Documentation/timers/highres.rst

2 High resolution timers and dynamic ticks design notes
8 https://www.kernel.org/doc/ols/2006/ols2006v1-pages-333-346.pdf
11 http://www.cs.columbia.edu/~nahum/w6998/papers/ols2006-hrtimers-slides.pdf
23 - hrtimer base infrastructure
24 - timeofday and clock source management
25 - clock event management
26 - high resolution timer functionality
27 - dynamic ticks
31 ---------------------------
40        - time ordered enqueueing into a rb-tree
41        - independent of ticks (the processing is based on nanoseconds)
45 -------------------------------------
48 code out of the architecture-specific areas into a generic management
70 ----------------------
81 to touch all the architecture-specific implementations in order to provide new
82 functionality like high resolution timers or dynamic ticks.
95 structure with clock-specific property parameters and callback functions. 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
102 accounting, profiling, and high resolution timers.
107       - system global periodic tick (jiffies update)
108       - cpu local update_process_times
109       - cpu local profiling
110       - cpu local next event interrupt (non periodic mode)
132 utilize the high resolution and dynamic tick functionalities without any change
134 enabling of high resolution timers and dynamic ticks is simply provided by
136 adding the dynamic tick specific calls to the idle routine (a total of 3 lines
142 high resolution timer functionality
143 -----------------------------------
145 During system boot it is not possible to use the high resolution timer
150 the high resolution functionality can work. Up to the point where hrtimers are
155 switching to high resolution mode. This ensures also that a kernel which is
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
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
169 decision is made per timer base and synchronized across per-cpu timer bases in
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
172 reprogrammable clock event device per-CPU is utilized.
176 red-black tree to a separate double linked list and invokes the softirq
182 context is the avoidance of up to two context switches - from the interrupted
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 -
190 The periodic tick functionality is provided by an per-cpu hrtimer. The callback
193 based periodic tick is designed to be extended with dynamic tick functionality.
194 This allows to use a single clock event device to schedule high resolution
200 separated from the tick bound timer softirq to allow accurate delivery of high
208 dynamic ticks
209 -------------
211 Dynamic ticks are the logical consequence of the hrtimer based periodic tick
215 - hrtimer_stop_sched_tick
216 - hrtimer_restart_sched_tick
217 - hrtimer_update_jiffies
237 The dynamic tick feature provides statistical values which are exported to