	for each function. The displayed address is the patch-site address
367	and can differ from /proc/kallsyms address.
368
369  dyn_ftrace_total_info:
370
371	This file is for debugging purposes. The number of functions that
372	have been converted to nops and are available to be traced.
373
374  enabled_functions:
375
376	This file is more for debugging ftrace, but can also be useful
377	in seeing if any function has a callback attached to it.
378	Not only does the trace infrastructure use ftrace function
379	trace utility, but other subsystems might too. This file
380	displays all functions that have a callback attached to them
381	as well as the number of callbacks that have been attached.
382	Note, a callback may also call multiple functions which will
383	not be listed in this count.
384
385	If the callback registered to be traced by a function with
386	the "save regs" attribute (thus even more overhead), a 'R'
387	will be displayed on the same line as the function that
388	is returning registers.
389
390	If the callback registered to be traced by a function with
391	the "ip modify" attribute (thus the regs->ip can be changed),
392	an 'I' will be displayed on the same line as the function that
393	can be overridden.
394
395	If a non ftrace trampoline is attached (BPF) a 'D' will be displayed.
396	Note, normal ftrace trampolines can also be attached, but only one
397	"direct" trampoline can be attached to a given function at a time.
398
399	Some architectures can not call direct trampolines, but instead have
400	the ftrace ops function located above the function entry point. In
401	such cases an 'O' will be displayed.
402
403	If a function had either the "ip modify" or a "direct" call attached to
404	it in the past, a 'M' will be shown. This flag is never cleared. It is
405	used to know if a function was every modified by the ftrace infrastructure,
406	and can be used for debugging.
407
408	If the architecture supports it, it will also show what callback
409	is being directly called by the function. If the count is greater
410	than 1 it most likely will be ftrace_ops_list_func().
411
412	If the callback of a function jumps to a trampoline that is
413	specific to the callback and which is not the standard trampoline,
414	its address will be printed as well as the function that the
415	trampoline calls.
416
417  touched_functions:
418
419	This file contains all the functions that ever had a function callback
420	to it via the ftrace infrastructure. It has the same format as
421	enabled_functions but shows all functions that have every been
422	traced.
423
424	To see any function that has every been modified by "ip modify" or a
425	direct trampoline, one can perform the following command:
426
427	grep ' M ' /sys/kernel/tracing/touched_functions
428
429  function_profile_enabled:
430
431	When set it will enable all functions with either the function
432	tracer, or if configured, the function graph tracer. It will
433	keep a histogram of the number of functions that were called
434	and if the function graph tracer was configured, it will also keep
435	track of the time spent in those functions. The histogram
436	content can be displayed in the files:
437
438	trace_stat/function<cpu> ( function0, function1, etc).
439
440  trace_stat:
441
442	A directory that holds different tracing stats.
443
444  kprobe_events:
445
446	Enable dynamic trace points. See kprobetrace.rst.
447
448  kprobe_profile:
449
450	Dynamic trace points stats. See kprobetrace.rst.
451
452  max_graph_depth:
453
454	Used with the function graph tracer. This is the max depth
455	it will trace into a function. Setting this to a value of
456	one will show only the first kernel function that is called
457	from user space.
458
459  printk_formats:
460
461	This is for tools that read the raw format files. If an event in
462	the ring buffer references a string, only a pointer to the string
463	is recorded into the buffer and not the string itself. This prevents
464	tools from knowing what that string was. This file displays the string
465	and address for	the string allowing tools to map the pointers to what
466	the strings were.
467
468  saved_cmdlines:
469
470	Only the pid of the task is recorded in a trace event unless
471	the event specifically saves the task comm as well. Ftrace
472	makes a cache of pid mappings to comms to try to display
473	comms for events. If a pid for a comm is not listed, then
474	"<...>" is displayed in the output.
475
476	If the option "record-cmd" is set to "0", then comms of tasks
477	will not be saved during recording. By default, it is enabled.
478
479  saved_cmdlines_size:
480
481	By default, 128 comms are saved (see "saved_cmdlines" above). To
482	increase or decrease the amount of comms that are cached, echo
483	the number of comms to cache into this file.
484
485  saved_tgids:
486
487	If the option "record-tgid" is set, on each scheduling context switch
488	the Task Group ID of a task is saved in a table mapping the PID of
489	the thread to its TGID. By default, the "record-tgid" option is
490	disabled.
491
492  snapshot:
493
494	This displays the "snapshot" buffer and also lets the user
495	take a snapshot of the current running trace.
496	See the "Snapshot" section below for more details.
497
498  stack_max_size:
499
500	When the stack tracer is activated, this will display the
501	maximum stack size it has encountered.
502	See the "Stack Trace" section below.
503
504  stack_trace:
505
506	This displays the stack back trace of the largest stack
507	that was encountered when the stack tracer is activated.
508	See the "Stack Trace" section below.
509
510  stack_trace_filter:
511
512	This is similar to "set_ftrace_filter" but it limits what
513	functions the stack tracer will check.
514
515  trace_clock:
516
517	Whenever an event is recorded into the ring buffer, a
518	"timestamp" is added. This stamp comes from a specified
519	clock. By default, ftrace uses the "local" clock. This
520	clock is very fast and strictly per cpu, but on some
521	systems it may not be monotonic with respect to other
522	CPUs. In other words, the local clocks may not be in sync
523	with local clocks on other CPUs.
524
525	Usual clocks for tracing::
526
527	  # cat trace_clock
528	  [local] global counter x86-tsc
529
530	The clock with the square brackets around it is the one in effect.
531
532	local:
533		Default clock, but may not be in sync across CPUs
534
535	global:
536		This clock is in sync with all CPUs but may
537		be a bit slower than the local clock.
538
539	counter:
540		This is not a clock at all, but literally an atomic
541		counter. It counts up one by one, but is in sync
542		with all CPUs. This is useful when you need to
543		know exactly the order events occurred with respect to
544		each other on different CPUs.
545
546	uptime:
547		This uses the jiffies counter and the time stamp
548		is relative to the time since boot up.
549
550	perf:
551		This makes ftrace use the same clock that perf uses.
552		Eventually perf will be able to read ftrace buffers
553		and this will help out in interleaving the data.
554
555	x86-tsc:
556		Architectures may define their own clocks. For
557		example, x86 uses its own TSC cycle clock here.
558
559	ppc-tb:
560		This uses the powerpc timebase register value.
561		This is in sync across CPUs and can also be used
562		to correlate events across hypervisor/guest if
563		tb_offset is known.
564
565	mono:
566		This uses the fast monotonic clock (CLOCK_MONOTONIC)
567		which is monotonic and is subject to NTP rate adjustments.
568
569	mono_raw:
570		This is the raw monotonic clock (CLOCK_MONOTONIC_RAW)
571		which is monotonic but is not subject to any rate adjustments
572		and ticks at the same rate as the hardware clocksource.
573
574	boot:
575		This is the boot clock (CLOCK_BOOTTIME) and is based on the
576		fast monotonic clock, but also accounts for time spent in
577		suspend. Since the clock access is designed for use in
578		tracing in the suspend path, some side effects are possible
579		if clock is accessed after the suspend time is accounted before
580		the fast mono clock is updated. In this case, the clock update
581		appears to happen slightly sooner than it normally would have.
582		Also on 32-bit systems, it's possible that the 64-bit boot offset
583		sees a partial update. These effects are rare and post
584		processing should be able to handle them. See comments in the
585		ktime_get_boot_fast_ns() function for more information.
586
587	tai:
588		This is the tai clock (CLOCK_TAI) and is derived from the wall-
589		clock time. However, this clock does not experience
590		discontinuities and backwards jumps caused by NTP inserting leap
591		seconds. Since the clock access is designed for use in tracing,
592		side effects are possible. The clock access may yield wrong
593		readouts in case the internal TAI offset is updated e.g., caused
594		by setting the system time or using adjtimex() with an offset.
595		These effects are rare and post processing should be able to
596		handle them. See comments in the ktime_get_tai_fast_ns()
597		function for more information.
598
599	To set a clock, simply echo the clock name into this file::
600
601	  # echo global > trace_clock
602
603	Setting a clock clears the ring buffer content as well as the
604	"snapshot" buffer.
605
606  trace_marker:
607
608	This is a very useful file for synchronizing user space
609	with events happening in the kernel. Writing strings into
610	this file will be written into the ftrace buffer.
611
612	It is useful in applications to open this file at the start
613	of the application and just reference the file descriptor
614	for the file::
615
616		void trace_write(const char *fmt, ...)
617		{
618			va_list ap;
619			char buf[256];
620			int n;
621
622			if (trace_fd < 0)
623				return;
624
625			va_start(ap, fmt);
626			n = vsnprintf(buf, 256, fmt, ap);
627			va_end(ap);
628
629			write(trace_fd, buf, n);
630		}
631
632	start::
633
634		trace_fd = open("trace_marker", O_WRONLY);
635
636	Note: Writing into the trace_marker file can also initiate triggers
637	      that are written into /sys/kernel/tracing/events/ftrace/print/trigger
638	      See "Event triggers" in Documentation/trace/events.rst and an
639              example in Documentation/trace/histogram.rst (Section 3.)
640
641  trace_marker_raw:
642
643	This is similar to trace_marker above, but is meant for binary data
644	to be written to it, where a tool can be used to parse the data
645	from trace_pipe_raw.
646
647  uprobe_events:
648
649	Add dynamic tracepoints in programs.
650	See uprobetracer.rst
651
652  uprobe_profile:
653
654	Uprobe statistics. See uprobetrace.txt
655
656  instances:
657
658	This is a way to make multiple trace buffers where different
659	events can be recorded in different buffers.
660	See "Instances" section below.
661
662  events:
663
664	This is the trace event directory. It holds event tracepoints
665	(also known as static tracepoints) that have been compiled
666	into the kernel. It shows what event tracepoints exist
667	and how they are grouped by system. There are "enable"
668	files at various levels that can enable the tracepoints
669	when a "1" is written to them.
670
671	See events.rst for more information.
672
673  set_event:
674
675	By echoing in the event into this file, will enable that event.
676
677	See events.rst for more information.
678
679  available_events:
680
681	A list of events that can be enabled in tracing.
682
683	See events.rst for more information.
684
685  timestamp_mode:
686
687	Certain tracers may change the timestamp mode used when
688	logging trace events into the event buffer.  Events with
689	different modes can coexist within a buffer but the mode in
690	effect when an event is logged determines which timestamp mode
691	is used for that event.  The default timestamp mode is
692	'delta'.
693
694	Usual timestamp modes for tracing:
695
696	  # cat timestamp_mode
697	  [delta] absolute
698
699	  The timestamp mode with the square brackets around it is the
700	  one in effect.
701
702	  delta: Default timestamp mode - timestamp is a delta against
703	         a per-buffer timestamp.
704
705	  absolute: The timestamp is a full timestamp, not a delta
706                 against some other value.  As such it takes up more
707                 space and is less efficient.
708
709  hwlat_detector:
710
711	Directory for the Hardware Latency Detector.
712	See "Hardware Latency Detector" section below.
713
714  per_cpu:
715
716	This is a directory that contains the trace per_cpu information.
717
718  per_cpu/cpu0/buffer_size_kb:
719
720	The ftrace buffer is defined per_cpu. That is, there's a separate
721	buffer for each CPU to allow writes to be done atomically,
722	and free from cache bouncing. These buffers may have different
723	size buffers. This file is similar to the buffer_size_kb
724	file, but it only displays or sets the buffer size for the
725	specific CPU. (here cpu0).
726
727  per_cpu/cpu0/trace:
728
729	This is similar to the "trace" file, but it will only display
730	the data specific for the CPU. If written to, it only clears
731	the specific CPU buffer.
732
733  per_cpu/cpu0/trace_pipe
734
735	This is similar to the "trace_pipe" file, and is a consuming
736	read, but it will only display (and consume) the data specific
737	for the CPU.
738
739  per_cpu/cpu0/trace_pipe_raw
740
741	For tools that can parse the ftrace ring buffer binary format,
742	the trace_pipe_raw file can be used to extract the data
743	from the ring buffer directly. With the use of the splice()
744	system call, the buffer data can be quickly transferred to
745	a file or to the network where a server is collecting the
746	data.
747
748	Like trace_pipe, this is a consuming reader, where multiple
749	reads will always produce different data.
750
751  per_cpu/cpu0/snapshot:
752
753	This is similar to the main "snapshot" file, but will only
754	snapshot the current CPU (if supported). It only displays
755	the content of the snapshot for a given CPU, and if
756	written to, only clears this CPU buffer.
757
758  per_cpu/cpu0/snapshot_raw:
759
760	Similar to the trace_pipe_raw, but will read the binary format
761	from the snapshot buffer for the given CPU.
762
763  per_cpu/cpu0/stats:
764
765	This displays certain stats about the ring buffer:
766
767	entries:
768		The number of events that are still in the buffer.
769
770	overrun:
771		The number of lost events due to overwriting when
772		the buffer was full.
773
774	commit overrun:
775		Should always be zero.
776		This gets set if so many events happened within a nested
777		event (ring buffer is re-entrant), that it fills the
778		buffer and starts dropping events.
779
780	bytes:
781		Bytes actually read (not overwritten).
782
783	oldest event ts:
784		The oldest timestamp in the buffer
785
786	now ts:
787		The current timestamp
788
789	dropped events:
790		Events lost due to overwrite option being off.
791
792	read events:
793		The number of events read.
794
795The Tracers
796-----------
797
798Here is the list of current tracers that may be configured.
799
800  "function"
801
802	Function call tracer to trace all kernel functions.
803
804  "function_graph"
805
806	Similar to the function tracer except that the
807	function tracer probes the functions on their entry
808	whereas the function graph tracer traces on both entry
809	and exit of the functions. It then provides the ability
810	to draw a graph of function calls similar to C code
811	source.
812
813  "blk"
814
815	The block tracer. The tracer used by the blktrace user
816	application.
817
818  "hwlat"
819
820	The Hardware Latency tracer is used to detect if the hardware
821	produces any latency. See "Hardware Latency Detector" section
822	below.
823
824  "irqsoff"
825
826	Traces the areas that disable interrupts and saves
827	the trace with the longest max latency.
828	See tracing_max_latency. When a new max is recorded,
829	it replaces the old trace. It is best to view this
830	trace with the latency-format option enabled, which
831	happens automatically when the tracer is selected.
832
833  "preemptoff"
834
835	Similar to irqsoff but traces and records the amount of
836	time for which preemption is disabled.
837
838  "preemptirqsoff"
839
840	Similar to irqsoff and preemptoff, but traces and
841	records the largest time for which irqs and/or preemption
842	is disabled.
843
844  "wakeup"
845
846	Traces and records the max latency that it takes for
847	the highest priority task to get scheduled after
848	it has been woken up.
849        Traces all tasks as an average developer would expect.
850
851  "wakeup_rt"
852
853        Traces and records the max latency that it takes for just
854        RT tasks (as the current "wakeup" does). This is useful
855        for those interested in wake up timings of RT tasks.
856
857  "wakeup_dl"
858
859	Traces and records the max latency that it takes for
860	a SCHED_DEADLINE task to be woken (as the "wakeup" and
861	"wakeup_rt" does).
862
863  "mmiotrace"
864
865	A special tracer that is used to trace binary module.
866	It will trace all the calls that a module makes to the
867	hardware. Everything it writes and reads from the I/O
868	as well.
869
870  "branch"
871
872	This tracer can be configured when tracing likely/unlikely
873	calls within the kernel. It will trace when a likely and
874	unlikely branch is hit and if it was correct in its prediction
875	of being correct.
876
877  "nop"
878
879	This is the "trace nothing" tracer. To remove all
880	tracers from tracing simply echo "nop" into
881	current_tracer.
882
883Error conditions
884----------------
885
886  For most ftrace commands, failure modes are obvious and communicated
887  using standard return codes.
888
889  For other more involved commands, extended error information may be
890  available via the tracing/error_log file.  For the commands that
891  support it, reading the tracing/error_log file after an error will
892  display more detailed information about what went wrong, if
893  information is available.  The tracing/error_log file is a circular
894  error log displaying a small number (currently, 8) of ftrace errors
895  for the last (8) failed commands.
896
897  The extended error information and usage takes the form shown in
898  this example::
899
900    # echo xxx > /sys/kernel/tracing/events/sched/sched_wakeup/trigger
901    echo: write error: Invalid argument
902
903    # cat /sys/kernel/tracing/error_log
904    [ 5348.887237] location: error: Couldn't yyy: zzz
905      Command: xxx
906               ^
907    [ 7517.023364] location: error: Bad rrr: sss
908      Command: ppp qqq
909                   ^
910
911  To clear the error log, echo the empty string into it::
912
913    # echo > /sys/kernel/tracing/error_log
914
915Examples of using the tracer
916----------------------------
917
918Here are typical examples of using the tracers when controlling
919them only with the tracefs interface (without using any
920user-land utilities).
921
922Output format:
923--------------
924
925Here is an example of the output format of the file "trace"::
926
927  # tracer: function
928  #
929  # entries-in-buffer/entries-written: 140080/250280   #P:4
930  #
931  #                              _-----=> irqs-off
932  #                             / _----=> need-resched
933  #                            | / _---=> hardirq/softirq
934  #                            || / _--=> preempt-depth
935  #                            ||| /     delay
936  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
937  #              | |       |   ||||       |         |
938              bash-1977  [000] .... 17284.993652: sys_close <-system_call_fastpath
939              bash-1977  [000] .... 17284.993653: __close_fd <-sys_close
940              bash-1977  [000] .... 17284.993653: _raw_spin_lock <-__close_fd
941              sshd-1974  [003] .... 17284.993653: __srcu_read_unlock <-fsnotify
942              bash-1977  [000] .... 17284.993654: add_preempt_count <-_raw_spin_lock
943              bash-1977  [000] ...1 17284.993655: _raw_spin_unlock <-__close_fd
944              bash-1977  [000] ...1 17284.993656: sub_preempt_count <-_raw_spin_unlock
945              bash-1977  [000] .... 17284.993657: filp_close <-__close_fd
946              bash-1977  [000] .... 17284.993657: dnotify_flush <-filp_close
947              sshd-1974  [003] .... 17284.993658: sys_select <-system_call_fastpath
948              ....
949
950A header is printed with the tracer name that is represented by
951the trace. In this case the tracer is "function". Then it shows the
952number of events in the buffer as well as the total number of entries
953that were written. The difference is the number of entries that were
954lost due to the buffer filling up (250280 - 140080 = 110200 events
955lost).
956
957The header explains the content of the events. Task name "bash", the task
958PID "1977", the CPU that it was running on "000", the latency format
959(explained below), the timestamp in <secs>.<usecs> format, the
960function name that was traced "sys_close" and the parent function that
961called this function "system_call_fastpath". The timestamp is the time
962at which the function was entered.
963
964Latency trace format
965--------------------
966
967When the latency-format option is enabled or when one of the latency
968tracers is set, the trace file gives somewhat more information to see
969why a latency happened. Here is a typical trace::
970
971  # tracer: irqsoff
972  #
973  # irqsoff latency trace v1.1.5 on 3.8.0-test+
974  # --------------------------------------------------------------------
975  # latency: 259 us, #4/4, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
976  #    -----------------
977  #    | task: ps-6143 (uid:0 nice:0 policy:0 rt_prio:0)
978  #    -----------------
979  #  => started at: __lock_task_sighand
980  #  => ended at:   _raw_spin_unlock_irqrestore
981  #
982  #
983  #                  _------=> CPU#
984  #                 / _-----=> irqs-off
985  #                | / _----=> need-resched
986  #                || / _---=> hardirq/softirq
987  #                ||| / _--=> preempt-depth
988  #                |||| /     delay
989  #  cmd     pid   ||||| time  |   caller
990  #     \   /      |||||  \    |   /
991        ps-6143    2d...    0us!: trace_hardirqs_off <-__lock_task_sighand
992        ps-6143    2d..1  259us+: trace_hardirqs_on <-_raw_spin_unlock_irqrestore
993        ps-6143    2d..1  263us+: time_hardirqs_on <-_raw_spin_unlock_irqrestore
994        ps-6143    2d..1  306us : <stack trace>
995   => trace_hardirqs_on_caller
996   => trace_hardirqs_on
997   => _raw_spin_unlock_irqrestore
998   => do_task_stat
999   => proc_tgid_stat
1000   => proc_single_show
1001   => seq_read
1002   => vfs_read
1003   => sys_read
1004   => system_call_fastpath
1005
1006
1007This shows that the current tracer is "irqsoff" tracing the time
1008for which interrupts were disabled. It gives the trace version (which
1009never changes) and the version of the kernel upon which this was executed on
1010(3.8). Then it displays the max latency in microseconds (259 us). The number
1011of trace entries displayed and the total number (both are four: #4/4).
1012VP, KP, SP, and HP are always zero and are reserved for later use.
1013#P is the number of online CPUs (#P:4).
1014
1015The task is the process that was running when the latency
1016occurred. (ps pid: 6143).
1017
1018The start and stop (the functions in which the interrupts were
1019disabled and enabled respectively) that caused the latencies:
1020
1021  - __lock_task_sighand is where the interrupts were disabled.
1022  - _raw_spin_unlock_irqrestore is where they were enabled again.
1023
1024The next lines after the header are the trace itself. The header
1025explains which is which.
1026
1027  cmd: The name of the process in the trace.
1028
1029  pid: The PID of that process.
1030
1031  CPU#: The CPU which the process was running on.
1032
1033  irqs-off: 'd' interrupts are disabled. '.' otherwise.
1034	.. caution:: If the architecture does not support a way to
1035		read the irq flags variable, an 'X' will always
1036		be printed here.
1037
1038  need-resched:
1039	- 'N' both TIF_NEED_RESCHED and PREEMPT_NEED_RESCHED is set,
1040	- 'n' only TIF_NEED_RESCHED is set,
1041	- 'p' only PREEMPT_NEED_RESCHED is set,
1042	- '.' otherwise.
1043
1044  hardirq/softirq:
1045	- 'Z' - NMI occurred inside a hardirq
1046	- 'z' - NMI is running
1047	- 'H' - hard irq occurred inside a softirq.
1048	- 'h' - hard irq is running
1049	- 's' - soft irq is running
1050	- '.' - normal context.
1051
1052  preempt-depth: The level of preempt_disabled
1053
1054The above is mostly meaningful for kernel developers.
1055
1056  time:
1057	When the latency-format option is enabled, the trace file
1058	output includes a timestamp relative to the start of the
1059	trace. This differs from the output when latency-format
1060	is disabled, which includes an absolute timestamp.
1061
1062  delay:
1063	This is just to help catch your eye a bit better. And
1064	needs to be fixed to be only relative to the same CPU.
1065	The marks are determined by the difference between this
1066	current trace and the next trace.
1067
1068	  - '$' - greater than 1 second
1069	  - '@' - greater than 100 millisecond
1070	  - '*' - greater than 10 millisecond
1071	  - '#' - greater than 1000 microsecond
1072	  - '!' - greater than 100 microsecond
1073	  - '+' - greater than 10 microsecond
1074	  - ' ' - less than or equal to 10 microsecond.
1075
1076  The rest is the same as the 'trace' file.
1077
1078  Note, the latency tracers will usually end with a back trace
1079  to easily find where the latency occurred.
1080
1081trace_options
1082-------------
1083
1084The trace_options file (or the options directory) is used to control
1085what gets printed in the trace output, or manipulate the tracers.
1086To see what is available, simply cat the file::
1087
1088  cat trace_options
1089	print-parent
1090	nosym-offset
1091	nosym-addr
1092	noverbose
1093	noraw
1094	nohex
1095	nobin
1096	noblock
1097	nofields
1098	trace_printk
1099	annotate
1100	nouserstacktrace
1101	nosym-userobj
1102	noprintk-msg-only
1103	context-info
1104	nolatency-format
1105	record-cmd
1106	norecord-tgid
1107	overwrite
1108	nodisable_on_free
1109	irq-info
1110	markers
1111	noevent-fork
1112	function-trace
1113	nofunction-fork
1114	nodisplay-graph
1115	nostacktrace
1116	nobranch
1117
1118To disable one of the options, echo in the option prepended with
1119"no"::
1120
1121  echo noprint-parent > trace_options
1122
1123To enable an option, leave off the "no"::
1124
1125  echo sym-offset > trace_options
1126
1127Here are the available options:
1128
1129  print-parent
1130	On function traces, display the calling (parent)
1131	function as well as the function being traced.
1132	::
1133
1134	  print-parent:
1135	   bash-4000  [01]  1477.606694: simple_strtoul <-kstrtoul
1136
1137	  noprint-parent:
1138	   bash-4000  [01]  1477.606694: simple_strtoul
1139
1140
1141  sym-offset
1142	Display not only the function name, but also the
1143	offset in the function. For example, instead of
1144	seeing just "ktime_get", you will see
1145	"ktime_get+0xb/0x20".
1146	::
1147
1148	  sym-offset:
1149	   bash-4000  [01]  1477.606694: simple_strtoul+0x6/0xa0
1150
1151  sym-addr
1152	This will also display the function address as well
1153	as the function name.
1154	::
1155
1156	  sym-addr:
1157	   bash-4000  [01]  1477.606694: simple_strtoul <c0339346>
1158
1159  verbose
1160	This deals with the trace file when the
1161        latency-format option is enabled.
1162	::
1163
1164	    bash  4000 1 0 00000000 00010a95 [58127d26] 1720.415ms \
1165	    (+0.000ms): simple_strtoul (kstrtoul)
1166
1167  raw
1168	This will display raw numbers. This option is best for
1169	use with user applications that can translate the raw
1170	numbers better than having it done in the kernel.
1171
1172  hex
1173	Similar to raw, but the numbers will be in a hexadecimal format.
1174
1175  bin
1176	This will print out the formats in raw binary.
1177
1178  block
1179	When set, reading trace_pipe will not block when polled.
1180
1181  fields
1182	Print the fields as described by their types. This is a better
1183	option than using hex, bin or raw, as it gives a better parsing
1184	of the content of the event.
1185
1186  trace_printk
1187	Can disable trace_printk() from writing into the buffer.
1188
1189  trace_printk_dest
1190	Set to have trace_printk() and similar internal tracing functions
1191	write into this instance. Note, only one trace instance can have
1192	this set. By setting this flag, it clears the trace_printk_dest flag
1193	of the instance that had it set previously. By default, the top
1194	level trace has this set, and will get it set again if another
1195	instance has it set then clears it.
1196
1197	This flag cannot be cleared by the top level instance, as it is the
1198	default instance. The only way the top level instance has this flag
1199	cleared, is by it being set in another instance.
1200
1201  annotate
1202	It is sometimes confusing when the CPU buffers are full
1203	and one CPU buffer had a lot of events recently, thus
1204	a shorter time frame, were another CPU may have only had
1205	a few events, which lets it have older events. When
1206	the trace is reported, it shows the oldest events first,
1207	and it may look like only one CPU ran (the one with the
1208	oldest events). When the annotate option is set, it will
1209	display when a new CPU buffer started::
1210
1211			  <idle>-0     [001] dNs4 21169.031481: wake_up_idle_cpu <-add_timer_on
1212			  <idle>-0     [001] dNs4 21169.031482: _raw_spin_unlock_irqrestore <-add_timer_on
1213			  <idle>-0     [001] .Ns4 21169.031484: sub_preempt_count <-_raw_spin_unlock_irqrestore
1214		##### CPU 2 buffer started ####
1215			  <idle>-0     [002] .N.1 21169.031484: rcu_idle_exit <-cpu_idle
1216			  <idle>-0     [001] .Ns3 21169.031484: _raw_spin_unlock <-clocksource_watchdog
1217			  <idle>-0     [001] .Ns3 21169.031485: sub_preempt_count <-_raw_spin_unlock
1218
1219  userstacktrace
1220	This option changes the trace. It records a
1221	stacktrace of the current user space thread after
1222	each trace event.
1223
1224  sym-userobj
1225	when user stacktrace are enabled, look up which
1226	object the address belongs to, and print a
1227	relative address. This is especially useful when
1228	ASLR is on, otherwise you don't get a chance to
1229	resolve the address to object/file/line after
1230	the app is no longer running
1231
1232	The lookup is performed when you read
1233	trace,trace_pipe. Example::
1234
1235		  a.out-1623  [000] 40874.465068: /root/a.out[+0x480] <-/root/a.out[+0
1236		  x494] <- /root/a.out[+0x4a8] <- /lib/libc-2.7.so[+0x1e1a6]
1237
1238
1239  printk-msg-only
1240	When set, trace_printk()s will only show the format
1241	and not their parameters (if trace_bprintk() or
1242	trace_bputs() was used to save the trace_printk()).
1243
1244  context-info
1245	Show only the event data. Hides the comm, PID,
1246	timestamp, CPU, and other useful data.
1247
1248  latency-format
1249	This option changes the trace output. When it is enabled,
1250	the trace displays additional information about the
1251	latency, as described in "Latency trace format".
1252
1253  pause-on-trace
1254	When set, opening the trace file for read, will pause
1255	writing to the ring buffer (as if tracing_on was set to zero).
1256	This simulates the original behavior of the trace file.
1257	When the file is closed, tracing will be enabled again.
1258
1259  hash-ptr
1260        When set, "%p" in the event printk format displays the
1261        hashed pointer value instead of real address.
1262        This will be useful if you want to find out which hashed
1263        value is corresponding to the real value in trace log.
1264
1265  record-cmd
1266	When any event or tracer is enabled, a hook is enabled
1267	in the sched_switch trace point to fill comm cache
1268	with mapped pids and comms. But this may cause some
1269	overhead, and if you only care about pids, and not the
1270	name of the task, disabling this option can lower the
1271	impact of tracing. See "saved_cmdlines".
1272
1273  record-tgid
1274	When any event or tracer is enabled, a hook is enabled
1275	in the sched_switch trace point to fill the cache of
1276	mapped Thread Group IDs (TGID) mapping to pids. See
1277	"saved_tgids".
1278
1279  overwrite
1280	This controls what happens when the trace buffer is
1281	full. If "1" (default), the oldest events are
1282	discarded and overwritten. If "0", then the newest
1283	events are discarded.
1284	(see per_cpu/cpu0/stats for overrun and dropped)
1285
1286  disable_on_free
1287	When the free_buffer is closed, tracing will
1288	stop (tracing_on set to 0).
1289
1290  irq-info
1291	Shows the interrupt, preempt count, need resched data.
1292	When disabled, the trace looks like::
1293
1294		# tracer: function
1295		#
1296		# entries-in-buffer/entries-written: 144405/9452052   #P:4
1297		#
1298		#           TASK-PID   CPU#      TIMESTAMP  FUNCTION
1299		#              | |       |          |         |
1300			  <idle>-0     [002]  23636.756054: ttwu_do_activate.constprop.89 <-try_to_wake_up
1301			  <idle>-0     [002]  23636.756054: activate_task <-ttwu_do_activate.constprop.89
1302			  <idle>-0     [002]  23636.756055: enqueue_task <-activate_task
1303
1304
1305  markers
1306	When set, the trace_marker is writable (only by root).
1307	When disabled, the trace_marker will error with EINVAL
1308	on write.
1309
1310  event-fork
1311	When set, tasks with PIDs listed in set_event_pid will have
1312	the PIDs of their children added to set_event_pid when those
1313	tasks fork. Also, when tasks with PIDs in set_event_pid exit,
1314	their PIDs will be removed from the file.
1315
1316        This affects PIDs listed in set_event_notrace_pid as well.
1317
1318  function-trace
1319	The latency tracers will enable function tracing
1320	if this option is enabled (default it is). When
1321	it is disabled, the latency tracers do not trace
1322	functions. This keeps the overhead of the tracer down
1323	when performing latency tests.
1324
1325  function-fork
1326	When set, tasks with PIDs listed in set_ftrace_pid will
1327	have the PIDs of their children added to set_ftrace_pid
1328	when those tasks fork. Also, when tasks with PIDs in
1329	set_ftrace_pid exit, their PIDs will be removed from the
1330	file.
1331
1332        This affects PIDs in set_ftrace_notrace_pid as well.
1333
1334  display-graph
1335	When set, the latency tracers (irqsoff, wakeup, etc) will
1336	use function graph tracing instead of function tracing.
1337
1338  stacktrace
1339	When set, a stack trace is recorded after any trace event
1340	is recorded.
1341
1342  branch
1343	Enable branch tracing with the tracer. This enables branch
1344	tracer along with the currently set tracer. Enabling this
1345	with the "nop" tracer is the same as just enabling the
1346	"branch" tracer.
1347
1348.. tip:: Some tracers have their own options. They only appear in this
1349       file when the tracer is active. They always appear in the
1350       options directory.
1351
1352
1353Here are the per tracer options:
1354
1355Options for function tracer:
1356
1357  func_stack_trace
1358	When set, a stack trace is recorded after every
1359	function that is recorded. NOTE! Limit the functions
1360	that are recorded before enabling this, with
1361	"set_ftrace_filter" otherwise the system performance
1362	will be critically degraded. Remember to disable
1363	this option before clearing the function filter.
1364
1365Options for function_graph tracer:
1366
1367 Since the function_graph tracer has a slightly different output
1368 it has its own options to control what is displayed.
1369
1370  funcgraph-overrun
1371	When set, the "overrun" of the graph stack is
1372	displayed after each function traced. The
1373	overrun, is when the stack depth of the calls
1374	is greater than what is reserved for each task.
1375	Each task has a fixed array of functions to
1376	trace in the call graph. If the depth of the
1377	calls exceeds that, the function is not traced.
1378	The overrun is the number of functions missed
1379	due to exceeding this array.
1380
1381  funcgraph-cpu
1382	When set, the CPU number of the CPU where the trace
1383	occurred is displayed.
1384
1385  funcgraph-overhead
1386	When set, if the function takes longer than
1387	A certain amount, then a delay marker is
1388	displayed. See "delay" above, under the
1389	header description.
1390
1391  funcgraph-proc
1392	Unlike other tracers, the process' command line
1393	is not displayed by default, but instead only
1394	when a task is traced in and out during a context
1395	switch. Enabling this options has the command
1396	of each process displayed at every line.
1397
1398  funcgraph-duration
1399	At the end of each function (the return)
1400	the duration of the amount of time in the
1401	function is displayed in microseconds.
1402
1403  funcgraph-abstime
1404	When set, the timestamp is displayed at each line.
1405
1406  funcgraph-irqs
1407	When disabled, functions that happen inside an
1408	interrupt will not be traced.
1409
1410  funcgraph-tail
1411	When set, the return event will include the function
1412	that it represents. By default this is off, and
1413	only a closing curly bracket "}" is displayed for
1414	the return of a function.
1415
1416  funcgraph-retval
1417	When set, the return value of each traced function
1418	will be printed after an equal sign "=". By default
1419	this is off.
1420
1421  funcgraph-retval-hex
1422	When set, the return value will always be printed
1423	in hexadecimal format. If the option is not set and
1424	the return value is an error code, it will be printed
1425	in signed decimal format; otherwise it will also be
1426	printed in hexadecimal format. By default, this option
1427	is off.
1428
1429  sleep-time
1430	When running function graph tracer, to include
1431	the time a task schedules out in its function.
1432	When enabled, it will account time the task has been
1433	scheduled out as part of the function call.
1434
1435  graph-time
1436	When running function profiler with function graph tracer,
1437	to include the time to call nested functions. When this is
1438	not set, the time reported for the function will only
1439	include the time the function itself executed for, not the
1440	time for functions that it called.
1441
1442Options for blk tracer:
1443
1444  blk_classic
1445	Shows a more minimalistic output.
1446
1447
1448irqsoff
1449-------
1450
1451When interrupts are disabled, the CPU can not react to any other
1452external event (besides NMIs and SMIs). This prevents the timer
1453interrupt from triggering or the mouse interrupt from letting
1454the kernel know of a new mouse event. The result is a latency
1455with the reaction time.
1456
1457The irqsoff tracer tracks the time for which interrupts are
1458disabled. When a new maximum latency is hit, the tracer saves
1459the trace leading up to that latency point so that every time a
1460new maximum is reached, the old saved trace is discarded and the
1461new trace is saved.
1462
1463To reset the maximum, echo 0 into tracing_max_latency. Here is
1464an example::
1465
1466  # echo 0 > options/function-trace
1467  # echo irqsoff > current_tracer
1468  # echo 1 > tracing_on
1469  # echo 0 > tracing_max_latency
1470  # ls -ltr
1471  [...]
1472  # echo 0 > tracing_on
1473  # cat trace
1474  # tracer: irqsoff
1475  #
1476  # irqsoff latency trace v1.1.5 on 3.8.0-test+
1477  # --------------------------------------------------------------------
1478  # latency: 16 us, #4/4, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1479  #    -----------------
1480  #    | task: swapper/0-0 (uid:0 nice:0 policy:0 rt_prio:0)
1481  #    -----------------
1482  #  => started at: run_timer_softirq
1483  #  => ended at:   run_timer_softirq
1484  #
1485  #
1486  #                  _------=> CPU#
1487  #                 / _-----=> irqs-off
1488  #                | / _----=> need-resched
1489  #                || / _---=> hardirq/softirq
1490  #                ||| / _--=> preempt-depth
1491  #                |||| /     delay
1492  #  cmd     pid   ||||| time  |   caller
1493  #     \   /      |||||  \    |   /
1494    <idle>-0       0d.s2    0us+: _raw_spin_lock_irq <-run_timer_softirq
1495    <idle>-0       0dNs3   17us : _raw_spin_unlock_irq <-run_timer_softirq
1496    <idle>-0       0dNs3   17us+: trace_hardirqs_on <-run_timer_softirq
1497    <idle>-0       0dNs3   25us : <stack trace>
1498   => _raw_spin_unlock_irq
1499   => run_timer_softirq
1500   => __do_softirq
1501   => call_softirq
1502   => do_softirq
1503   => irq_exit
1504   => smp_apic_timer_interrupt
1505   => apic_timer_interrupt
1506   => rcu_idle_exit
1507   => cpu_idle
1508   => rest_init
1509   => start_kernel
1510   => x86_64_start_reservations
1511   => x86_64_start_kernel
1512
1513Here we see that we had a latency of 16 microseconds (which is
1514very good). The _raw_spin_lock_irq in run_timer_softirq disabled
1515interrupts. The difference between the 16 and the displayed
1516timestamp 25us occurred because the clock was incremented
1517between the time of recording the max latency and the time of
1518recording the function that had that latency.
1519
1520Note the above example had function-trace not set. If we set
1521function-trace, we get a much larger output::
1522
1523 with echo 1 > options/function-trace
1524
1525  # tracer: irqsoff
1526  #
1527  # irqsoff latency trace v1.1.5 on 3.8.0-test+
1528  # --------------------------------------------------------------------
1529  # latency: 71 us, #168/168, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1530  #    -----------------
1531  #    | task: bash-2042 (uid:0 nice:0 policy:0 rt_prio:0)
1532  #    -----------------
1533  #  => started at: ata_scsi_queuecmd
1534  #  => ended at:   ata_scsi_queuecmd
1535  #
1536  #
1537  #                  _------=> CPU#
1538  #                 / _-----=> irqs-off
1539  #                | / _----=> need-resched
1540  #                || / _---=> hardirq/softirq
1541  #                ||| / _--=> preempt-depth
1542  #                |||| /     delay
1543  #  cmd     pid   ||||| time  |   caller
1544  #     \   /      |||||  \    |   /
1545      bash-2042    3d...    0us : _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1546      bash-2042    3d...    0us : add_preempt_count <-_raw_spin_lock_irqsave
1547      bash-2042    3d..1    1us : ata_scsi_find_dev <-ata_scsi_queuecmd
1548      bash-2042    3d..1    1us : __ata_scsi_find_dev <-ata_scsi_find_dev
1549      bash-2042    3d..1    2us : ata_find_dev.part.14 <-__ata_scsi_find_dev
1550      bash-2042    3d..1    2us : ata_qc_new_init <-__ata_scsi_queuecmd
1551      bash-2042    3d..1    3us : ata_sg_init <-__ata_scsi_queuecmd
1552      bash-2042    3d..1    4us : ata_scsi_rw_xlat <-__ata_scsi_queuecmd
1553      bash-2042    3d..1    4us : ata_build_rw_tf <-ata_scsi_rw_xlat
1554  [...]
1555      bash-2042    3d..1   67us : delay_tsc <-__delay
1556      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1557      bash-2042    3d..2   67us : sub_preempt_count <-delay_tsc
1558      bash-2042    3d..1   67us : add_preempt_count <-delay_tsc
1559      bash-2042    3d..2   68us : sub_preempt_count <-delay_tsc
1560      bash-2042    3d..1   68us+: ata_bmdma_start <-ata_bmdma_qc_issue
1561      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1562      bash-2042    3d..1   71us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1563      bash-2042    3d..1   72us+: trace_hardirqs_on <-ata_scsi_queuecmd
1564      bash-2042    3d..1  120us : <stack trace>
1565   => _raw_spin_unlock_irqrestore
1566   => ata_scsi_queuecmd
1567   => scsi_dispatch_cmd
1568   => scsi_request_fn
1569   => __blk_run_queue_uncond
1570   => __blk_run_queue
1571   => blk_queue_bio
1572   => submit_bio_noacct
1573   => submit_bio
1574   => submit_bh
1575   => __ext3_get_inode_loc
1576   => ext3_iget
1577   => ext3_lookup
1578   => lookup_real
1579   => __lookup_hash
1580   => walk_component
1581   => lookup_last
1582   => path_lookupat
1583   => filename_lookup
1584   => user_path_at_empty
1585   => user_path_at
1586   => vfs_fstatat
1587   => vfs_stat
1588   => sys_newstat
1589   => system_call_fastpath
1590
1591
1592Here we traced a 71 microsecond latency. But we also see all the
1593functions that were called during that time. Note that by
1594enabling function tracing, we incur an added overhead. This
1595overhead may extend the latency times. But nevertheless, this
1596trace has provided some very helpful debugging information.
1597
1598If we prefer function graph output instead of function, we can set
1599display-graph option::
1600
1601 with echo 1 > options/display-graph
1602
1603  # tracer: irqsoff
1604  #
1605  # irqsoff latency trace v1.1.5 on 4.20.0-rc6+
1606  # --------------------------------------------------------------------
1607  # latency: 3751 us, #274/274, CPU#0 | (M:desktop VP:0, KP:0, SP:0 HP:0 #P:4)
1608  #    -----------------
1609  #    | task: bash-1507 (uid:0 nice:0 policy:0 rt_prio:0)
1610  #    -----------------
1611  #  => started at: free_debug_processing
1612  #  => ended at:   return_to_handler
1613  #
1614  #
1615  #                                       _-----=> irqs-off
1616  #                                      / _----=> need-resched
1617  #                                     | / _---=> hardirq/softirq
1618  #                                     || / _--=> preempt-depth
1619  #                                     ||| /
1620  #   REL TIME      CPU  TASK/PID       ||||     DURATION                  FUNCTION CALLS
1621  #      |          |     |    |        ||||      |   |                     |   |   |   |
1622          0 us |   0)   bash-1507    |  d... |   0.000 us    |  _raw_spin_lock_irqsave();
1623          0 us |   0)   bash-1507    |  d..1 |   0.378 us    |    do_raw_spin_trylock();
1624          1 us |   0)   bash-1507    |  d..2 |               |    set_track() {
1625          2 us |   0)   bash-1507    |  d..2 |               |      save_stack_trace() {
1626          2 us |   0)   bash-1507    |  d..2 |               |        __save_stack_trace() {
1627          3 us |   0)   bash-1507    |  d..2 |               |          __unwind_start() {
1628          3 us |   0)   bash-1507    |  d..2 |               |            get_stack_info() {
1629          3 us |   0)   bash-1507    |  d..2 |   0.351 us    |              in_task_stack();
1630          4 us |   0)   bash-1507    |  d..2 |   1.107 us    |            }
1631  [...]
1632       3750 us |   0)   bash-1507    |  d..1 |   0.516 us    |      do_raw_spin_unlock();
1633       3750 us |   0)   bash-1507    |  d..1 |   0.000 us    |  _raw_spin_unlock_irqrestore();
1634       3764 us |   0)   bash-1507    |  d..1 |   0.000 us    |  tracer_hardirqs_on();
1635      bash-1507    0d..1 3792us : <stack trace>
1636   => free_debug_processing
1637   => __slab_free
1638   => kmem_cache_free
1639   => vm_area_free
1640   => remove_vma
1641   => exit_mmap
1642   => mmput
1643   => begin_new_exec
1644   => load_elf_binary
1645   => search_binary_handler
1646   => __do_execve_file.isra.32
1647   => __x64_sys_execve
1648   => do_syscall_64
1649   => entry_SYSCALL_64_after_hwframe
1650
1651preemptoff
1652----------
1653
1654When preemption is disabled, we may be able to receive
1655interrupts but the task cannot be preempted and a higher
1656priority task must wait for preemption to be enabled again
1657before it can preempt a lower priority task.
1658
1659The preemptoff tracer traces the places that disable preemption.
1660Like the irqsoff tracer, it records the maximum latency for
1661which preemption was disabled. The control of preemptoff tracer
1662is much like the irqsoff tracer.
1663::
1664
1665  # echo 0 > options/function-trace
1666  # echo preemptoff > current_tracer
1667  # echo 1 > tracing_on
1668  # echo 0 > tracing_max_latency
1669  # ls -ltr
1670  [...]
1671  # echo 0 > tracing_on
1672  # cat trace
1673  # tracer: preemptoff
1674  #
1675  # preemptoff latency trace v1.1.5 on 3.8.0-test+
1676  # --------------------------------------------------------------------
1677  # latency: 46 us, #4/4, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1678  #    -----------------
1679  #    | task: sshd-1991 (uid:0 nice:0 policy:0 rt_prio:0)
1680  #    -----------------
1681  #  => started at: do_IRQ
1682  #  => ended at:   do_IRQ
1683  #
1684  #
1685  #                  _------=> CPU#
1686  #                 / _-----=> irqs-off
1687  #                | / _----=> need-resched
1688  #                || / _---=> hardirq/softirq
1689  #                ||| / _--=> preempt-depth
1690  #                |||| /     delay
1691  #  cmd     pid   ||||| time  |   caller
1692  #     \   /      |||||  \    |   /
1693      sshd-1991    1d.h.    0us+: irq_enter <-do_IRQ
1694      sshd-1991    1d..1   46us : irq_exit <-do_IRQ
1695      sshd-1991    1d..1   47us+: trace_preempt_on <-do_IRQ
1696      sshd-1991    1d..1   52us : <stack trace>
1697   => sub_preempt_count
1698   => irq_exit
1699   => do_IRQ
1700   => ret_from_intr
1701
1702
1703This has some more changes. Preemption was disabled when an
1704interrupt came in (notice the 'h'), and was enabled on exit.
1705But we also see that interrupts have been disabled when entering
1706the preempt off section and leaving it (the 'd'). We do not know if
1707interrupts were enabled in the mean time or shortly after this
1708was over.
1709::
1710
1711  # tracer: preemptoff
1712  #
1713  # preemptoff latency trace v1.1.5 on 3.8.0-test+
1714  # --------------------------------------------------------------------
1715  # latency: 83 us, #241/241, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1716  #    -----------------
1717  #    | task: bash-1994 (uid:0 nice:0 policy:0 rt_prio:0)
1718  #    -----------------
1719  #  => started at: wake_up_new_task
1720  #  => ended at:   task_rq_unlock
1721  #
1722  #
1723  #                  _------=> CPU#
1724  #                 / _-----=> irqs-off
1725  #                | / _----=> need-resched
1726  #                || / _---=> hardirq/softirq
1727  #                ||| / _--=> preempt-depth
1728  #                |||| /     delay
1729  #  cmd     pid   ||||| time  |   caller
1730  #     \   /      |||||  \    |   /
1731      bash-1994    1d..1    0us : _raw_spin_lock_irqsave <-wake_up_new_task
1732      bash-1994    1d..1    0us : select_task_rq_fair <-select_task_rq
1733      bash-1994    1d..1    1us : __rcu_read_lock <-select_task_rq_fair
1734      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1735      bash-1994    1d..1    1us : source_load <-select_task_rq_fair
1736  [...]
1737      bash-1994    1d..1   12us : irq_enter <-smp_apic_timer_interrupt
1738      bash-1994    1d..1   12us : rcu_irq_enter <-irq_enter
1739      bash-1994    1d..1   13us : add_preempt_count <-irq_enter
1740      bash-1994    1d.h1   13us : exit_idle <-smp_apic_timer_interrupt
1741      bash-1994    1d.h1   13us : hrtimer_interrupt <-smp_apic_timer_interrupt
1742      bash-1994    1d.h1   13us : _raw_spin_lock <-hrtimer_interrupt
1743      bash-1994    1d.h1   14us : add_preempt_count <-_raw_spin_lock
1744      bash-1994    1d.h2   14us : ktime_get_update_offsets <-hrtimer_interrupt
1745  [...]
1746      bash-1994    1d.h1   35us : lapic_next_event <-clockevents_program_event
1747      bash-1994    1d.h1   35us : irq_exit <-smp_apic_timer_interrupt
1748      bash-1994    1d.h1   36us : sub_preempt_count <-irq_exit
1749      bash-1994    1d..2   36us : do_softirq <-irq_exit
1750      bash-1994    1d..2   36us : __do_softirq <-call_softirq
1751      bash-1994    1d..2   36us : __local_bh_disable <-__do_softirq
1752      bash-1994    1d.s2   37us : add_preempt_count <-_raw_spin_lock_irq
1753      bash-1994    1d.s3   38us : _raw_spin_unlock <-run_timer_softirq
1754      bash-1994    1d.s3   39us : sub_preempt_count <-_raw_spin_unlock
1755      bash-1994    1d.s2   39us : call_timer_fn <-run_timer_softirq
1756  [...]
1757      bash-1994    1dNs2   81us : cpu_needs_another_gp <-rcu_process_callbacks
1758      bash-1994    1dNs2   82us : __local_bh_enable <-__do_softirq
1759      bash-1994    1dNs2   82us : sub_preempt_count <-__local_bh_enable
1760      bash-1994    1dN.2   82us : idle_cpu <-irq_exit
1761      bash-1994    1dN.2   83us : rcu_irq_exit <-irq_exit
1762      bash-1994    1dN.2   83us : sub_preempt_count <-irq_exit
1763      bash-1994    1.N.1   84us : _raw_spin_unlock_irqrestore <-task_rq_unlock
1764      bash-1994    1.N.1   84us+: trace_preempt_on <-task_rq_unlock
1765      bash-1994    1.N.1  104us : <stack trace>
1766   => sub_preempt_count
1767   => _raw_spin_unlock_irqrestore
1768   => task_rq_unlock
1769   => wake_up_new_task
1770   => do_fork
1771   => sys_clone
1772   => stub_clone
1773
1774
1775The above is an example of the preemptoff trace with
1776function-trace set. Here we see that interrupts were not disabled
1777the entire time. The irq_enter code lets us know that we entered
1778an interrupt 'h'. Before that, the functions being traced still
1779show that it is not in an interrupt, but we can see from the
1780functions themselves that this is not the case.
1781
1782preemptirqsoff
1783--------------
1784
1785Knowing the locations that have interrupts disabled or
1786preemption disabled for the longest times is helpful. But
1787sometimes we would like to know when either preemption and/or
1788interrupts are disabled.
1789
1790Consider the following code::
1791
1792    local_irq_disable();
1793    call_function_with_irqs_off();
1794    preempt_disable();
1795    call_function_with_irqs_and_preemption_off();
1796    local_irq_enable();
1797    call_function_with_preemption_off();
1798    preempt_enable();
1799
1800The irqsoff tracer will record the total length of
1801call_function_with_irqs_off() and
1802call_function_with_irqs_and_preemption_off().
1803
1804The preemptoff tracer will record the total length of
1805call_function_with_irqs_and_preemption_off() and
1806call_function_with_preemption_off().
1807
1808But neither will trace the time that interrupts and/or
1809preemption is disabled. This total time is the time that we can
1810not schedule. To record this time, use the preemptirqsoff
1811tracer.
1812
1813Again, using this trace is much like the irqsoff and preemptoff
1814tracers.
1815::
1816
1817  # echo 0 > options/function-trace
1818  # echo preemptirqsoff > current_tracer
1819  # echo 1 > tracing_on
1820  # echo 0 > tracing_max_latency
1821  # ls -ltr
1822  [...]
1823  # echo 0 > tracing_on
1824  # cat trace
1825  # tracer: preemptirqsoff
1826  #
1827  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1828  # --------------------------------------------------------------------
1829  # latency: 100 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1830  #    -----------------
1831  #    | task: ls-2230 (uid:0 nice:0 policy:0 rt_prio:0)
1832  #    -----------------
1833  #  => started at: ata_scsi_queuecmd
1834  #  => ended at:   ata_scsi_queuecmd
1835  #
1836  #
1837  #                  _------=> CPU#
1838  #                 / _-----=> irqs-off
1839  #                | / _----=> need-resched
1840  #                || / _---=> hardirq/softirq
1841  #                ||| / _--=> preempt-depth
1842  #                |||| /     delay
1843  #  cmd     pid   ||||| time  |   caller
1844  #     \   /      |||||  \    |   /
1845        ls-2230    3d...    0us+: _raw_spin_lock_irqsave <-ata_scsi_queuecmd
1846        ls-2230    3...1  100us : _raw_spin_unlock_irqrestore <-ata_scsi_queuecmd
1847        ls-2230    3...1  101us+: trace_preempt_on <-ata_scsi_queuecmd
1848        ls-2230    3...1  111us : <stack trace>
1849   => sub_preempt_count
1850   => _raw_spin_unlock_irqrestore
1851   => ata_scsi_queuecmd
1852   => scsi_dispatch_cmd
1853   => scsi_request_fn
1854   => __blk_run_queue_uncond
1855   => __blk_run_queue
1856   => blk_queue_bio
1857   => submit_bio_noacct
1858   => submit_bio
1859   => submit_bh
1860   => ext3_bread
1861   => ext3_dir_bread
1862   => htree_dirblock_to_tree
1863   => ext3_htree_fill_tree
1864   => ext3_readdir
1865   => vfs_readdir
1866   => sys_getdents
1867   => system_call_fastpath
1868
1869
1870The trace_hardirqs_off_thunk is called from assembly on x86 when
1871interrupts are disabled in the assembly code. Without the
1872function tracing, we do not know if interrupts were enabled
1873within the preemption points. We do see that it started with
1874preemption enabled.
1875
1876Here is a trace with function-trace set::
1877
1878  # tracer: preemptirqsoff
1879  #
1880  # preemptirqsoff latency trace v1.1.5 on 3.8.0-test+
1881  # --------------------------------------------------------------------
1882  # latency: 161 us, #339/339, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1883  #    -----------------
1884  #    | task: ls-2269 (uid:0 nice:0 policy:0 rt_prio:0)
1885  #    -----------------
1886  #  => started at: schedule
1887  #  => ended at:   mutex_unlock
1888  #
1889  #
1890  #                  _------=> CPU#
1891  #                 / _-----=> irqs-off
1892  #                | / _----=> need-resched
1893  #                || / _---=> hardirq/softirq
1894  #                ||| / _--=> preempt-depth
1895  #                |||| /     delay
1896  #  cmd     pid   ||||| time  |   caller
1897  #     \   /      |||||  \    |   /
1898  kworker/-59      3...1    0us : __schedule <-schedule
1899  kworker/-59      3d..1    0us : rcu_preempt_qs <-rcu_note_context_switch
1900  kworker/-59      3d..1    1us : add_preempt_count <-_raw_spin_lock_irq
1901  kworker/-59      3d..2    1us : deactivate_task <-__schedule
1902  kworker/-59      3d..2    1us : dequeue_task <-deactivate_task
1903  kworker/-59      3d..2    2us : update_rq_clock <-dequeue_task
1904  kworker/-59      3d..2    2us : dequeue_task_fair <-dequeue_task
1905  kworker/-59      3d..2    2us : update_curr <-dequeue_task_fair
1906  kworker/-59      3d..2    2us : update_min_vruntime <-update_curr
1907  kworker/-59      3d..2    3us : cpuacct_charge <-update_curr
1908  kworker/-59      3d..2    3us : __rcu_read_lock <-cpuacct_charge
1909  kworker/-59      3d..2    3us : __rcu_read_unlock <-cpuacct_charge
1910  kworker/-59      3d..2    3us : update_cfs_rq_blocked_load <-dequeue_task_fair
1911  kworker/-59      3d..2    4us : clear_buddies <-dequeue_task_fair
1912  kworker/-59      3d..2    4us : account_entity_dequeue <-dequeue_task_fair
1913  kworker/-59      3d..2    4us : update_min_vruntime <-dequeue_task_fair
1914  kworker/-59      3d..2    4us : update_cfs_shares <-dequeue_task_fair
1915  kworker/-59      3d..2    5us : hrtick_update <-dequeue_task_fair
1916  kworker/-59      3d..2    5us : wq_worker_sleeping <-__schedule
1917  kworker/-59      3d..2    5us : kthread_data <-wq_worker_sleeping
1918  kworker/-59      3d..2    5us : put_prev_task_fair <-__schedule
1919  kworker/-59      3d..2    6us : pick_next_task_fair <-pick_next_task
1920  kworker/-59      3d..2    6us : clear_buddies <-pick_next_task_fair
1921  kworker/-59      3d..2    6us : set_next_entity <-pick_next_task_fair
1922  kworker/-59      3d..2    6us : update_stats_wait_end <-set_next_entity
1923        ls-2269    3d..2    7us : finish_task_switch <-__schedule
1924        ls-2269    3d..2    7us : _raw_spin_unlock_irq <-finish_task_switch
1925        ls-2269    3d..2    8us : do_IRQ <-ret_from_intr
1926        ls-2269    3d..2    8us : irq_enter <-do_IRQ
1927        ls-2269    3d..2    8us : rcu_irq_enter <-irq_enter
1928        ls-2269    3d..2    9us : add_preempt_count <-irq_enter
1929        ls-2269    3d.h2    9us : exit_idle <-do_IRQ
1930  [...]
1931        ls-2269    3d.h3   20us : sub_preempt_count <-_raw_spin_unlock
1932        ls-2269    3d.h2   20us : irq_exit <-do_IRQ
1933        ls-2269    3d.h2   21us : sub_preempt_count <-irq_exit
1934        ls-2269    3d..3   21us : do_softirq <-irq_exit
1935        ls-2269    3d..3   21us : __do_softirq <-call_softirq
1936        ls-2269    3d..3   21us+: __local_bh_disable <-__do_softirq
1937        ls-2269    3d.s4   29us : sub_preempt_count <-_local_bh_enable_ip
1938        ls-2269    3d.s5   29us : sub_preempt_count <-_local_bh_enable_ip
1939        ls-2269    3d.s5   31us : do_IRQ <-ret_from_intr
1940        ls-2269    3d.s5   31us : irq_enter <-do_IRQ
1941        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1942  [...]
1943        ls-2269    3d.s5   31us : rcu_irq_enter <-irq_enter
1944        ls-2269    3d.s5   32us : add_preempt_count <-irq_enter
1945        ls-2269    3d.H5   32us : exit_idle <-do_IRQ
1946        ls-2269    3d.H5   32us : handle_irq <-do_IRQ
1947        ls-2269    3d.H5   32us : irq_to_desc <-handle_irq
1948        ls-2269    3d.H5   33us : handle_fasteoi_irq <-handle_irq
1949  [...]
1950        ls-2269    3d.s5  158us : _raw_spin_unlock_irqrestore <-rtl8139_poll
1951        ls-2269    3d.s3  158us : net_rps_action_and_irq_enable.isra.65 <-net_rx_action
1952        ls-2269    3d.s3  159us : __local_bh_enable <-__do_softirq
1953        ls-2269    3d.s3  159us : sub_preempt_count <-__local_bh_enable
1954        ls-2269    3d..3  159us : idle_cpu <-irq_exit
1955        ls-2269    3d..3  159us : rcu_irq_exit <-irq_exit
1956        ls-2269    3d..3  160us : sub_preempt_count <-irq_exit
1957        ls-2269    3d...  161us : __mutex_unlock_slowpath <-mutex_unlock
1958        ls-2269    3d...  162us+: trace_hardirqs_on <-mutex_unlock
1959        ls-2269    3d...  186us : <stack trace>
1960   => __mutex_unlock_slowpath
1961   => mutex_unlock
1962   => process_output
1963   => n_tty_write
1964   => tty_write
1965   => vfs_write
1966   => sys_write
1967   => system_call_fastpath
1968
1969This is an interesting trace. It started with kworker running and
1970scheduling out and ls taking over. But as soon as ls released the
1971rq lock and enabled interrupts (but not preemption) an interrupt
1972triggered. When the interrupt finished, it started running softirqs.
1973But while the softirq was running, another interrupt triggered.
1974When an interrupt is running inside a softirq, the annotation is 'H'.
1975
1976
1977wakeup
1978------
1979
1980One common case that people are interested in tracing is the
1981time it takes for a task that is woken to actually wake up.
1982Now for non Real-Time tasks, this can be arbitrary. But tracing
1983it nonetheless can be interesting.
1984
1985Without function tracing::
1986
1987  # echo 0 > options/function-trace
1988  # echo wakeup > current_tracer
1989  # echo 1 > tracing_on
1990  # echo 0 > tracing_max_latency
1991  # chrt -f 5 sleep 1
1992  # echo 0 > tracing_on
1993  # cat trace
1994  # tracer: wakeup
1995  #
1996  # wakeup latency trace v1.1.5 on 3.8.0-test+
1997  # --------------------------------------------------------------------
1998  # latency: 15 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
1999  #    -----------------
2000  #    | task: kworker/3:1H-312 (uid:0 nice:-20 policy:0 rt_prio:0)
2001  #    -----------------
2002  #
2003  #                  _------=> CPU#
2004  #                 / _-----=> irqs-off
2005  #                | / _----=> need-resched
2006  #                || / _---=> hardirq/softirq
2007  #                ||| / _--=> preempt-depth
2008  #                |||| /     delay
2009  #  cmd     pid   ||||| time  |   caller
2010  #     \   /      |||||  \    |   /
2011    <idle>-0       3dNs7    0us :      0:120:R   + [003]   312:100:R kworker/3:1H
2012    <idle>-0       3dNs7    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2013    <idle>-0       3d..3   15us : __schedule <-schedule
2014    <idle>-0       3d..3   15us :      0:120:R ==> [003]   312:100:R kworker/3:1H
2015
2016The tracer only traces the highest priority task in the system
2017to avoid tracing the normal circumstances. Here we see that
2018the kworker with a nice priority of -20 (not very nice), took
2019just 15 microseconds from the time it woke up, to the time it
2020ran.
2021
2022Non Real-Time tasks are not that interesting. A more interesting
2023trace is to concentrate only on Real-Time tasks.
2024
2025wakeup_rt
2026---------
2027
2028In a Real-Time environment it is very important to know the
2029wakeup time it takes for the highest priority task that is woken
2030up to the time that it executes. This is also known as "schedule
2031latency". I stress the point that this is about RT tasks. It is
2032also important to know the scheduling latency of non-RT tasks,
2033but the average schedule latency is better for non-RT tasks.
2034Tools like LatencyTop are more appropriate for such
2035measurements.
2036
2037Real-Time environments are interested in the worst case latency.
2038That is the longest latency it takes for something to happen,
2039and not the average. We can have a very fast scheduler that may
2040only have a large latency once in a while, but that would not
2041work well with Real-Time tasks.  The wakeup_rt tracer was designed
2042to record the worst case wakeups of RT tasks. Non-RT tasks are
2043not recorded because the tracer only records one worst case and
2044tracing non-RT tasks that are unpredictable will overwrite the
2045worst case latency of RT tasks (just run the normal wakeup
2046tracer for a while to see that effect).
2047
2048Since this tracer only deals with RT tasks, we will run this
2049slightly differently than we did with the previous tracers.
2050Instead of performing an 'ls', we will run 'sleep 1' under
2051'chrt' which changes the priority of the task.
2052::
2053
2054  # echo 0 > options/function-trace
2055  # echo wakeup_rt > current_tracer
2056  # echo 1 > tracing_on
2057  # echo 0 > tracing_max_latency
2058  # chrt -f 5 sleep 1
2059  # echo 0 > tracing_on
2060  # cat trace
2061  # tracer: wakeup
2062  #
2063  # tracer: wakeup_rt
2064  #
2065  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2066  # --------------------------------------------------------------------
2067  # latency: 5 us, #4/4, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2068  #    -----------------
2069  #    | task: sleep-2389 (uid:0 nice:0 policy:1 rt_prio:5)
2070  #    -----------------
2071  #
2072  #                  _------=> CPU#
2073  #                 / _-----=> irqs-off
2074  #                | / _----=> need-resched
2075  #                || / _---=> hardirq/softirq
2076  #                ||| / _--=> preempt-depth
2077  #                |||| /     delay
2078  #  cmd     pid   ||||| time  |   caller
2079  #     \   /      |||||  \    |   /
2080    <idle>-0       3d.h4    0us :      0:120:R   + [003]  2389: 94:R sleep
2081    <idle>-0       3d.h4    1us+: ttwu_do_activate.constprop.87 <-try_to_wake_up
2082    <idle>-0       3d..3    5us : __schedule <-schedule
2083    <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
2084
2085
2086Running this on an idle system, we see that it only took 5 microseconds
2087to perform the task switch.  Note, since the trace point in the schedule
2088is before the actual "switch", we stop the tracing when the recorded task
2089is about to schedule in. This may change if we add a new marker at the
2090end of the scheduler.
2091
2092Notice that the recorded task is 'sleep' with the PID of 2389
2093and it has an rt_prio of 5. This priority is user-space priority
2094and not the internal kernel priority. The policy is 1 for
2095SCHED_FIFO and 2 for SCHED_RR.
2096
2097Note, that the trace data shows the internal priority (99 - rtprio).
2098::
2099
2100  <idle>-0       3d..3    5us :      0:120:R ==> [003]  2389: 94:R sleep
2101
2102The 0:120:R means idle was running with a nice priority of 0 (120 - 120)
2103and in the running state 'R'. The sleep task was scheduled in with
21042389: 94:R. That is the priority is the kernel rtprio (99 - 5 = 94)
2105and it too is in the running state.
2106
2107Doing the same with chrt -r 5 and function-trace set.
2108::
2109
2110  echo 1 > options/function-trace
2111
2112  # tracer: wakeup_rt
2113  #
2114  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2115  # --------------------------------------------------------------------
2116  # latency: 29 us, #85/85, CPU#3 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2117  #    -----------------
2118  #    | task: sleep-2448 (uid:0 nice:0 policy:1 rt_prio:5)
2119  #    -----------------
2120  #
2121  #                  _------=> CPU#
2122  #                 / _-----=> irqs-off
2123  #                | / _----=> need-resched
2124  #                || / _---=> hardirq/softirq
2125  #                ||| / _--=> preempt-depth
2126  #                |||| /     delay
2127  #  cmd     pid   ||||| time  |   caller
2128  #     \   /      |||||  \    |   /
2129    <idle>-0       3d.h4    1us+:      0:120:R   + [003]  2448: 94:R sleep
2130    <idle>-0       3d.h4    2us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2131    <idle>-0       3d.h3    3us : check_preempt_curr <-ttwu_do_wakeup
2132    <idle>-0       3d.h3    3us : resched_curr <-check_preempt_curr
2133    <idle>-0       3dNh3    4us : task_woken_rt <-ttwu_do_wakeup
2134    <idle>-0       3dNh3    4us : _raw_spin_unlock <-try_to_wake_up
2135    <idle>-0       3dNh3    4us : sub_preempt_count <-_raw_spin_unlock
2136    <idle>-0       3dNh2    5us : ttwu_stat <-try_to_wake_up
2137    <idle>-0       3dNh2    5us : _raw_spin_unlock_irqrestore <-try_to_wake_up
2138    <idle>-0       3dNh2    6us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2139    <idle>-0       3dNh1    6us : _raw_spin_lock <-__run_hrtimer
2140    <idle>-0       3dNh1    6us : add_preempt_count <-_raw_spin_lock
2141    <idle>-0       3dNh2    7us : _raw_spin_unlock <-hrtimer_interrupt
2142    <idle>-0       3dNh2    7us : sub_preempt_count <-_raw_spin_unlock
2143    <idle>-0       3dNh1    7us : tick_program_event <-hrtimer_interrupt
2144    <idle>-0       3dNh1    7us : clockevents_program_event <-tick_program_event
2145    <idle>-0       3dNh1    8us : ktime_get <-clockevents_program_event
2146    <idle>-0       3dNh1    8us : lapic_next_event <-clockevents_program_event
2147    <idle>-0       3dNh1    8us : irq_exit <-smp_apic_timer_interrupt
2148    <idle>-0       3dNh1    9us : sub_preempt_count <-irq_exit
2149    <idle>-0       3dN.2    9us : idle_cpu <-irq_exit
2150    <idle>-0       3dN.2    9us : rcu_irq_exit <-irq_exit
2151    <idle>-0       3dN.2   10us : rcu_eqs_enter_common.isra.45 <-rcu_irq_exit
2152    <idle>-0       3dN.2   10us : sub_preempt_count <-irq_exit
2153    <idle>-0       3.N.1   11us : rcu_idle_exit <-cpu_idle
2154    <idle>-0       3dN.1   11us : rcu_eqs_exit_common.isra.43 <-rcu_idle_exit
2155    <idle>-0       3.N.1   11us : tick_nohz_idle_exit <-cpu_idle
2156    <idle>-0       3dN.1   12us : menu_hrtimer_cancel <-tick_nohz_idle_exit
2157    <idle>-0       3dN.1   12us : ktime_get <-tick_nohz_idle_exit
2158    <idle>-0       3dN.1   12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit
2159    <idle>-0       3dN.1   13us : cpu_load_update_nohz <-tick_nohz_idle_exit
2160    <idle>-0       3dN.1   13us : _raw_spin_lock <-cpu_load_update_nohz
2161    <idle>-0       3dN.1   13us : add_preempt_count <-_raw_spin_lock
2162    <idle>-0       3dN.2   13us : __cpu_load_update <-cpu_load_update_nohz
2163    <idle>-0       3dN.2   14us : sched_avg_update <-__cpu_load_update
2164    <idle>-0       3dN.2   14us : _raw_spin_unlock <-cpu_load_update_nohz
2165    <idle>-0       3dN.2   14us : sub_preempt_count <-_raw_spin_unlock
2166    <idle>-0       3dN.1   15us : calc_load_nohz_stop <-tick_nohz_idle_exit
2167    <idle>-0       3dN.1   15us : touch_softlockup_watchdog <-tick_nohz_idle_exit
2168    <idle>-0       3dN.1   15us : hrtimer_cancel <-tick_nohz_idle_exit
2169    <idle>-0       3dN.1   15us : hrtimer_try_to_cancel <-hrtimer_cancel
2170    <idle>-0       3dN.1   16us : lock_hrtimer_base.isra.18 <-hrtimer_try_to_cancel
2171    <idle>-0       3dN.1   16us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2172    <idle>-0       3dN.1   16us : add_preempt_count <-_raw_spin_lock_irqsave
2173    <idle>-0       3dN.2   17us : __remove_hrtimer <-remove_hrtimer.part.16
2174    <idle>-0       3dN.2   17us : hrtimer_force_reprogram <-__remove_hrtimer
2175    <idle>-0       3dN.2   17us : tick_program_event <-hrtimer_force_reprogram
2176    <idle>-0       3dN.2   18us : clockevents_program_event <-tick_program_event
2177    <idle>-0       3dN.2   18us : ktime_get <-clockevents_program_event
2178    <idle>-0       3dN.2   18us : lapic_next_event <-clockevents_program_event
2179    <idle>-0       3dN.2   19us : _raw_spin_unlock_irqrestore <-hrtimer_try_to_cancel
2180    <idle>-0       3dN.2   19us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2181    <idle>-0       3dN.1   19us : hrtimer_forward <-tick_nohz_idle_exit
2182    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
2183    <idle>-0       3dN.1   20us : ktime_add_safe <-hrtimer_forward
2184    <idle>-0       3dN.1   20us : hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
2185    <idle>-0       3dN.1   20us : __hrtimer_start_range_ns <-hrtimer_start_range_ns
2186    <idle>-0       3dN.1   21us : lock_hrtimer_base.isra.18 <-__hrtimer_start_range_ns
2187    <idle>-0       3dN.1   21us : _raw_spin_lock_irqsave <-lock_hrtimer_base.isra.18
2188    <idle>-0       3dN.1   21us : add_preempt_count <-_raw_spin_lock_irqsave
2189    <idle>-0       3dN.2   22us : ktime_add_safe <-__hrtimer_start_range_ns
2190    <idle>-0       3dN.2   22us : enqueue_hrtimer <-__hrtimer_start_range_ns
2191    <idle>-0       3dN.2   22us : tick_program_event <-__hrtimer_start_range_ns
2192    <idle>-0       3dN.2   23us : clockevents_program_event <-tick_program_event
2193    <idle>-0       3dN.2   23us : ktime_get <-clockevents_program_event
2194    <idle>-0       3dN.2   23us : lapic_next_event <-clockevents_program_event
2195    <idle>-0       3dN.2   24us : _raw_spin_unlock_irqrestore <-__hrtimer_start_range_ns
2196    <idle>-0       3dN.2   24us : sub_preempt_count <-_raw_spin_unlock_irqrestore
2197    <idle>-0       3dN.1   24us : account_idle_ticks <-tick_nohz_idle_exit
2198    <idle>-0       3dN.1   24us : account_idle_time <-account_idle_ticks
2199    <idle>-0       3.N.1   25us : sub_preempt_count <-cpu_idle
2200    <idle>-0       3.N..   25us : schedule <-cpu_idle
2201    <idle>-0       3.N..   25us : __schedule <-preempt_schedule
2202    <idle>-0       3.N..   26us : add_preempt_count <-__schedule
2203    <idle>-0       3.N.1   26us : rcu_note_context_switch <-__schedule
2204    <idle>-0       3.N.1   26us : rcu_sched_qs <-rcu_note_context_switch
2205    <idle>-0       3dN.1   27us : rcu_preempt_qs <-rcu_note_context_switch
2206    <idle>-0       3.N.1   27us : _raw_spin_lock_irq <-__schedule
2207    <idle>-0       3dN.1   27us : add_preempt_count <-_raw_spin_lock_irq
2208    <idle>-0       3dN.2   28us : put_prev_task_idle <-__schedule
2209    <idle>-0       3dN.2   28us : pick_next_task_stop <-pick_next_task
2210    <idle>-0       3dN.2   28us : pick_next_task_rt <-pick_next_task
2211    <idle>-0       3dN.2   29us : dequeue_pushable_task <-pick_next_task_rt
2212    <idle>-0       3d..3   29us : __schedule <-preempt_schedule
2213    <idle>-0       3d..3   30us :      0:120:R ==> [003]  2448: 94:R sleep
2214
2215This isn't that big of a trace, even with function tracing enabled,
2216so I included the entire trace.
2217
2218The interrupt went off while when the system was idle. Somewhere
2219before task_woken_rt() was called, the NEED_RESCHED flag was set,
2220this is indicated by the first occurrence of the 'N' flag.
2221
2222Latency tracing and events
2223--------------------------
2224As function tracing can induce a much larger latency, but without
2225seeing what happens within the latency it is hard to know what
2226caused it. There is a middle ground, and that is with enabling
2227events.
2228::
2229
2230  # echo 0 > options/function-trace
2231  # echo wakeup_rt > current_tracer
2232  # echo 1 > events/enable
2233  # echo 1 > tracing_on
2234  # echo 0 > tracing_max_latency
2235  # chrt -f 5 sleep 1
2236  # echo 0 > tracing_on
2237  # cat trace
2238  # tracer: wakeup_rt
2239  #
2240  # wakeup_rt latency trace v1.1.5 on 3.8.0-test+
2241  # --------------------------------------------------------------------
2242  # latency: 6 us, #12/12, CPU#2 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:4)
2243  #    -----------------
2244  #    | task: sleep-5882 (uid:0 nice:0 policy:1 rt_prio:5)
2245  #    -----------------
2246  #
2247  #                  _------=> CPU#
2248  #                 / _-----=> irqs-off
2249  #                | / _----=> need-resched
2250  #                || / _---=> hardirq/softirq
2251  #                ||| / _--=> preempt-depth
2252  #                |||| /     delay
2253  #  cmd     pid   ||||| time  |   caller
2254  #     \   /      |||||  \    |   /
2255    <idle>-0       2d.h4    0us :      0:120:R   + [002]  5882: 94:R sleep
2256    <idle>-0       2d.h4    0us : ttwu_do_activate.constprop.87 <-try_to_wake_up
2257    <idle>-0       2d.h4    1us : sched_wakeup: comm=sleep pid=5882 prio=94 success=1 target_cpu=002
2258    <idle>-0       2dNh2    1us : hrtimer_expire_exit: hrtimer=ffff88007796feb8
2259    <idle>-0       2.N.2    2us : power_end: cpu_id=2
2260    <idle>-0       2.N.2    3us : cpu_idle: state=4294967295 cpu_id=2
2261    <idle>-0       2dN.3    4us : hrtimer_cancel: hrtimer=ffff88007d50d5e0
2262    <idle>-0       2dN.3    4us : hrtimer_start: hrtimer=ffff88007d50d5e0 function=tick_sched_timer expires=34311211000000 softexpires=34311211000000
2263    <idle>-0       2.N.2    5us : rcu_utilization: Start context switch
2264    <idle>-0       2.N.2    5us : rcu_utilization: End context switch
2265    <idle>-0       2d..3    6us : __schedule <-schedule
2266    <idle>-0       2d..3    6us :      0:120:R ==> [002]  5882: 94:R sleep
2267
2268
2269Hardware Latency Detector
2270-------------------------
2271
2272The hardware latency detector is executed by enabling the "hwlat" tracer.
2273
2274NOTE, this tracer will affect the performance of the system as it will
2275periodically make a CPU constantly busy with interrupts disabled.
2276::
2277
2278  # echo hwlat > current_tracer
2279  # sleep 100
2280  # cat trace
2281  # tracer: hwlat
2282  #
2283  # entries-in-buffer/entries-written: 13/13   #P:8
2284  #
2285  #                              _-----=> irqs-off
2286  #                             / _----=> need-resched
2287  #                            | / _---=> hardirq/softirq
2288  #                            || / _--=> preempt-depth
2289  #                            ||| /     delay
2290  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2291  #              | |       |   ||||       |         |
2292             <...>-1729  [001] d...   678.473449: #1     inner/outer(us):   11/12    ts:1581527483.343962693 count:6
2293             <...>-1729  [004] d...   689.556542: #2     inner/outer(us):   16/9     ts:1581527494.889008092 count:1
2294             <...>-1729  [005] d...   714.756290: #3     inner/outer(us):   16/16    ts:1581527519.678961629 count:5
2295             <...>-1729  [001] d...   718.788247: #4     inner/outer(us):    9/17    ts:1581527523.889012713 count:1
2296             <...>-1729  [002] d...   719.796341: #5     inner/outer(us):   13/9     ts:1581527524.912872606 count:1
2297             <...>-1729  [006] d...   844.787091: #6     inner/outer(us):    9/12    ts:1581527649.889048502 count:2
2298             <...>-1729  [003] d...   849.827033: #7     inner/outer(us):   18/9     ts:1581527654.889013793 count:1
2299             <...>-1729  [007] d...   853.859002: #8     inner/outer(us):    9/12    ts:1581527658.889065736 count:1
2300             <...>-1729  [001] d...   855.874978: #9     inner/outer(us):    9/11    ts:1581527660.861991877 count:1
2301             <...>-1729  [001] d...   863.938932: #10    inner/outer(us):    9/11    ts:1581527668.970010500 count:1 nmi-total:7 nmi-count:1
2302             <...>-1729  [007] d...   878.050780: #11    inner/outer(us):    9/12    ts:1581527683.385002600 count:1 nmi-total:5 nmi-count:1
2303             <...>-1729  [007] d...   886.114702: #12    inner/outer(us):    9/12    ts:1581527691.385001600 count:1
2304
2305
2306The above output is somewhat the same in the header. All events will have
2307interrupts disabled 'd'. Under the FUNCTION title there is:
2308
2309 #1
2310	This is the count of events recorded that were greater than the
2311	tracing_threshold (See below).
2312
2313 inner/outer(us):   11/11
2314
2315      This shows two numbers as "inner latency" and "outer latency". The test
2316      runs in a loop checking a timestamp twice. The latency detected within
2317      the two timestamps is the "inner latency" and the latency detected
2318      after the previous timestamp and the next timestamp in the loop is
2319      the "outer latency".
2320
2321 ts:1581527483.343962693
2322
2323      The absolute timestamp that the first latency was recorded in the window.
2324
2325 count:6
2326
2327      The number of times a latency was detected during the window.
2328
2329 nmi-total:7 nmi-count:1
2330
2331      On architectures that support it, if an NMI comes in during the
2332      test, the time spent in NMI is reported in "nmi-total" (in
2333      microseconds).
2334
2335      All architectures that have NMIs will show the "nmi-count" if an
2336      NMI comes in during the test.
2337
2338hwlat files:
2339
2340  tracing_threshold
2341	This gets automatically set to "10" to represent 10
2342	microseconds. This is the threshold of latency that
2343	needs to be detected before the trace will be recorded.
2344
2345	Note, when hwlat tracer is finished (another tracer is
2346	written into "current_tracer"), the original value for
2347	tracing_threshold is placed back into this file.
2348
2349  hwlat_detector/width
2350	The length of time the test runs with interrupts disabled.
2351
2352  hwlat_detector/window
2353	The length of time of the window which the test
2354	runs. That is, the test will run for "width"
2355	microseconds per "window" microseconds
2356
2357  tracing_cpumask
2358	When the test is started. A kernel thread is created that
2359	runs the test. This thread will alternate between CPUs
2360	listed in the tracing_cpumask between each period
2361	(one "window"). To limit the test to specific CPUs
2362	set the mask in this file to only the CPUs that the test
2363	should run on.
2364
2365function
2366--------
2367
2368This tracer is the function tracer. Enabling the function tracer
2369can be done from the debug file system. Make sure the
2370ftrace_enabled is set; otherwise this tracer is a nop.
2371See the "ftrace_enabled" section below.
2372::
2373
2374  # sysctl kernel.ftrace_enabled=1
2375  # echo function > current_tracer
2376  # echo 1 > tracing_on
2377  # usleep 1
2378  # echo 0 > tracing_on
2379  # cat trace
2380  # tracer: function
2381  #
2382  # entries-in-buffer/entries-written: 24799/24799   #P:4
2383  #
2384  #                              _-----=> irqs-off
2385  #                             / _----=> need-resched
2386  #                            | / _---=> hardirq/softirq
2387  #                            || / _--=> preempt-depth
2388  #                            ||| /     delay
2389  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
2390  #              | |       |   ||||       |         |
2391              bash-1994  [002] ....  3082.063030: mutex_unlock <-rb_simple_write
2392              bash-1994  [002] ....  3082.063031: __mutex_unlock_slowpath <-mutex_unlock
2393              bash-1994  [002] ....  3082.063031: __fsnotify_parent <-fsnotify_modify
2394              bash-1994  [002] ....  3082.063032: fsnotify <-fsnotify_modify
2395              bash-1994  [002] ....  3082.063032: __srcu_read_lock <-fsnotify
2396              bash-1994  [002] ....  3082.063032: add_preempt_count <-__srcu_read_lock
2397              bash-1994  [002] ...1  3082.063032: sub_preempt_count <-__srcu_read_lock
2398              bash-1994  [002] ....  3082.063033: __srcu_read_unlock <-fsnotify
2399  [...]
2400
2401
2402Note: function tracer uses ring buffers to store the above
2403entries. The newest data may overwrite the oldest data.
2404Sometimes using echo to stop the trace is not sufficient because
2405the tracing could have overwritten the data that you wanted to
2406record. For this reason, it is sometimes better to disable
2407tracing directly from a program. This allows you to stop the
2408tracing at the point that you hit the part that you are
2409interested in. To disable the tracing directly from a C program,
2410something like following code snippet can be used::
2411
2412	int trace_fd;
2413	[...]
2414	int main(int argc, char *argv[]) {
2415		[...]
2416		trace_fd = open(tracing_file("tracing_on"), O_WRONLY);
2417		[...]
2418		if (condition_hit()) {
2419			write(trace_fd, "0", 1);
2420		}
2421		[...]
2422	}
2423
2424
2425Single thread tracing
2426---------------------
2427
2428By writing into set_ftrace_pid you can trace a
2429single thread. For example::
2430
2431  # cat set_ftrace_pid
2432  no pid
2433  # echo 3111 > set_ftrace_pid
2434  # cat set_ftrace_pid
2435  3111
2436  # echo function > current_tracer
2437  # cat trace | head
2438  # tracer: function
2439  #
2440  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
2441  #              | |       |          |         |
2442      yum-updatesd-3111  [003]  1637.254676: finish_task_switch <-thread_return
2443      yum-updatesd-3111  [003]  1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
2444      yum-updatesd-3111  [003]  1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
2445      yum-updatesd-3111  [003]  1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
2446      yum-updatesd-3111  [003]  1637.254685: fget_light <-do_sys_poll
2447      yum-updatesd-3111  [003]  1637.254686: pipe_poll <-do_sys_poll
2448  # echo > set_ftrace_pid
2449  # cat trace |head
2450  # tracer: function
2451  #
2452  #           TASK-PID    CPU#    TIMESTAMP  FUNCTION
2453  #              | |       |          |         |
2454  ##### CPU 3 buffer started ####
2455      yum-updatesd-3111  [003]  1701.957688: free_poll_entry <-poll_freewait
2456      yum-updatesd-3111  [003]  1701.957689: remove_wait_queue <-free_poll_entry
2457      yum-updatesd-3111  [003]  1701.957691: fput <-free_poll_entry
2458      yum-updatesd-3111  [003]  1701.957692: audit_syscall_exit <-sysret_audit
2459      yum-updatesd-3111  [003]  1701.957693: path_put <-audit_syscall_exit
2460
2461If you want to trace a function when executing, you could use
2462something like this simple program.
2463::
2464
2465	#include <stdio.h>
2466	#include <stdlib.h>
2467	#include <sys/types.h>
2468	#include <sys/stat.h>
2469	#include <fcntl.h>
2470	#include <unistd.h>
2471	#include <string.h>
2472
2473	#define _STR(x) #x
2474	#define STR(x) _STR(x)
2475	#define MAX_PATH 256
2476
2477	const char *find_tracefs(void)
2478	{
2479	       static char tracefs[MAX_PATH+1];
2480	       static int tracefs_found;
2481	       char type[100];
2482	       FILE *fp;
2483
2484	       if (tracefs_found)
2485		       return tracefs;
2486
2487	       if ((fp = fopen("/proc/mounts","r")) == NULL) {
2488		       perror("/proc/mounts");
2489		       return NULL;
2490	       }
2491
2492	       while (fscanf(fp, "%*s %"
2493		             STR(MAX_PATH)
2494		             "s %99s %*s %*d %*d\n",
2495		             tracefs, type) == 2) {
2496		       if (strcmp(type, "tracefs") == 0)
2497		               break;
2498	       }
2499	       fclose(fp);
2500
2501	       if (strcmp(type, "tracefs") != 0) {
2502		       fprintf(stderr, "tracefs not mounted");
2503		       return NULL;
2504	       }
2505
2506	       strcat(tracefs, "/tracing/");
2507	       tracefs_found = 1;
2508
2509	       return tracefs;
2510	}
2511
2512	const char *tracing_file(const char *file_name)
2513	{
2514	       static char trace_file[MAX_PATH+1];
2515	       snprintf(trace_file, MAX_PATH, "%s/%s", find_tracefs(), file_name);
2516	       return trace_file;
2517	}
2518
2519	int main (int argc, char **argv)
2520	{
2521		if (argc < 1)
2522		        exit(-1);
2523
2524		if (fork() > 0) {
2525		        int fd, ffd;
2526		        char line[64];
2527		        int s;
2528
2529		        ffd = open(tracing_file("current_tracer"), O_WRONLY);
2530		        if (ffd < 0)
2531		                exit(-1);
2532		        write(ffd, "nop", 3);
2533
2534		        fd = open(tracing_file("set_ftrace_pid"), O_WRONLY);
2535		        s = sprintf(line, "%d\n", getpid());
2536		        write(fd, line, s);
2537
2538		        write(ffd, "function", 8);
2539
2540		        close(fd);
2541		        close(ffd);
2542
2543		        execvp(argv[1], argv+1);
2544		}
2545
2546		return 0;
2547	}
2548
2549Or this simple script!
2550::
2551
2552  #!/bin/bash
2553
2554  tracefs=`sed -ne 's/^tracefs \(.*\) tracefs.*/\1/p' /proc/mounts`
2555  echo 0 > $tracefs/tracing_on
2556  echo $$ > $tracefs/set_ftrace_pid
2557  echo function > $tracefs/current_tracer
2558  echo 1 > $tracefs/tracing_on
2559  exec "$@"
2560
2561
2562function graph tracer
2563---------------------------
2564
2565This tracer is similar to the function tracer except that it
2566probes a function on its entry and its exit. This is done by
2567using a dynamically allocated stack of return addresses in each
2568task_struct. On function entry the tracer overwrites the return
2569address of each function traced to set a custom probe. Thus the
2570original return address is stored on the stack of return address
2571in the task_struct.
2572
2573Probing on both ends of a function leads to special features
2574such as:
2575
2576- measure of a function's time execution
2577- having a reliable call stack to draw function calls graph
2578
2579This tracer is useful in several situations:
2580
2581- you want to find the reason of a strange kernel behavior and
2582  need to see what happens in detail on any areas (or specific
2583  ones).
2584
2585- you are experiencing weird latencies but it's difficult to
2586  find its origin.
2587
2588- you want to find quickly which path is taken by a specific
2589  function
2590
2591- you just want to peek inside a working kernel and want to see
2592  what happens there.
2593
2594::
2595
2596  # tracer: function_graph
2597  #
2598  # CPU  DURATION                  FUNCTION CALLS
2599  # |     |   |                     |   |   |   |
2600
2601   0)               |  sys_open() {
2602   0)               |    do_sys_open() {
2603   0)               |      getname() {
2604   0)               |        kmem_cache_alloc() {
2605   0)   1.382 us    |          __might_sleep();
2606   0)   2.478 us    |        }
2607   0)               |        strncpy_from_user() {
2608   0)               |          might_fault() {
2609   0)   1.389 us    |            __might_sleep();
2610   0)   2.553 us    |          }
2611   0)   3.807 us    |        }
2612   0)   7.876 us    |      }
2613   0)               |      alloc_fd() {
2614   0)   0.668 us    |        _spin_lock();
2615   0)   0.570 us    |        expand_files();
2616   0)   0.586 us    |        _spin_unlock();
2617
2618
2619There are several columns that can be dynamically
2620enabled/disabled. You can use every combination of options you
2621want, depending on your needs.
2622
2623- The cpu number on which the function executed is default
2624  enabled.  It is sometimes better to only trace one cpu (see
2625  tracing_cpumask file) or you might sometimes see unordered
2626  function calls while cpu tracing switch.
2627
2628	- hide: echo nofuncgraph-cpu > trace_options
2629	- show: echo funcgraph-cpu > trace_options
2630
2631- The duration (function's time of execution) is displayed on
2632  the closing bracket line of a function or on the same line
2633  than the current function in case of a leaf one. It is default
2634  enabled.
2635
2636	- hide: echo nofuncgraph-duration > trace_options
2637	- show: echo funcgraph-duration > trace_options
2638
2639- The overhead field precedes the duration field in case of
2640  reached duration thresholds.
2641
2642	- hide: echo nofuncgraph-overhead > trace_options
2643	- show: echo funcgraph-overhead > trace_options
2644	- depends on: funcgraph-duration
2645
2646  ie::
2647
2648    3) # 1837.709 us |          } /* __switch_to */
2649    3)               |          finish_task_switch() {
2650    3)   0.313 us    |            _raw_spin_unlock_irq();
2651    3)   3.177 us    |          }
2652    3) # 1889.063 us |        } /* __schedule */
2653    3) ! 140.417 us  |      } /* __schedule */
2654    3) # 2034.948 us |    } /* schedule */
2655    3) * 33998.59 us |  } /* schedule_preempt_disabled */
2656
2657    [...]
2658
2659    1)   0.260 us    |              msecs_to_jiffies();
2660    1)   0.313 us    |              __rcu_read_unlock();
2661    1) + 61.770 us   |            }
2662    1) + 64.479 us   |          }
2663    1)   0.313 us    |          rcu_bh_qs();
2664    1)   0.313 us    |          __local_bh_enable();
2665    1) ! 217.240 us  |        }
2666    1)   0.365 us    |        idle_cpu();
2667    1)               |        rcu_irq_exit() {
2668    1)   0.417 us    |          rcu_eqs_enter_common.isra.47();
2669    1)   3.125 us    |        }
2670    1) ! 227.812 us  |      }
2671    1) ! 457.395 us  |    }
2672    1) @ 119760.2 us |  }
2673
2674    [...]
2675
2676    2)               |    handle_IPI() {
2677    1)   6.979 us    |                  }
2678    2)   0.417 us    |      scheduler_ipi();
2679    1)   9.791 us    |                }
2680    1) + 12.917 us   |              }
2681    2)   3.490 us    |    }
2682    1) + 15.729 us   |            }
2683    1) + 18.542 us   |          }
2684    2) $ 3594274 us  |  }
2685
2686Flags::
2687
2688  + means that the function exceeded 10 usecs.
2689  ! means that the function exceeded 100 usecs.
2690  # means that the function exceeded 1000 usecs.
2691  * means that the function exceeded 10 msecs.
2692  @ means that the function exceeded 100 msecs.
2693  $ means that the function exceeded 1 sec.
2694
2695
2696- The task/pid field displays the thread cmdline and pid which
2697  executed the function. It is default disabled.
2698
2699	- hide: echo nofuncgraph-proc > trace_options
2700	- show: echo funcgraph-proc > trace_options
2701
2702  ie::
2703
2704    # tracer: function_graph
2705    #
2706    # CPU  TASK/PID        DURATION                  FUNCTION CALLS
2707    # |    |    |           |   |                     |   |   |   |
2708    0)    sh-4802     |               |                  d_free() {
2709    0)    sh-4802     |               |                    call_rcu() {
2710    0)    sh-4802     |               |                      __call_rcu() {
2711    0)    sh-4802     |   0.616 us    |                        rcu_process_gp_end();
2712    0)    sh-4802     |   0.586 us    |                        check_for_new_grace_period();
2713    0)    sh-4802     |   2.899 us    |                      }
2714    0)    sh-4802     |   4.040 us    |                    }
2715    0)    sh-4802     |   5.151 us    |                  }
2716    0)    sh-4802     | + 49.370 us   |                }
2717
2718
2719- The absolute time field is an absolute timestamp given by the
2720  system clock since it started. A snapshot of this time is
2721  given on each entry/exit of functions
2722
2723	- hide: echo nofuncgraph-abstime > trace_options
2724	- show: echo funcgraph-abstime > trace_options
2725
2726  ie::
2727
2728    #
2729    #      TIME       CPU  DURATION                  FUNCTION CALLS
2730    #       |         |     |   |                     |   |   |   |
2731    360.774522 |   1)   0.541 us    |                                          }
2732    360.774522 |   1)   4.663 us    |                                        }
2733    360.774523 |   1)   0.541 us    |                                        __wake_up_bit();
2734    360.774524 |   1)   6.796 us    |                                      }
2735    360.774524 |   1)   7.952 us    |                                    }
2736    360.774525 |   1)   9.063 us    |                                  }
2737    360.774525 |   1)   0.615 us    |                                  journal_mark_dirty();
2738    360.774527 |   1)   0.578 us    |                                  __brelse();
2739    360.774528 |   1)               |                                  reiserfs_prepare_for_journal() {
2740    360.774528 |   1)               |                                    unlock_buffer() {
2741    360.774529 |   1)               |                                      wake_up_bit() {
2742    360.774529 |   1)               |                                        bit_waitqueue() {
2743    360.774530 |   1)   0.594 us    |                                          __phys_addr();
2744
2745
2746The function name is always displayed after the closing bracket
2747for a function if the start of that function is not in the
2748trace buffer.
2749
2750Display of the function name after the closing bracket may be
2751enabled for functions whose start is in the trace buffer,
2752allowing easier searching with grep for function durations.
2753It is default disabled.
2754
2755	- hide: echo nofuncgraph-tail > trace_options
2756	- show: echo funcgraph-tail > trace_options
2757
2758  Example with nofuncgraph-tail (default)::
2759
2760    0)               |      putname() {
2761    0)               |        kmem_cache_free() {
2762    0)   0.518 us    |          __phys_addr();
2763    0)   1.757 us    |        }
2764    0)   2.861 us    |      }
2765
2766  Example with funcgraph-tail::
2767
2768    0)               |      putname() {
2769    0)               |        kmem_cache_free() {
2770    0)   0.518 us    |          __phys_addr();
2771    0)   1.757 us    |        } /* kmem_cache_free() */
2772    0)   2.861 us    |      } /* putname() */
2773
2774The return value of each traced function can be displayed after
2775an equal sign "=". When encountering system call failures, it
2776can be very helpful to quickly locate the function that first
2777returns an error code.
2778
2779	- hide: echo nofuncgraph-retval > trace_options
2780	- show: echo funcgraph-retval > trace_options
2781
2782  Example with funcgraph-retval::
2783
2784    1)               |    cgroup_migrate() {
2785    1)   0.651 us    |      cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2786    1)               |      cgroup_migrate_execute() {
2787    1)               |        cpu_cgroup_can_attach() {
2788    1)               |          cgroup_taskset_first() {
2789    1)   0.732 us    |            cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2790    1)   1.232 us    |          } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2791    1)   0.380 us    |          sched_rt_can_attach(); /* = 0x0 */
2792    1)   2.335 us    |        } /* cpu_cgroup_can_attach = -22 */
2793    1)   4.369 us    |      } /* cgroup_migrate_execute = -22 */
2794    1)   7.143 us    |    } /* cgroup_migrate = -22 */
2795
2796The above example shows that the function cpu_cgroup_can_attach
2797returned the error code -22 firstly, then we can read the code
2798of this function to get the root cause.
2799
2800When the option funcgraph-retval-hex is not set, the return value can
2801be displayed in a smart way. Specifically, if it is an error code,
2802it will be printed in signed decimal format, otherwise it will
2803printed in hexadecimal format.
2804
2805	- smart: echo nofuncgraph-retval-hex > trace_options
2806	- hexadecimal: echo funcgraph-retval-hex > trace_options
2807
2808  Example with funcgraph-retval-hex::
2809
2810    1)               |      cgroup_migrate() {
2811    1)   0.651 us    |        cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
2812    1)               |        cgroup_migrate_execute() {
2813    1)               |          cpu_cgroup_can_attach() {
2814    1)               |            cgroup_taskset_first() {
2815    1)   0.732 us    |              cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
2816    1)   1.232 us    |            } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
2817    1)   0.380 us    |            sched_rt_can_attach(); /* = 0x0 */
2818    1)   2.335 us    |          } /* cpu_cgroup_can_attach = 0xffffffea */
2819    1)   4.369 us    |        } /* cgroup_migrate_execute = 0xffffffea */
2820    1)   7.143 us    |      } /* cgroup_migrate = 0xffffffea */
2821
2822At present, there are some limitations when using the funcgraph-retval
2823option, and these limitations will be eliminated in the future:
2824
2825- Even if the function return type is void, a return value will still
2826  be printed, and you can just ignore it.
2827
2828- Even if return values are stored in multiple registers, only the
2829  value contained in the first register will be recorded and printed.
2830  To illustrate, in the x86 architecture, eax and edx are used to store
2831  a 64-bit return value, with the lower 32 bits saved in eax and the
2832  upper 32 bits saved in edx. However, only the value stored in eax
2833  will be recorded and printed.
2834
2835- In certain procedure call standards, such as arm64's AAPCS64, when a
2836  type is smaller than a GPR, it is the responsibility of the consumer
2837  to perform the narrowing, and the upper bits may contain UNKNOWN values.
2838  Therefore, it is advisable to check the code for such cases. For instance,
2839  when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
2840  especially when larger types are truncated, whether explicitly or implicitly.
2841  Here are some specific cases to illustrate this point:
2842
2843  **Case One**:
2844
2845  The function narrow_to_u8 is defined as follows::
2846
2847	u8 narrow_to_u8(u64 val)
2848	{
2849		// implicitly truncated
2850		return val;
2851	}
2852
2853  It may be compiled to::
2854
2855	narrow_to_u8:
2856		< ... ftrace instrumentation ... >
2857		RET
2858
2859  If you pass 0x123456789abcdef to this function and want to narrow it,
2860  it may be recorded as 0x123456789abcdef instead of 0xef.
2861
2862  **Case Two**:
2863
2864  The function error_if_not_4g_aligned is defined as follows::
2865
2866	int error_if_not_4g_aligned(u64 val)
2867	{
2868		if (val & GENMASK(31, 0))
2869			return -EINVAL;
2870
2871		return 0;
2872	}
2873
2874  It could be compiled to::
2875
2876	error_if_not_4g_aligned:
2877		CBNZ    w0, .Lnot_aligned
2878		RET			// bits [31:0] are zero, bits
2879					// [63:32] are UNKNOWN
2880	.Lnot_aligned:
2881		MOV    x0, #-EINVAL
2882		RET
2883
2884  When passing 0x2_0000_0000 to it, the return value may be recorded as
2885  0x2_0000_0000 instead of 0.
2886
2887You can put some comments on specific functions by using
2888trace_printk() For example, if you want to put a comment inside
2889the __might_sleep() function, you just have to include
2890<linux/ftrace.h> and call trace_printk() inside __might_sleep()::
2891
2892	trace_printk("I'm a comment!\n")
2893
2894will produce::
2895
2896   1)               |             __might_sleep() {
2897   1)               |                /* I'm a comment! */
2898   1)   1.449 us    |             }
2899
2900
2901You might find other useful features for this tracer in the
2902following "dynamic ftrace" section such as tracing only specific
2903functions or tasks.
2904
2905dynamic ftrace
2906--------------
2907
2908If CONFIG_DYNAMIC_FTRACE is set, the system will run with
2909virtually no overhead when function tracing is disabled. The way
2910this works is the mcount function call (placed at the start of
2911every kernel function, produced by the -pg switch in gcc),
2912starts of pointing to a simple return. (Enabling FTRACE will
2913include the -pg switch in the compiling of the kernel.)
2914
2915At compile time every C file object is run through the
2916recordmcount program (located in the scripts directory). This
2917program will parse the ELF headers in the C object to find all
2918the locations in the .text section that call mcount. Starting
2919with gcc version 4.6, the -mfentry has been added for x86, which
2920calls "__fentry__" instead of "mcount". Which is called before
2921the creation of the stack frame.
2922
2923Note, not all sections are traced. They may be prevented by either
2924a notrace, or blocked another way and all inline functions are not
2925traced. Check the "available_filter_functions" file to see what functions
2926can be traced.
2927
2928A section called "__mcount_loc" is created that holds
2929references to all the mcount/fentry call sites in the .text section.
2930The recordmcount program re-links this section back into the
2931original object. The final linking stage of the kernel will add all these
2932references into a single table.
2933
2934On boot up, before SMP is initialized, the dynamic ftrace code
2935scans this table and updates all the locations into nops. It
2936also records the locations, which are added to the
2937available_filter_functions list.  Modules are processed as they
2938are loaded and before they are executed.  When a module is
2939unloaded, it also removes its functions from the ftrace function
2940list. This is automatic in the module unload code, and the
2941module author does not need to worry about it.
2942
2943When tracing is enabled, the process of modifying the function
2944tracepoints is dependent on architecture. The old method is to use
2945kstop_machine to prevent races with the CPUs executing code being
2946modified (which can cause the CPU to do undesirable things, especially
2947if the modified code crosses cache (or page) boundaries), and the nops are
2948patched back to calls. But this time, they do not call mcount
2949(which is just a function stub). They now call into the ftrace
2950infrastructure.
2951
2952The new method of modifying the function tracepoints is to place
2953a breakpoint at the location to be modified, sync all CPUs, modify
2954the rest of the instruction not covered by the breakpoint. Sync
2955all CPUs again, and then remove the breakpoint with the finished
2956version to the ftrace call site.
2957
2958Some archs do not even need to monkey around with the synchronization,
2959and can just slap the new code on top of the old without any
2960problems with other CPUs executing it at the same time.
2961
2962One special side-effect to the recording of the functions being
2963traced is that we can now selectively choose which functions we
2964wish to trace and which ones we want the mcount calls to remain
2965as nops.
2966
2967Two files are used, one for enabling and one for disabling the
2968tracing of specified functions. They are:
2969
2970  set_ftrace_filter
2971
2972and
2973
2974  set_ftrace_notrace
2975
2976A list of available functions that you can add to these files is
2977listed in:
2978
2979   available_filter_functions
2980
2981::
2982
2983  # cat available_filter_functions
2984  put_prev_task_idle
2985  kmem_cache_create
2986  pick_next_task_rt
2987  cpus_read_lock
2988  pick_next_task_fair
2989  mutex_lock
2990  [...]
2991
2992If I am only interested in sys_nanosleep and hrtimer_interrupt::
2993
2994  # echo sys_nanosleep hrtimer_interrupt > set_ftrace_filter
2995  # echo function > current_tracer
2996  # echo 1 > tracing_on
2997  # usleep 1
2998  # echo 0 > tracing_on
2999  # cat trace
3000  # tracer: function
3001  #
3002  # entries-in-buffer/entries-written: 5/5   #P:4
3003  #
3004  #                              _-----=> irqs-off
3005  #                             / _----=> need-resched
3006  #                            | / _---=> hardirq/softirq
3007  #                            || / _--=> preempt-depth
3008  #                            ||| /     delay
3009  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3010  #              | |       |   ||||       |         |
3011            usleep-2665  [001] ....  4186.475355: sys_nanosleep <-system_call_fastpath
3012            <idle>-0     [001] d.h1  4186.475409: hrtimer_interrupt <-smp_apic_timer_interrupt
3013            usleep-2665  [001] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
3014            <idle>-0     [003] d.h1  4186.475426: hrtimer_interrupt <-smp_apic_timer_interrupt
3015            <idle>-0     [002] d.h1  4186.475427: hrtimer_interrupt <-smp_apic_timer_interrupt
3016
3017To see which functions are being traced, you can cat the file:
3018::
3019
3020  # cat set_ftrace_filter
3021  hrtimer_interrupt
3022  sys_nanosleep
3023
3024
3025Perhaps this is not enough. The filters also allow glob(7) matching.
3026
3027  ``<match>*``
3028	will match functions that begin with <match>
3029  ``*<match>``
3030	will match functions that end with <match>
3031  ``*<match>*``
3032	will match functions that have <match> in it
3033  ``<match1>*<match2>``
3034	will match functions that begin with <match1> and end with <match2>
3035
3036.. note::
3037      It is better to use quotes to enclose the wild cards,
3038      otherwise the shell may expand the parameters into names
3039      of files in the local directory.
3040
3041::
3042
3043  # echo 'hrtimer_*' > set_ftrace_filter
3044
3045Produces::
3046
3047  # tracer: function
3048  #
3049  # entries-in-buffer/entries-written: 897/897   #P:4
3050  #
3051  #                              _-----=> irqs-off
3052  #                             / _----=> need-resched
3053  #                            | / _---=> hardirq/softirq
3054  #                            || / _--=> preempt-depth
3055  #                            ||| /     delay
3056  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3057  #              | |       |   ||||       |         |
3058            <idle>-0     [003] dN.1  4228.547803: hrtimer_cancel <-tick_nohz_idle_exit
3059            <idle>-0     [003] dN.1  4228.547804: hrtimer_try_to_cancel <-hrtimer_cancel
3060            <idle>-0     [003] dN.2  4228.547805: hrtimer_force_reprogram <-__remove_hrtimer
3061            <idle>-0     [003] dN.1  4228.547805: hrtimer_forward <-tick_nohz_idle_exit
3062            <idle>-0     [003] dN.1  4228.547805: hrtimer_start_range_ns <-hrtimer_start_expires.constprop.11
3063            <idle>-0     [003] d..1  4228.547858: hrtimer_get_next_event <-get_next_timer_interrupt
3064            <idle>-0     [003] d..1  4228.547859: hrtimer_start <-__tick_nohz_idle_enter
3065            <idle>-0     [003] d..2  4228.547860: hrtimer_force_reprogram <-__rem
3066
3067Notice that we lost the sys_nanosleep.
3068::
3069
3070  # cat set_ftrace_filter
3071  hrtimer_run_queues
3072  hrtimer_run_pending
3073  hrtimer_init
3074  hrtimer_cancel
3075  hrtimer_try_to_cancel
3076  hrtimer_forward
3077  hrtimer_start
3078  hrtimer_reprogram
3079  hrtimer_force_reprogram
3080  hrtimer_get_next_event
3081  hrtimer_interrupt
3082  hrtimer_nanosleep
3083  hrtimer_wakeup
3084  hrtimer_get_remaining
3085  hrtimer_get_res
3086  hrtimer_init_sleeper
3087
3088
3089This is because the '>' and '>>' act just like they do in bash.
3090To rewrite the filters, use '>'
3091To append to the filters, use '>>'
3092
3093To clear out a filter so that all functions will be recorded
3094again::
3095
3096 # echo > set_ftrace_filter
3097 # cat set_ftrace_filter
3098 #
3099
3100Again, now we want to append.
3101
3102::
3103
3104  # echo sys_nanosleep > set_ftrace_filter
3105  # cat set_ftrace_filter
3106  sys_nanosleep
3107  # echo 'hrtimer_*' >> set_ftrace_filter
3108  # cat set_ftrace_filter
3109  hrtimer_run_queues
3110  hrtimer_run_pending
3111  hrtimer_init
3112  hrtimer_cancel
3113  hrtimer_try_to_cancel
3114  hrtimer_forward
3115  hrtimer_start
3116  hrtimer_reprogram
3117  hrtimer_force_reprogram
3118  hrtimer_get_next_event
3119  hrtimer_interrupt
3120  sys_nanosleep
3121  hrtimer_nanosleep
3122  hrtimer_wakeup
3123  hrtimer_get_remaining
3124  hrtimer_get_res
3125  hrtimer_init_sleeper
3126
3127
3128The set_ftrace_notrace prevents those functions from being
3129traced.
3130::
3131
3132  # echo '*preempt*' '*lock*' > set_ftrace_notrace
3133
3134Produces::
3135
3136  # tracer: function
3137  #
3138  # entries-in-buffer/entries-written: 39608/39608   #P:4
3139  #
3140  #                              _-----=> irqs-off
3141  #                             / _----=> need-resched
3142  #                            | / _---=> hardirq/softirq
3143  #                            || / _--=> preempt-depth
3144  #                            ||| /     delay
3145  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3146  #              | |       |   ||||       |         |
3147              bash-1994  [000] ....  4342.324896: file_ra_state_init <-do_dentry_open
3148              bash-1994  [000] ....  4342.324897: open_check_o_direct <-do_last
3149              bash-1994  [000] ....  4342.324897: ima_file_check <-do_last
3150              bash-1994  [000] ....  4342.324898: process_measurement <-ima_file_check
3151              bash-1994  [000] ....  4342.324898: ima_get_action <-process_measurement
3152              bash-1994  [000] ....  4342.324898: ima_match_policy <-ima_get_action
3153              bash-1994  [000] ....  4342.324899: do_truncate <-do_last
3154              bash-1994  [000] ....  4342.324899: setattr_should_drop_suidgid <-do_truncate
3155              bash-1994  [000] ....  4342.324899: notify_change <-do_truncate
3156              bash-1994  [000] ....  4342.324900: current_fs_time <-notify_change
3157              bash-1994  [000] ....  4342.324900: current_kernel_time <-current_fs_time
3158              bash-1994  [000] ....  4342.324900: timespec_trunc <-current_fs_time
3159
3160We can see that there's no more lock or preempt tracing.
3161
3162Selecting function filters via index
3163------------------------------------
3164
3165Because processing of strings is expensive (the address of the function
3166needs to be looked up before comparing to the string being passed in),
3167an index can be used as well to enable functions. This is useful in the
3168case of setting thousands of specific functions at a time. By passing
3169in a list of numbers, no string processing will occur. Instead, the function
3170at the specific location in the internal array (which corresponds to the
3171functions in the "available_filter_functions" file), is selected.
3172
3173::
3174
3175  # echo 1 > set_ftrace_filter
3176
3177Will select the first function listed in "available_filter_functions"
3178
3179::
3180
3181  # head -1 available_filter_functions
3182  trace_initcall_finish_cb
3183
3184  # cat set_ftrace_filter
3185  trace_initcall_finish_cb
3186
3187  # head -50 available_filter_functions | tail -1
3188  x86_pmu_commit_txn
3189
3190  # echo 1 50 > set_ftrace_filter
3191  # cat set_ftrace_filter
3192  trace_initcall_finish_cb
3193  x86_pmu_commit_txn
3194
3195Dynamic ftrace with the function graph tracer
3196---------------------------------------------
3197
3198Although what has been explained above concerns both the
3199function tracer and the function-graph-tracer, there are some
3200special features only available in the function-graph tracer.
3201
3202If you want to trace only one function and all of its children,
3203you just have to echo its name into set_graph_function::
3204
3205 echo __do_fault > set_graph_function
3206
3207will produce the following "expanded" trace of the __do_fault()
3208function::
3209
3210   0)               |  __do_fault() {
3211   0)               |    filemap_fault() {
3212   0)               |      find_lock_page() {
3213   0)   0.804 us    |        find_get_page();
3214   0)               |        __might_sleep() {
3215   0)   1.329 us    |        }
3216   0)   3.904 us    |      }
3217   0)   4.979 us    |    }
3218   0)   0.653 us    |    _spin_lock();
3219   0)   0.578 us    |    page_add_file_rmap();
3220   0)   0.525 us    |    native_set_pte_at();
3221   0)   0.585 us    |    _spin_unlock();
3222   0)               |    unlock_page() {
3223   0)   0.541 us    |      page_waitqueue();
3224   0)   0.639 us    |      __wake_up_bit();
3225   0)   2.786 us    |    }
3226   0) + 14.237 us   |  }
3227   0)               |  __do_fault() {
3228   0)               |    filemap_fault() {
3229   0)               |      find_lock_page() {
3230   0)   0.698 us    |        find_get_page();
3231   0)               |        __might_sleep() {
3232   0)   1.412 us    |        }
3233   0)   3.950 us    |      }
3234   0)   5.098 us    |    }
3235   0)   0.631 us    |    _spin_lock();
3236   0)   0.571 us    |    page_add_file_rmap();
3237   0)   0.526 us    |    native_set_pte_at();
3238   0)   0.586 us    |    _spin_unlock();
3239   0)               |    unlock_page() {
3240   0)   0.533 us    |      page_waitqueue();
3241   0)   0.638 us    |      __wake_up_bit();
3242   0)   2.793 us    |    }
3243   0) + 14.012 us   |  }
3244
3245You can also expand several functions at once::
3246
3247 echo sys_open > set_graph_function
3248 echo sys_close >> set_graph_function
3249
3250Now if you want to go back to trace all functions you can clear
3251this special filter via::
3252
3253 echo > set_graph_function
3254
3255
3256ftrace_enabled
3257--------------
3258
3259Note, the proc sysctl ftrace_enable is a big on/off switch for the
3260function tracer. By default it is enabled (when function tracing is
3261enabled in the kernel). If it is disabled, all function tracing is
3262disabled. This includes not only the function tracers for ftrace, but
3263also for any other uses (perf, kprobes, stack tracing, profiling, etc). It
3264cannot be disabled if there is a callback with FTRACE_OPS_FL_PERMANENT set
3265registered.
3266
3267Please disable this with care.
3268
3269This can be disable (and enabled) with::
3270
3271  sysctl kernel.ftrace_enabled=0
3272  sysctl kernel.ftrace_enabled=1
3273
3274 or
3275
3276  echo 0 > /proc/sys/kernel/ftrace_enabled
3277  echo 1 > /proc/sys/kernel/ftrace_enabled
3278
3279
3280Filter commands
3281---------------
3282
3283A few commands are supported by the set_ftrace_filter interface.
3284Trace commands have the following format::
3285
3286  <function>:<command>:<parameter>
3287
3288The following commands are supported:
3289
3290- mod:
3291  This command enables function filtering per module. The
3292  parameter defines the module. For example, if only the write*
3293  functions in the ext3 module are desired, run:
3294
3295   echo 'write*:mod:ext3' > set_ftrace_filter
3296
3297  This command interacts with the filter in the same way as
3298  filtering based on function names. Thus, adding more functions
3299  in a different module is accomplished by appending (>>) to the
3300  filter file. Remove specific module functions by prepending
3301  '!'::
3302
3303   echo '!writeback*:mod:ext3' >> set_ftrace_filter
3304
3305  Mod command supports module globbing. Disable tracing for all
3306  functions except a specific module::
3307
3308   echo '!*:mod:!ext3' >> set_ftrace_filter
3309
3310  Disable tracing for all modules, but still trace kernel::
3311
3312   echo '!*:mod:*' >> set_ftrace_filter
3313
3314  Enable filter only for kernel::
3315
3316   echo '*write*:mod:!*' >> set_ftrace_filter
3317
3318  Enable filter for module globbing::
3319
3320   echo '*write*:mod:*snd*' >> set_ftrace_filter
3321
3322- traceon/traceoff:
3323  These commands turn tracing on and off when the specified
3324  functions are hit. The parameter determines how many times the
3325  tracing system is turned on and off. If unspecified, there is
3326  no limit. For example, to disable tracing when a schedule bug
3327  is hit the first 5 times, run::
3328
3329   echo '__schedule_bug:traceoff:5' > set_ftrace_filter
3330
3331  To always disable tracing when __schedule_bug is hit::
3332
3333   echo '__schedule_bug:traceoff' > set_ftrace_filter
3334
3335  These commands are cumulative whether or not they are appended
3336  to set_ftrace_filter. To remove a command, prepend it by '!'
3337  and drop the parameter::
3338
3339   echo '!__schedule_bug:traceoff:0' > set_ftrace_filter
3340
3341  The above removes the traceoff command for __schedule_bug
3342  that have a counter. To remove commands without counters::
3343
3344   echo '!__schedule_bug:traceoff' > set_ftrace_filter
3345
3346- snapshot:
3347  Will cause a snapshot to be triggered when the function is hit.
3348  ::
3349
3350   echo 'native_flush_tlb_others:snapshot' > set_ftrace_filter
3351
3352  To only snapshot once:
3353  ::
3354
3355   echo 'native_flush_tlb_others:snapshot:1' > set_ftrace_filter
3356
3357  To remove the above commands::
3358
3359   echo '!native_flush_tlb_others:snapshot' > set_ftrace_filter
3360   echo '!native_flush_tlb_others:snapshot:0' > set_ftrace_filter
3361
3362- enable_event/disable_event:
3363  These commands can enable or disable a trace event. Note, because
3364  function tracing callbacks are very sensitive, when these commands
3365  are registered, the trace point is activated, but disabled in
3366  a "soft" mode. That is, the tracepoint will be called, but
3367  just will not be traced. The event tracepoint stays in this mode
3368  as long as there's a command that triggers it.
3369  ::
3370
3371   echo 'try_to_wake_up:enable_event:sched:sched_switch:2' > \
3372   	 set_ftrace_filter
3373
3374  The format is::
3375
3376    <function>:enable_event:<system>:<event>[:count]
3377    <function>:disable_event:<system>:<event>[:count]
3378
3379  To remove the events commands::
3380
3381   echo '!try_to_wake_up:enable_event:sched:sched_switch:0' > \
3382   	 set_ftrace_filter
3383   echo '!schedule:disable_event:sched:sched_switch' > \
3384   	 set_ftrace_filter
3385
3386- dump:
3387  When the function is hit, it will dump the contents of the ftrace
3388  ring buffer to the console. This is useful if you need to debug
3389  something, and want to dump the trace when a certain function
3390  is hit. Perhaps it's a function that is called before a triple
3391  fault happens and does not allow you to get a regular dump.
3392
3393- cpudump:
3394  When the function is hit, it will dump the contents of the ftrace
3395  ring buffer for the current CPU to the console. Unlike the "dump"
3396  command, it only prints out the contents of the ring buffer for the
3397  CPU that executed the function that triggered the dump.
3398
3399- stacktrace:
3400  When the function is hit, a stack trace is recorded.
3401
3402trace_pipe
3403----------
3404
3405The trace_pipe outputs the same content as the trace file, but
3406the effect on the tracing is different. Every read from
3407trace_pipe is consumed. This means that subsequent reads will be
3408different. The trace is live.
3409::
3410
3411  # echo function > current_tracer
3412  # cat trace_pipe > /tmp/trace.out &
3413  [1] 4153
3414  # echo 1 > tracing_on
3415  # usleep 1
3416  # echo 0 > tracing_on
3417  # cat trace
3418  # tracer: function
3419  #
3420  # entries-in-buffer/entries-written: 0/0   #P:4
3421  #
3422  #                              _-----=> irqs-off
3423  #                             / _----=> need-resched
3424  #                            | / _---=> hardirq/softirq
3425  #                            || / _--=> preempt-depth
3426  #                            ||| /     delay
3427  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3428  #              | |       |   ||||       |         |
3429
3430  #
3431  # cat /tmp/trace.out
3432             bash-1994  [000] ....  5281.568961: mutex_unlock <-rb_simple_write
3433             bash-1994  [000] ....  5281.568963: __mutex_unlock_slowpath <-mutex_unlock
3434             bash-1994  [000] ....  5281.568963: __fsnotify_parent <-fsnotify_modify
3435             bash-1994  [000] ....  5281.568964: fsnotify <-fsnotify_modify
3436             bash-1994  [000] ....  5281.568964: __srcu_read_lock <-fsnotify
3437             bash-1994  [000] ....  5281.568964: add_preempt_count <-__srcu_read_lock
3438             bash-1994  [000] ...1  5281.568965: sub_preempt_count <-__srcu_read_lock
3439             bash-1994  [000] ....  5281.568965: __srcu_read_unlock <-fsnotify
3440             bash-1994  [000] ....  5281.568967: sys_dup2 <-system_call_fastpath
3441
3442
3443Note, reading the trace_pipe file will block until more input is
3444added. This is contrary to the trace file. If any process opened
3445the trace file for reading, it will actually disable tracing and
3446prevent new entries from being added. The trace_pipe file does
3447not have this limitation.
3448
3449trace entries
3450-------------
3451
3452Having too much or not enough data can be troublesome in
3453diagnosing an issue in the kernel. The file buffer_size_kb is
3454used to modify the size of the internal trace buffers. The
3455number listed is the number of entries that can be recorded per
3456CPU. To know the full size, multiply the number of possible CPUs
3457with the number of entries.
3458::
3459
3460  # cat buffer_size_kb
3461  1408 (units kilobytes)
3462
3463Or simply read buffer_total_size_kb
3464::
3465
3466  # cat buffer_total_size_kb
3467  5632
3468
3469To modify the buffer, simple echo in a number (in 1024 byte segments).
3470::
3471
3472  # echo 10000 > buffer_size_kb
3473  # cat buffer_size_kb
3474  10000 (units kilobytes)
3475
3476It will try to allocate as much as possible. If you allocate too
3477much, it can cause Out-Of-Memory to trigger.
3478::
3479
3480  # echo 1000000000000 > buffer_size_kb
3481  -bash: echo: write error: Cannot allocate memory
3482  # cat buffer_size_kb
3483  85
3484
3485The per_cpu buffers can be changed individually as well:
3486::
3487
3488  # echo 10000 > per_cpu/cpu0/buffer_size_kb
3489  # echo 100 > per_cpu/cpu1/buffer_size_kb
3490
3491When the per_cpu buffers are not the same, the buffer_size_kb
3492at the top level will just show an X
3493::
3494
3495  # cat buffer_size_kb
3496  X
3497
3498This is where the buffer_total_size_kb is useful:
3499::
3500
3501  # cat buffer_total_size_kb
3502  12916
3503
3504Writing to the top level buffer_size_kb will reset all the buffers
3505to be the same again.
3506
3507Snapshot
3508--------
3509CONFIG_TRACER_SNAPSHOT makes a generic snapshot feature
3510available to all non latency tracers. (Latency tracers which
3511record max latency, such as "irqsoff" or "wakeup", can't use
3512this feature, since those are already using the snapshot
3513mechanism internally.)
3514
3515Snapshot preserves a current trace buffer at a particular point
3516in time without stopping tracing. Ftrace swaps the current
3517buffer with a spare buffer, and tracing continues in the new
3518current (=previous spare) buffer.
3519
3520The following tracefs files in "tracing" are related to this
3521feature:
3522
3523  snapshot:
3524
3525	This is used to take a snapshot and to read the output
3526	of the snapshot. Echo 1 into this file to allocate a
3527	spare buffer and to take a snapshot (swap), then read
3528	the snapshot from this file in the same format as
3529	"trace" (described above in the section "The File
3530	System"). Both reads snapshot and tracing are executable
3531	in parallel. When the spare buffer is allocated, echoing
3532	0 frees it, and echoing else (positive) values clear the
3533	snapshot contents.
3534	More details are shown in the table below.
3535
3536	+--------------+------------+------------+------------+
3537	|status\\input |     0      |     1      |    else    |
3538	+==============+============+============+============+
3539	|not allocated |(do nothing)| alloc+swap |(do nothing)|
3540	+--------------+------------+------------+------------+
3541	|allocated     |    free    |    swap    |   clear    |
3542	+--------------+------------+------------+------------+
3543
3544Here is an example of using the snapshot feature.
3545::
3546
3547  # echo 1 > events/sched/enable
3548  # echo 1 > snapshot
3549  # cat snapshot
3550  # tracer: nop
3551  #
3552  # entries-in-buffer/entries-written: 71/71   #P:8
3553  #
3554  #                              _-----=> irqs-off
3555  #                             / _----=> need-resched
3556  #                            | / _---=> hardirq/softirq
3557  #                            || / _--=> preempt-depth
3558  #                            ||| /     delay
3559  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3560  #              | |       |   ||||       |         |
3561            <idle>-0     [005] d...  2440.603828: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120   prev_state=R ==> next_comm=snapshot-test-2 next_pid=2242 next_prio=120
3562             sleep-2242  [005] d...  2440.603846: sched_switch: prev_comm=snapshot-test-2 prev_pid=2242 prev_prio=120   prev_state=R ==> next_comm=kworker/5:1 next_pid=60 next_prio=120
3563  [...]
3564          <idle>-0     [002] d...  2440.707230: sched_switch: prev_comm=swapper/2 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2229 next_prio=120
3565
3566  # cat trace
3567  # tracer: nop
3568  #
3569  # entries-in-buffer/entries-written: 77/77   #P:8
3570  #
3571  #                              _-----=> irqs-off
3572  #                             / _----=> need-resched
3573  #                            | / _---=> hardirq/softirq
3574  #                            || / _--=> preempt-depth
3575  #                            ||| /     delay
3576  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3577  #              | |       |   ||||       |         |
3578            <idle>-0     [007] d...  2440.707395: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=snapshot-test-2 next_pid=2243 next_prio=120
3579   snapshot-test-2-2229  [002] d...  2440.707438: sched_switch: prev_comm=snapshot-test-2 prev_pid=2229 prev_prio=120 prev_state=S ==> next_comm=swapper/2 next_pid=0 next_prio=120
3580  [...]
3581
3582
3583If you try to use this snapshot feature when current tracer is
3584one of the latency tracers, you will get the following results.
3585::
3586
3587  # echo wakeup > current_tracer
3588  # echo 1 > snapshot
3589  bash: echo: write error: Device or resource busy
3590  # cat snapshot
3591  cat: snapshot: Device or resource busy
3592
3593
3594Instances
3595---------
3596In the tracefs tracing directory, there is a directory called "instances".
3597This directory can have new directories created inside of it using
3598mkdir, and removing directories with rmdir. The directory created
3599with mkdir in this directory will already contain files and other
3600directories after it is created.
3601::
3602
3603  # mkdir instances/foo
3604  # ls instances/foo
3605  buffer_size_kb  buffer_total_size_kb  events  free_buffer  per_cpu
3606  set_event  snapshot  trace  trace_clock  trace_marker  trace_options
3607  trace_pipe  tracing_on
3608
3609As you can see, the new directory looks similar to the tracing directory
3610itself. In fact, it is very similar, except that the buffer and
3611events are agnostic from the main directory, or from any other
3612instances that are created.
3613
3614The files in the new directory work just like the files with the
3615same name in the tracing directory except the buffer that is used
3616is a separate and new buffer. The files affect that buffer but do not
3617affect the main buffer with the exception of trace_options. Currently,
3618the trace_options affect all instances and the top level buffer
3619the same, but this may change in future releases. That is, options
3620may become specific to the instance they reside in.
3621
3622Notice that none of the function tracer files are there, nor is
3623current_tracer and available_tracers. This is because the buffers
3624can currently only have events enabled for them.
3625::
3626
3627  # mkdir instances/foo
3628  # mkdir instances/bar
3629  # mkdir instances/zoot
3630  # echo 100000 > buffer_size_kb
3631  # echo 1000 > instances/foo/buffer_size_kb
3632  # echo 5000 > instances/bar/per_cpu/cpu1/buffer_size_kb
3633  # echo function > current_trace
3634  # echo 1 > instances/foo/events/sched/sched_wakeup/enable
3635  # echo 1 > instances/foo/events/sched/sched_wakeup_new/enable
3636  # echo 1 > instances/foo/events/sched/sched_switch/enable
3637  # echo 1 > instances/bar/events/irq/enable
3638  # echo 1 > instances/zoot/events/syscalls/enable
3639  # cat trace_pipe
3640  CPU:2 [LOST 11745 EVENTS]
3641              bash-2044  [002] .... 10594.481032: _raw_spin_lock_irqsave <-get_page_from_freelist
3642              bash-2044  [002] d... 10594.481032: add_preempt_count <-_raw_spin_lock_irqsave
3643              bash-2044  [002] d..1 10594.481032: __rmqueue <-get_page_from_freelist
3644              bash-2044  [002] d..1 10594.481033: _raw_spin_unlock <-get_page_from_freelist
3645              bash-2044  [002] d..1 10594.481033: sub_preempt_count <-_raw_spin_unlock
3646              bash-2044  [002] d... 10594.481033: get_pageblock_flags_group <-get_pageblock_migratetype
3647              bash-2044  [002] d... 10594.481034: __mod_zone_page_state <-get_page_from_freelist
3648              bash-2044  [002] d... 10594.481034: zone_statistics <-get_page_from_freelist
3649              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3650              bash-2044  [002] d... 10594.481034: __inc_zone_state <-zone_statistics
3651              bash-2044  [002] .... 10594.481035: arch_dup_task_struct <-copy_process
3652  [...]
3653
3654  # cat instances/foo/trace_pipe
3655              bash-1998  [000] d..4   136.676759: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3656              bash-1998  [000] dN.4   136.676760: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3657            <idle>-0     [003] d.h3   136.676906: sched_wakeup: comm=rcu_preempt pid=9 prio=120 success=1 target_cpu=003
3658            <idle>-0     [003] d..3   136.676909: sched_switch: prev_comm=swapper/3 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=rcu_preempt next_pid=9 next_prio=120
3659       rcu_preempt-9     [003] d..3   136.676916: sched_switch: prev_comm=rcu_preempt prev_pid=9 prev_prio=120 prev_state=S ==> next_comm=swapper/3 next_pid=0 next_prio=120
3660              bash-1998  [000] d..4   136.677014: sched_wakeup: comm=kworker/0:1 pid=59 prio=120 success=1 target_cpu=000
3661              bash-1998  [000] dN.4   136.677016: sched_wakeup: comm=bash pid=1998 prio=120 success=1 target_cpu=000
3662              bash-1998  [000] d..3   136.677018: sched_switch: prev_comm=bash prev_pid=1998 prev_prio=120 prev_state=R+ ==> next_comm=kworker/0:1 next_pid=59 next_prio=120
3663       kworker/0:1-59    [000] d..4   136.677022: sched_wakeup: comm=sshd pid=1995 prio=120 success=1 target_cpu=001
3664       kworker/0:1-59    [000] d..3   136.677025: sched_switch: prev_comm=kworker/0:1 prev_pid=59 prev_prio=120 prev_state=S ==> next_comm=bash next_pid=1998 next_prio=120
3665  [...]
3666
3667  # cat instances/bar/trace_pipe
3668       migration/1-14    [001] d.h3   138.732674: softirq_raise: vec=3 [action=NET_RX]
3669            <idle>-0     [001] dNh3   138.732725: softirq_raise: vec=3 [action=NET_RX]
3670              bash-1998  [000] d.h1   138.733101: softirq_raise: vec=1 [action=TIMER]
3671              bash-1998  [000] d.h1   138.733102: softirq_raise: vec=9 [action=RCU]
3672              bash-1998  [000] ..s2   138.733105: softirq_entry: vec=1 [action=TIMER]
3673              bash-1998  [000] ..s2   138.733106: softirq_exit: vec=1 [action=TIMER]
3674              bash-1998  [000] ..s2   138.733106: softirq_entry: vec=9 [action=RCU]
3675              bash-1998  [000] ..s2   138.733109: softirq_exit: vec=9 [action=RCU]
3676              sshd-1995  [001] d.h1   138.733278: irq_handler_entry: irq=21 name=uhci_hcd:usb4
3677              sshd-1995  [001] d.h1   138.733280: irq_handler_exit: irq=21 ret=unhandled
3678              sshd-1995  [001] d.h1   138.733281: irq_handler_entry: irq=21 name=eth0
3679              sshd-1995  [001] d.h1   138.733283: irq_handler_exit: irq=21 ret=handled
3680  [...]
3681
3682  # cat instances/zoot/trace
3683  # tracer: nop
3684  #
3685  # entries-in-buffer/entries-written: 18996/18996   #P:4
3686  #
3687  #                              _-----=> irqs-off
3688  #                             / _----=> need-resched
3689  #                            | / _---=> hardirq/softirq
3690  #                            || / _--=> preempt-depth
3691  #                            ||| /     delay
3692  #           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
3693  #              | |       |   ||||       |         |
3694              bash-1998  [000] d...   140.733501: sys_write -> 0x2
3695              bash-1998  [000] d...   140.733504: sys_dup2(oldfd: a, newfd: 1)
3696              bash-1998  [000] d...   140.733506: sys_dup2 -> 0x1
3697              bash-1998  [000] d...   140.733508: sys_fcntl(fd: a, cmd: 1, arg: 0)
3698              bash-1998  [000] d...   140.733509: sys_fcntl -> 0x1
3699              bash-1998  [000] d...   140.733510: sys_close(fd: a)
3700              bash-1998  [000] d...   140.733510: sys_close -> 0x0
3701              bash-1998  [000] d...   140.733514: sys_rt_sigprocmask(how: 0, nset: 0, oset: 6e2768, sigsetsize: 8)
3702              bash-1998  [000] d...   140.733515: sys_rt_sigprocmask -> 0x0
3703              bash-1998  [000] d...   140.733516: sys_rt_sigaction(sig: 2, act: 7fff718846f0, oact: 7fff71884650, sigsetsize: 8)
3704              bash-1998  [000] d...   140.733516: sys_rt_sigaction -> 0x0
3705
3706You can see that the trace of the top most trace buffer shows only
3707the function tracing. The foo instance displays wakeups and task
3708switches.
3709
3710To remove the instances, simply delete their directories:
3711::
3712
3713  # rmdir instances/foo
3714  # rmdir instances/bar
3715  # rmdir instances/zoot
3716
3717Note, if a process has a trace file open in one of the instance
3718directories, the rmdir will fail with EBUSY.
3719
3720
3721Stack trace
3722-----------
3723Since the kernel has a fixed sized stack, it is important not to
3724waste it in functions. A kernel developer must be conscious of
3725what they allocate on the stack. If they add too much, the system
3726can be in danger of a stack overflow, and corruption will occur,
3727usually leading to a system panic.
3728
3729There are some tools that check this, usually with interrupts
3730periodically checking usage. But if you can perform a check
3731at every function call that will become very useful. As ftrace provides
3732a function tracer, it makes it convenient to check the stack size
3733at every function call. This is enabled via the stack tracer.
3734
3735CONFIG_STACK_TRACER enables the ftrace stack tracing functionality.
3736To enable it, write a '1' into /proc/sys/kernel/stack_tracer_enabled.
3737::
3738
3739 # echo 1 > /proc/sys/kernel/stack_tracer_enabled
3740
3741You can also enable it from the kernel command line to trace
3742the stack size of the kernel during boot up, by adding "stacktrace"
3743to the kernel command line parameter.
3744
3745After running it for a few minutes, the output looks like:
3746::
3747
3748  # cat stack_max_size
3749  2928
3750
3751  # cat stack_trace
3752          Depth    Size   Location    (18 entries)
3753          -----    ----   --------
3754    0)     2928     224   update_sd_lb_stats+0xbc/0x4ac
3755    1)     2704     160   find_busiest_group+0x31/0x1f1
3756    2)     2544     256   load_balance+0xd9/0x662
3757    3)     2288      80   idle_balance+0xbb/0x130
3758    4)     2208     128   __schedule+0x26e/0x5b9
3759    5)     2080      16   schedule+0x64/0x66
3760    6)     2064     128   schedule_timeout+0x34/0xe0
3761    7)     1936     112   wait_for_common+0x97/0xf1
3762    8)     1824      16   wait_for_completion+0x1d/0x1f
3763    9)     1808     128   flush_work+0xfe/0x119
3764   10)     1680      16   tty_flush_to_ldisc+0x1e/0x20
3765   11)     1664      48   input_available_p+0x1d/0x5c
3766   12)     1616      48   n_tty_poll+0x6d/0x134
3767   13)     1568      64   tty_poll+0x64/0x7f
3768   14)     1504     880   do_select+0x31e/0x511
3769   15)      624     400   core_sys_select+0x177/0x216
3770   16)      224      96   sys_select+0x91/0xb9
3771   17)      128     128   system_call_fastpath+0x16/0x1b
3772
3773Note, if -mfentry is being used by gcc, functions get traced before
3774they set up the stack frame. This means that leaf level functions
3775are not tested by the stack tracer when -mfentry is used.
3776
3777Currently, -mfentry is used by gcc 4.6.0 and above on x86 only.
3778
3779More
3780----
3781More details can be found in the source code, in the `kernel/trace/*.c` files.
3782