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Linux/Documentation/timers/highres.rst

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  1 =====================================================
  2 High resolution timers and dynamic ticks design notes
  3 =====================================================
  4 
  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:
  8 https://www.kernel.org/doc/ols/2006/ols2006v1-pages-333-346.pdf
  9 
 10 The slides to this talk are available from:
 11 http://www.cs.columbia.edu/~nahum/w6998/papers/ols2006-hrtimers-slides.pdf
 12 
 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
 16 got merged into mainline.
 17 
 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.
 20 
 21 The design contains the following basic building blocks:
 22 
 23 - hrtimer base infrastructure
 24 - timeofday and clock source management
 25 - clock event management
 26 - high resolution timer functionality
 27 - dynamic ticks
 28 
 29 
 30 hrtimer base infrastructure
 31 ---------------------------
 32 
 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
 35 also figure #2 (OLS slides p. 15)
 36 
 37 The main differences to the timer wheel, which holds the armed timer_list type
 38 timers are:
 39 
 40        - time ordered enqueueing into a rb-tree
 41        - independent of ticks (the processing is based on nanoseconds)
 42 
 43 
 44 timeofday and clock source management
 45 -------------------------------------
 46 
 47 John Stultz's Generic Time Of Day (GTOD) framework moves a large portion of
 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.
 57 
 58 Further information about the Generic Time Of Day framework is available in the
 59 OLS 2005 Proceedings Volume 1:
 60 
 61         http://www.linuxsymposium.org/2005/linuxsymposium_procv1.pdf
 62 
 63 The paper "We Are Not Getting Any Younger: A New Approach to Time and
 64 Timers" was written by J. Stultz, D.V. Hart, & N. Aravamudan.
 65 
 66 Figure #3 (OLS slides p.18) illustrates the transformation.
 67 
 68 
 69 clock event management
 70 ----------------------
 71 
 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
 77 dependent code. This results in duplicated code across all architectures and
 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
 82 functionality like high resolution timers or dynamic ticks.
 83 
 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
 88 hardware related handling and to allow easy addition and utilization of new
 89 clock event devices. It also minimizes the duplicated code across the
 90 architectures as it provides generic functionality down to the interrupt
 91 service handler, which is almost inherently hardware dependent.
 92 
 93 Clock event devices are registered either by the architecture dependent boot
 94 code or at module insertion time. Each clock event device fills a data
 95 structure with clock-specific property parameters and callback functions. The
 96 clock event management decides, by using the specified property parameters, the
 97 set of system functions a clock event device will be used to support. This
 98 includes the distinction of per-CPU and per-system global event devices.
 99 
100 System-level global event devices are used for the Linux periodic tick. Per-CPU
101 event devices are used to provide local CPU functionality such as process
102 accounting, profiling, and high resolution timers.
103 
104 The management layer assigns one or more of the following functions to a clock
105 event device:
106 
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)
111 
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.
119 
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
122 interrupt, clock event device notification service and support for suspend and
123 resume.
124 
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.
130 
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
134 enabling of high resolution timers and dynamic ticks is simply provided by
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)
138 
139 Figure #4 (OLS slides p.20) illustrates the transformation.
140 
141 
142 high resolution timer functionality
143 -----------------------------------
144 
145 During system boot it is not possible to use the high resolution timer
146 functionality, while making it possible would be difficult and would serve no
147 useful function. The initialization of the clock event device framework, the
148 clock source framework (GTOD) and hrtimers itself has to be done and
149 appropriate clock sources and clock event devices have to be registered before
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
153 which inform hrtimers about availability of new hardware. hrtimers validates
154 the usability of the registered clock sources and clock event devices before
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
157 necessary hardware support.
158 
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
165 tackled yet.
166 
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
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.
173 
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
184 timer.
185 
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.
189 
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
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
195 timer and periodic events (jiffies tick, profiling, process accounting) on UP
196 systems. This has been proved to work with the PIT on i386 and the Incrementer
197 on PPC.
198 
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
201 resolution timer signals which are used by itimer and POSIX interval
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.
204 
205 Figure #5 (OLS slides p.22) illustrates the transformation.
206 
207 
208 dynamic ticks
209 -------------
210 
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
213 extended by three functions:
214 
215 - hrtimer_stop_sched_tick
216 - hrtimer_restart_sched_tick
217 - hrtimer_update_jiffies
218 
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.
228 
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().
232 
233 hrtimer_update_jiffies() is called from irq_enter() when an interrupt happens
234 in the idle period to make sure that jiffies are up to date and the interrupt
235 handler has not to deal with an eventually stale jiffy value.
236 
237 The dynamic tick feature provides statistical values which are exported to
238 userspace via /proc/stat and can be made available for enhanced power
239 management control.
240 
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.
244 
245 
246 Aside the current initial submission of i386 support, the patchset has been
247 extended to x86_64 and ARM already. Initial (work in progress) support is also
248 available for MIPS and PowerPC.
249 
250           Thomas, Ingo

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