|From:||Linus Walleij <email@example.com>|
|To:||firstname.lastname@example.org, Thomas Gleixner <email@example.com>, John Stultz <firstname.lastname@example.org>|
|Subject:||[PATCH v2] clocksource: document some basic timekeeping concepts|
|Date:||Tue, 24 Jun 2014 10:51:12 +0200|
|Cc:||Linus Walleij <email@example.com>, Nicolas Pitre <firstname.lastname@example.org>, Colin Cross <email@example.com>, Peter Zijlstra <firstname.lastname@example.org>, Ingo Molnar <email@example.com>|
This adds some documentation about clock sources, clock events, the weak sched_clock() function and delay timers that answers questions that repeatedly arise on the mailing lists. Cc: Thomas Gleixner <firstname.lastname@example.org> Cc: Nicolas Pitre <email@example.com> Cc: Colin Cross <firstname.lastname@example.org> Cc: John Stultz <email@example.com> Cc: Peter Zijlstra <firstname.lastname@example.org> Cc: Ingo Molnar <email@example.com> Signed-off-by: Linus Walleij <firstname.lastname@example.org> --- ChangeLog v1->v2: - Included paragraphs and minor edits to account for PeterZ's comments on addressing SMP use cases, which makes especially the semantics of sched_clock() much clearer. --- Documentation/timers/00-INDEX | 2 + Documentation/timers/timekeeping.txt | 179 +++++++++++++++++++++++++++++++++++ 2 files changed, 181 insertions(+) create mode 100644 Documentation/timers/timekeeping.txt diff --git a/Documentation/timers/00-INDEX b/Documentation/timers/00-INDEX index 6d042dc1cce0..ee212a27772f 100644 --- a/Documentation/timers/00-INDEX +++ b/Documentation/timers/00-INDEX @@ -12,6 +12,8 @@ Makefile - Build and link hpet_example NO_HZ.txt - Summary of the different methods for the scheduler clock-interrupts management. +timekeeping.txt + - Clock sources, clock events, sched_clock() and delay timer notes timers-howto.txt - how to insert delays in the kernel the right (tm) way. timer_stats.txt diff --git a/Documentation/timers/timekeeping.txt b/Documentation/timers/timekeeping.txt new file mode 100644 index 000000000000..89ff5c39edcc --- /dev/null +++ b/Documentation/timers/timekeeping.txt @@ -0,0 +1,179 @@ +Clock sources, Clock events, sched_clock() and delay timers +----------------------------------------------------------- + +This document tries to briefly explain some basic kernel timekeeping +abstractions. It partly pertains to the drivers usually found in +drivers/clocksource in the kernel tree, but the code may be spread out +across the kernel. + +If you grep through the kernel source you will find a number of architecture- +specific implementations of clock sources, clockevents and several likewise +architecture-specific overrides of the sched_clock() function and some +delay timers. + +To provide timekeeping for your platform, the clock source provides +the basic timeline, whereas clock events shoot interrupts on certain points +on this timeline, providing facilities such as high-resolution timers. +sched_clock() is used for scheduling and timestamping, and delay timers +provide an accurate delay source using hardware counters. + + +Clock sources +------------- + +The purpose of the clock source is to provide a timeline for the system that +tells you where you are in time. For example issuing the command 'date' on +a Linux system will eventually read the clock source to determine exactly +what time it is. + +Typically the clock source is a monotonic, atomic counter which will provide +n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over. +It will ideally NEVER stop ticking as long as the system is functional. + +The clock source shall have as high resolution as possible, and shall be as +stable and correct as possible as compared to a real-world wall clock. It +should not move unpredictably back and forth in time or miss a few cycles +here and there. + +It must be immune to the kind of effects that occur in hardware where e.g. +the counter register is read in two phases on the bus lowest 16 bits first +and the higher 16 bits in a second bus cycle with the counter bits +potentially being updated inbetween leading to the risk of very strange +values from the counter. + +When the wall-clock accuracy of the clock source isn't satisfactory, there +are various quirks and layers in the timekeeping code for e.g. synchronizing +the user-visible time to RTC clocks in the system or against networked time +servers using NTP, but all they do is basically to update an offset against +the clock source, which provides the fundamental timeline for the system. +These measures does not affect the clock source per se, they only adapt the +system to the shortcomings of it. + +The clock source struct shall provide means to translate the provided counter +into a rough nanosecond value as an unsigned long long (unsigned 64 bit) number. +Since this operation may be invoked very often, doing this in a strict +mathematical sense is not desireable: instead the number is taken as close as +possible to a nanosecond value using only the arithmetic operations +mult and shift, so in clocksource_cyc2ns() you find: + + ns ~= (clocksource * mult) >> shift + +You will find a number of helper functions in the clock source code intended +to aid in providing these mult and shift values, such as +clocksource_khz2mult(), clocksource_hz2mult() that help determinining the +mult factor from a fixed shift, and clocksource_calc_mult_shift() and +clocksource_register_hz() which will help out assigning both shift and mult +factors using the frequency of the clock source and desirable minimum idle +time as the only input. + +For real simple clock sources accessed from a single I/O memory location +there is nowadays even clocksource_mmio_init() which will take a memory +location, bit width, a parameter telling whether the counter in the +register counts up or down, and the timer clock rate, and then conjure all +necessary parameters. + +In the past, the timekeeping authors would come up with the shift and mult +values by hand, which is why you will sometimes find hard-coded shift and +mult values in the code. + +Since a 32 bit counter at say 100 MHz will wrap around to zero after some 43 +seconds, the code handling the clock source will have to compensate for this. +That is the reason to why the clock source struct also contains a 'mask' +member telling how many bits of the source are valid. This way the timekeeping +code knows when the counter will wrap around and can insert the necessary +compensation code on both sides of the wrap point so that the system timeline +remains monotonic. + + +Clock events +------------ + +Clock events are conceptually orthogonal to clock sources. The same hardware +and register range may be used for the clock event, but it is essentially +a different thing. The hardware driving clock events have to be able to +fire interrupts, so as to trigger events on the system timeline. On a SMP +system, it is ideal (and custom) to have one such event driving timer per +CPU core, so that each core can trigger events independently of any other +core. + +You will notice that the clock event device code is based on the same basic +idea about translating counters to nanoseconds using mult and shift +arithmetics, and you find the same family of helper functions again for +assigning these values. The clock event driver does not need a 'mask' +attribute however: the system will not try to plan events beyond the time +horizon of the clock event. + + +sched_clock() +------------- + +In addition to the clock sources and clock events there is a special weak +function in the kernel called sched_clock(). This function shall return the +number of nanoseconds since the system was started. An architecture may or +may not provide an implementation of sched_clock() on its own. If a local +implementation is not provided, the system jiffy counter will be used as +sched_clock(). + +As the name suggests, sched_clock() is used for scheduling the system, +determining the absolute timeslice for a certain process in the CFS scheduler +for example. It is also used for printk timestamps when you have selected to +include time information in printk for things like bootcharts. + +Compared to clock sources, sched_clock() has to be very fast: it is called +much more often, especially by the scheduler. If you have to do trade-offs +between accuracy compared to the clock source, you may sacrifice accuracy +for speed in sched_clock(). It however require some of the same basic +characteristics as the clock source, i.e. it has to be monotonic. + +The sched_clock() function may wrap only on unsigned long long boundaries, +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps +after circa 585 years. (For most practical systems this means "never".) + +If an architecture does not provide its own implementation of this function, +it will fall back to using jiffies, making its maximum resolution 1/HZ of the +jiffy frequency for the architecture. This will affect scheduling accuracy +and will likely show up in system benchmarks. + +The clock driving sched_clock() may stop or reset to zero during system +suspend/sleep. This does not matter to the function it serves of scheduling +events on the system. However it may result in interesting timestamps in +printk(). + +The sched_clock() function should be callable in any context, IRQ- and +NMI-safe and return a sane value in any context. + +Some architectures may have a limited set of time sources and lack a nice +counter to derive a 64-bit nanosecond value, so for example on the ARM +architecture, special helper functions have been created to provide a +sched_clock() nanosecond base from a 16- or 32-bit counter. Sometimes the +same counter that is also used as clock source is used for this purpose. + +On SMP systems, it is crucial for performance that sched_clock() can be called +independently on each CPU without any synchronization performance hits. +Some hardware (such as the x86 TSC) will cause the sched_clock() function to +drift between the CPUs on the system. The kernel can work around this by +enabling the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK option. This is another aspect +that makes sched_clock() different from the ordinary clock source. + + +Delay timers (some architectures only) +-------------------------------------- + +On systems with variable CPU frequency, the various kernel delay() function +will sometimes behave strangely. Basically these delays usually use a hard +loop to delay a certain number of jiffy fractions using a "lpj" (loops per +jiffy) value, calibrated on boot. + +Let's hope that your system is running on maximum frequency when this value +is calibrated: as an effect when the frequency is geared down to half the +full frequency, any delay() will be twice as long. Usually this does not +hurt, as you're commonly requesting that amount of delay *or more*. But +basically the sematics are quite unpredictable on such systems. + +Enter timer-based delays. Using these, a timer read may be used instead of +a hard-coded loop for providing the desired delay. + +This is done by declaring a struct delay_timer and assigning the apropriate +function pointers and rate settings for this delay timer. + +This is available on some architectures like OpenRISC or ARM. -- 1.9.3
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