Root/Documentation/timers/highres.txt

1High resolution timers and dynamic ticks design notes
2-----------------------------------------------------
3
4Further information can be found in the paper of the OLS 2006 talk "hrtimers
5and beyond". The paper is part of the OLS 2006 Proceedings Volume 1, which can
6be found on the OLS website:
7http://www.linuxsymposium.org/2006/linuxsymposium_procv1.pdf
8
9The slides to this talk are available from:
10http://tglx.de/projects/hrtimers/ols2006-hrtimers.pdf
11
12The slides contain five figures (pages 2, 15, 18, 20, 22), which illustrate the
13changes in the time(r) related Linux subsystems. Figure #1 (p. 2) shows the
14design of the Linux time(r) system before hrtimers and other building blocks
15got merged into mainline.
16
17Note: the paper and the slides are talking about "clock event source", while we
18switched to the name "clock event devices" in meantime.
19
20The design contains the following basic building blocks:
21
22- hrtimer base infrastructure
23- timeofday and clock source management
24- clock event management
25- high resolution timer functionality
26- dynamic ticks
27
28
29hrtimer base infrastructure
30---------------------------
31
32The hrtimer base infrastructure was merged into the 2.6.16 kernel. Details of
33the base implementation are covered in Documentation/hrtimers/hrtimer.txt. See
34also figure #2 (OLS slides p. 15)
35
36The main differences to the timer wheel, which holds the armed timer_list type
37timers are:
38       - time ordered enqueueing into a rb-tree
39       - independent of ticks (the processing is based on nanoseconds)
40
41
42timeofday and clock source management
43-------------------------------------
44
45John Stultz's Generic Time Of Day (GTOD) framework moves a large portion of
46code out of the architecture-specific areas into a generic management
47framework, as illustrated in figure #3 (OLS slides p. 18). The architecture
48specific portion is reduced to the low level hardware details of the clock
49sources, which are registered in the framework and selected on a quality based
50decision. The low level code provides hardware setup and readout routines and
51initializes data structures, which are used by the generic time keeping code to
52convert the clock ticks to nanosecond based time values. All other time keeping
53related functionality is moved into the generic code. The GTOD base patch got
54merged into the 2.6.18 kernel.
55
56Further information about the Generic Time Of Day framework is available in the
57OLS 2005 Proceedings Volume 1:
58http://www.linuxsymposium.org/2005/linuxsymposium_procv1.pdf
59
60The paper "We Are Not Getting Any Younger: A New Approach to Time and
61Timers" was written by J. Stultz, D.V. Hart, & N. Aravamudan.
62
63Figure #3 (OLS slides p.18) illustrates the transformation.
64
65
66clock event management
67----------------------
68
69While clock sources provide read access to the monotonically increasing time
70value, clock event devices are used to schedule the next event
71interrupt(s). The next event is currently defined to be periodic, with its
72period defined at compile time. The setup and selection of the event device
73for various event driven functionalities is hardwired into the architecture
74dependent code. This results in duplicated code across all architectures and
75makes it extremely difficult to change the configuration of the system to use
76event interrupt devices other than those already built into the
77architecture. Another implication of the current design is that it is necessary
78to touch all the architecture-specific implementations in order to provide new
79functionality like high resolution timers or dynamic ticks.
80
81The clock events subsystem tries to address this problem by providing a generic
82solution to manage clock event devices and their usage for the various clock
83event driven kernel functionalities. The goal of the clock event subsystem is
84to minimize the clock event related architecture dependent code to the pure
85hardware related handling and to allow easy addition and utilization of new
86clock event devices. It also minimizes the duplicated code across the
87architectures as it provides generic functionality down to the interrupt
88service handler, which is almost inherently hardware dependent.
89
90Clock event devices are registered either by the architecture dependent boot
91code or at module insertion time. Each clock event device fills a data
92structure with clock-specific property parameters and callback functions. The
93clock event management decides, by using the specified property parameters, the
94set of system functions a clock event device will be used to support. This
95includes the distinction of per-CPU and per-system global event devices.
96
97System-level global event devices are used for the Linux periodic tick. Per-CPU
98event devices are used to provide local CPU functionality such as process
99accounting, profiling, and high resolution timers.
100
101The management layer assigns one or more of the following functions to a clock
102event device:
103      - system global periodic tick (jiffies update)
104      - cpu local update_process_times
105      - cpu local profiling
106      - cpu local next event interrupt (non periodic mode)
107
108The clock event device delegates the selection of those timer interrupt related
109functions completely to the management layer. The clock management layer stores
110a function pointer in the device description structure, which has to be called
111from the hardware level handler. This removes a lot of duplicated code from the
112architecture specific timer interrupt handlers and hands the control over the
113clock event devices and the assignment of timer interrupt related functionality
114to the core code.
115
116The clock event layer API is rather small. Aside from the clock event device
117registration interface it provides functions to schedule the next event
118interrupt, clock event device notification service and support for suspend and
119resume.
120
121The framework adds about 700 lines of code which results in a 2KB increase of
122the kernel binary size. The conversion of i386 removes about 100 lines of
123code. The binary size decrease is in the range of 400 byte. We believe that the
124increase of flexibility and the avoidance of duplicated code across
125architectures justifies the slight increase of the binary size.
126
127The conversion of an architecture has no functional impact, but allows to
128utilize the high resolution and dynamic tick functionalities without any change
129to the clock event device and timer interrupt code. After the conversion the
130enabling of high resolution timers and dynamic ticks is simply provided by
131adding the kernel/time/Kconfig file to the architecture specific Kconfig and
132adding the dynamic tick specific calls to the idle routine (a total of 3 lines
133added to the idle function and the Kconfig file)
134
135Figure #4 (OLS slides p.20) illustrates the transformation.
136
137
138high resolution timer functionality
139-----------------------------------
140
141During system boot it is not possible to use the high resolution timer
142functionality, while making it possible would be difficult and would serve no
143useful function. The initialization of the clock event device framework, the
144clock source framework (GTOD) and hrtimers itself has to be done and
145appropriate clock sources and clock event devices have to be registered before
146the high resolution functionality can work. Up to the point where hrtimers are
147initialized, the system works in the usual low resolution periodic mode. The
148clock source and the clock event device layers provide notification functions
149which inform hrtimers about availability of new hardware. hrtimers validates
150the usability of the registered clock sources and clock event devices before
151switching to high resolution mode. This ensures also that a kernel which is
152configured for high resolution timers can run on a system which lacks the
153necessary hardware support.
154
155The high resolution timer code does not support SMP machines which have only
156global clock event devices. The support of such hardware would involve IPI
157calls when an interrupt happens. The overhead would be much larger than the
158benefit. This is the reason why we currently disable high resolution and
159dynamic ticks on i386 SMP systems which stop the local APIC in C3 power
160state. A workaround is available as an idea, but the problem has not been
161tackled yet.
162
163The time ordered insertion of timers provides all the infrastructure to decide
164whether the event device has to be reprogrammed when a timer is added. The
165decision is made per timer base and synchronized across per-cpu timer bases in
166a support function. The design allows the system to utilize separate per-CPU
167clock event devices for the per-CPU timer bases, but currently only one
168reprogrammable clock event device per-CPU is utilized.
169
170When the timer interrupt happens, the next event interrupt handler is called
171from the clock event distribution code and moves expired timers from the
172red-black tree to a separate double linked list and invokes the softirq
173handler. An additional mode field in the hrtimer structure allows the system to
174execute callback functions directly from the next event interrupt handler. This
175is restricted to code which can safely be executed in the hard interrupt
176context. This applies, for example, to the common case of a wakeup function as
177used by nanosleep. The advantage of executing the handler in the interrupt
178context is the avoidance of up to two context switches - from the interrupted
179context to the softirq and to the task which is woken up by the expired
180timer.
181
182Once a system has switched to high resolution mode, the periodic tick is
183switched off. This disables the per system global periodic clock event device -
184e.g. the PIT on i386 SMP systems.
185
186The periodic tick functionality is provided by an per-cpu hrtimer. The callback
187function is executed in the next event interrupt context and updates jiffies
188and calls update_process_times and profiling. The implementation of the hrtimer
189based periodic tick is designed to be extended with dynamic tick functionality.
190This allows to use a single clock event device to schedule high resolution
191timer and periodic events (jiffies tick, profiling, process accounting) on UP
192systems. This has been proved to work with the PIT on i386 and the Incrementer
193on PPC.
194
195The softirq for running the hrtimer queues and executing the callbacks has been
196separated from the tick bound timer softirq to allow accurate delivery of high
197resolution timer signals which are used by itimer and POSIX interval
198timers. The execution of this softirq can still be delayed by other softirqs,
199but the overall latencies have been significantly improved by this separation.
200
201Figure #5 (OLS slides p.22) illustrates the transformation.
202
203
204dynamic ticks
205-------------
206
207Dynamic ticks are the logical consequence of the hrtimer based periodic tick
208replacement (sched_tick). The functionality of the sched_tick hrtimer is
209extended by three functions:
210
211- hrtimer_stop_sched_tick
212- hrtimer_restart_sched_tick
213- hrtimer_update_jiffies
214
215hrtimer_stop_sched_tick() is called when a CPU goes into idle state. The code
216evaluates the next scheduled timer event (from both hrtimers and the timer
217wheel) and in case that the next event is further away than the next tick it
218reprograms the sched_tick to this future event, to allow longer idle sleeps
219without worthless interruption by the periodic tick. The function is also
220called when an interrupt happens during the idle period, which does not cause a
221reschedule. The call is necessary as the interrupt handler might have armed a
222new timer whose expiry time is before the time which was identified as the
223nearest event in the previous call to hrtimer_stop_sched_tick.
224
225hrtimer_restart_sched_tick() is called when the CPU leaves the idle state before
226it calls schedule(). hrtimer_restart_sched_tick() resumes the periodic tick,
227which is kept active until the next call to hrtimer_stop_sched_tick().
228
229hrtimer_update_jiffies() is called from irq_enter() when an interrupt happens
230in the idle period to make sure that jiffies are up to date and the interrupt
231handler has not to deal with an eventually stale jiffy value.
232
233The dynamic tick feature provides statistical values which are exported to
234userspace via /proc/stats and can be made available for enhanced power
235management control.
236
237The implementation leaves room for further development like full tickless
238systems, where the time slice is controlled by the scheduler, variable
239frequency profiling, and a complete removal of jiffies in the future.
240
241
242Aside the current initial submission of i386 support, the patchset has been
243extended to x86_64 and ARM already. Initial (work in progress) support is also
244available for MIPS and PowerPC.
245
246      Thomas, Ingo
247
248
249
250

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