Documentation/timers/highres.rst

5 Further information can be found in the paper of the OLS 2006 talk "hrtimers
6 and beyond". The paper is part of the OLS 2006 Proceedings Volume 1, which can
7 be found on the OLS website:
10 The slides to this talk are available from:
13 The slides contain five figures (pages 2, 15, 18, 20, 22), which illustrate the
14 changes in the time(r) related Linux subsystems. Figure #1 (p. 2) shows the
15 design of the Linux time(r) system before hrtimers and other building blocks
18 Note: the paper and the slides are talking about "clock event source", while we
19 switched to the name "clock event devices" in meantime.
21 The design contains the following basic building blocks:
33 The hrtimer base infrastructure was merged into the 2.6.16 kernel. Details of
34 the base implementation are covered in Documentation/timers/hrtimers.rst. See
37 The main differences to the timer wheel, which holds the armed timer_list type
41        - independent of ticks (the processing is based on nanoseconds)
48 code out of the architecture-specific areas into a generic management
49 framework, as illustrated in figure #3 (OLS slides p. 18). The architecture
50 specific portion is reduced to the low level hardware details of the clock
51 sources, which are registered in the framework and selected on a quality based
52 decision. The low level code provides hardware setup and readout routines and
53 initializes data structures, which are used by the generic time keeping code to
54 convert the clock ticks to nanosecond based time values. All other time keeping
55 related functionality is moved into the generic code. The GTOD base patch got
56 merged into the 2.6.18 kernel.
58 Further information about the Generic Time Of Day framework is available in the
63 The paper "We Are Not Getting Any Younger: A New Approach to Time and
66 Figure #3 (OLS slides p.18) illustrates the transformation.
72 While clock sources provide read access to the monotonically increasing time
73 value, clock event devices are used to schedule the next event
74 interrupt(s). The next event is currently defined to be periodic, with its
75 period defined at compile time. The setup and selection of the event device
76 for various event driven functionalities is hardwired into the architecture
78 makes it extremely difficult to change the configuration of the system to use
79 event interrupt devices other than those already built into the
80 architecture. Another implication of the current design is that it is necessary
81 to touch all the architecture-specific implementations in order to provide new
84 The clock events subsystem tries to address this problem by providing a generic
85 solution to manage clock event devices and their usage for the various clock
86 event driven kernel functionalities. The goal of the clock event subsystem is
87 to minimize the clock event related architecture dependent code to the pure
89 clock event devices. It also minimizes the duplicated code across the
90 architectures as it provides generic functionality down to the interrupt
93 Clock event devices are registered either by the architecture dependent boot
95 structure with clock-specific property parameters and callback functions. The
96 clock event management decides, by using the specified property parameters, the
98 includes the distinction of per-CPU and per-system global event devices.
100 System-level global event devices are used for the Linux periodic tick. Per-CPU
104 The management layer assigns one or more of the following functions to a clock
112 The clock event device delegates the selection of those timer interrupt related
113 functions completely to the management layer. The clock management layer stores
114 a function pointer in the device description structure, which has to be called
115 from the hardware level handler. This removes a lot of duplicated code from the
116 architecture specific timer interrupt handlers and hands the control over the
117 clock event devices and the assignment of timer interrupt related functionality
118 to the core code.
120 The clock event layer API is rather small. Aside from the clock event device
121 registration interface it provides functions to schedule the next event
125 The framework adds about 700 lines of code which results in a 2KB increase of
126 the kernel binary size. The conversion of i386 removes about 100 lines of
127 code. The binary size decrease is in the range of 400 byte. We believe that the
128 increase of flexibility and the avoidance of duplicated code across
129 architectures justifies the slight increase of the binary size.
131 The conversion of an architecture has no functional impact, but allows to
132 utilize the high resolution and dynamic tick functionalities without any change
133 to the clock event device and timer interrupt code. After the conversion the
135 adding the kernel/time/Kconfig file to the architecture specific Kconfig and
136 adding the dynamic tick specific calls to the idle routine (a total of 3 lines
137 added to the idle function and the Kconfig file)
139 Figure #4 (OLS slides p.20) illustrates the transformation.
145 During system boot it is not possible to use the high resolution timer
147 useful function. The initialization of the clock event device framework, the
150 the high resolution functionality can work. Up to the point where hrtimers are
151 initialized, the system works in the usual low resolution periodic mode. The
152 clock source and the clock event device layers provide notification functions
154 the usability of the registered clock sources and clock event devices before
156 configured for high resolution timers can run on a system which lacks the
159 The high resolution timer code does not support SMP machines which have only
160 global clock event devices. The support of such hardware would involve IPI
161 calls when an interrupt happens. The overhead would be much larger than the
162 benefit. This is the reason why we currently disable high resolution and
163 dynamic ticks on i386 SMP systems which stop the local APIC in C3 power
164 state. A workaround is available as an idea, but the problem has not been
167 The time ordered insertion of timers provides all the infrastructure to decide
168 whether the event device has to be reprogrammed when a timer is added. The
170 a support function. The design allows the system to utilize separate per-CPU
171 clock event devices for the per-CPU timer bases, but currently only one
174 When the timer interrupt happens, the next event interrupt handler is called
175 from the clock event distribution code and moves expired timers from the
176 red-black tree to a separate double linked list and invokes the softirq
177 handler. An additional mode field in the hrtimer structure allows the system to
178 execute callback functions directly from the next event interrupt handler. This
179 is restricted to code which can safely be executed in the hard interrupt
180 context. This applies, for example, to the common case of a wakeup function as
181 used by nanosleep. The advantage of executing the handler in the interrupt
182 context is the avoidance of up to two context switches - from the interrupted
183 context to the softirq and to the task which is woken up by the expired
186 Once a system has switched to high resolution mode, the periodic tick is
187 switched off. This disables the per system global periodic clock event device -
188 e.g. the PIT on i386 SMP systems.
190 The periodic tick functionality is provided by an per-cpu hrtimer. The callback
191 function is executed in the next event interrupt context and updates jiffies
192 and calls update_process_times and profiling. The implementation of the hrtimer
196 systems. This has been proved to work with the PIT on i386 and the Incrementer
199 The softirq for running the hrtimer queues and executing the callbacks has been
200 separated from the tick bound timer softirq to allow accurate delivery of high
202 timers. The execution of this softirq can still be delayed by other softirqs,
203 but the overall latencies have been significantly improved by this separation.
205 Figure #5 (OLS slides p.22) illustrates the transformation.
211 Dynamic ticks are the logical consequence of the hrtimer based periodic tick
212 replacement (sched_tick). The functionality of the sched_tick hrtimer is
219 hrtimer_stop_sched_tick() is called when a CPU goes into idle state. The code
220 evaluates the next scheduled timer event (from both hrtimers and the timer
221 wheel) and in case that the next event is further away than the next tick it
222 reprograms the sched_tick to this future event, to allow longer idle sleeps
223 without worthless interruption by the periodic tick. The function is also
224 called when an interrupt happens during the idle period, which does not cause a
225 reschedule. The call is necessary as the interrupt handler might have armed a
226 new timer whose expiry time is before the time which was identified as the
227 nearest event in the previous call to hrtimer_stop_sched_tick.
229 hrtimer_restart_sched_tick() is called when the CPU leaves the idle state before
230 it calls schedule(). hrtimer_restart_sched_tick() resumes the periodic tick,
231 which is kept active until the next call to hrtimer_stop_sched_tick().
234 in the idle period to make sure that jiffies are up to date and the interrupt
237 The dynamic tick feature provides statistical values which are exported to
241 The implementation leaves room for further development like full tickless
242 systems, where the time slice is controlled by the scheduler, variable
243 frequency profiling, and a complete removal of jiffies in the future.
246 Aside the current initial submission of i386 support, the patchset has been