181 lines
8.8 KiB
ReStructuredText
181 lines
8.8 KiB
ReStructuredText
===========================================================
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Clock sources, Clock events, sched_clock() and delay timers
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===========================================================
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This document tries to briefly explain some basic kernel timekeeping
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abstractions. It partly pertains to the drivers usually found in
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drivers/clocksource in the kernel tree, but the code may be spread out
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across the kernel.
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If you grep through the kernel source you will find a number of architecture-
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specific implementations of clock sources, clockevents and several likewise
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architecture-specific overrides of the sched_clock() function and some
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delay timers.
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To provide timekeeping for your platform, the clock source provides
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the basic timeline, whereas clock events shoot interrupts on certain points
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on this timeline, providing facilities such as high-resolution timers.
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sched_clock() is used for scheduling and timestamping, and delay timers
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provide an accurate delay source using hardware counters.
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Clock sources
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-------------
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The purpose of the clock source is to provide a timeline for the system that
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tells you where you are in time. For example issuing the command 'date' on
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a Linux system will eventually read the clock source to determine exactly
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what time it is.
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Typically the clock source is a monotonic, atomic counter which will provide
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n bits which count from 0 to (2^n)-1 and then wraps around to 0 and start over.
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It will ideally NEVER stop ticking as long as the system is running. It
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may stop during system suspend.
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The clock source shall have as high resolution as possible, and the frequency
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shall be as stable and correct as possible as compared to a real-world wall
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clock. It should not move unpredictably back and forth in time or miss a few
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cycles here and there.
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It must be immune to the kind of effects that occur in hardware where e.g.
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the counter register is read in two phases on the bus lowest 16 bits first
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and the higher 16 bits in a second bus cycle with the counter bits
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potentially being updated in between leading to the risk of very strange
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values from the counter.
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When the wall-clock accuracy of the clock source isn't satisfactory, there
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are various quirks and layers in the timekeeping code for e.g. synchronizing
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the user-visible time to RTC clocks in the system or against networked time
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servers using NTP, but all they do basically is update an offset against
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the clock source, which provides the fundamental timeline for the system.
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These measures does not affect the clock source per se, they only adapt the
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system to the shortcomings of it.
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The clock source struct shall provide means to translate the provided counter
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into a nanosecond value as an unsigned long long (unsigned 64 bit) number.
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Since this operation may be invoked very often, doing this in a strict
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mathematical sense is not desirable: instead the number is taken as close as
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possible to a nanosecond value using only the arithmetic operations
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multiply and shift, so in clocksource_cyc2ns() you find:
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ns ~= (clocksource * mult) >> shift
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You will find a number of helper functions in the clock source code intended
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to aid in providing these mult and shift values, such as
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clocksource_khz2mult(), clocksource_hz2mult() that help determine the
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mult factor from a fixed shift, and clocksource_register_hz() and
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clocksource_register_khz() which will help out assigning both shift and mult
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factors using the frequency of the clock source as the only input.
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For real simple clock sources accessed from a single I/O memory location
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there is nowadays even clocksource_mmio_init() which will take a memory
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location, bit width, a parameter telling whether the counter in the
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register counts up or down, and the timer clock rate, and then conjure all
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necessary parameters.
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Since a 32-bit counter at say 100 MHz will wrap around to zero after some 43
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seconds, the code handling the clock source will have to compensate for this.
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That is the reason why the clock source struct also contains a 'mask'
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member telling how many bits of the source are valid. This way the timekeeping
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code knows when the counter will wrap around and can insert the necessary
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compensation code on both sides of the wrap point so that the system timeline
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remains monotonic.
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Clock events
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------------
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Clock events are the conceptual reverse of clock sources: they take a
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desired time specification value and calculate the values to poke into
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hardware timer registers.
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Clock events are orthogonal to clock sources. The same hardware
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and register range may be used for the clock event, but it is essentially
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a different thing. The hardware driving clock events has to be able to
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fire interrupts, so as to trigger events on the system timeline. On an SMP
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system, it is ideal (and customary) to have one such event driving timer per
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CPU core, so that each core can trigger events independently of any other
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core.
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You will notice that the clock event device code is based on the same basic
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idea about translating counters to nanoseconds using mult and shift
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arithmetic, and you find the same family of helper functions again for
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assigning these values. The clock event driver does not need a 'mask'
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attribute however: the system will not try to plan events beyond the time
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horizon of the clock event.
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sched_clock()
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-------------
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In addition to the clock sources and clock events there is a special weak
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function in the kernel called sched_clock(). This function shall return the
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number of nanoseconds since the system was started. An architecture may or
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may not provide an implementation of sched_clock() on its own. If a local
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implementation is not provided, the system jiffy counter will be used as
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sched_clock().
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As the name suggests, sched_clock() is used for scheduling the system,
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determining the absolute timeslice for a certain process in the CFS scheduler
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for example. It is also used for printk timestamps when you have selected to
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include time information in printk for things like bootcharts.
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Compared to clock sources, sched_clock() has to be very fast: it is called
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much more often, especially by the scheduler. If you have to do trade-offs
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between accuracy compared to the clock source, you may sacrifice accuracy
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for speed in sched_clock(). It however requires some of the same basic
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characteristics as the clock source, i.e. it should be monotonic.
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The sched_clock() function may wrap only on unsigned long long boundaries,
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i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps
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after circa 585 years. (For most practical systems this means "never".)
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If an architecture does not provide its own implementation of this function,
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it will fall back to using jiffies, making its maximum resolution 1/HZ of the
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jiffy frequency for the architecture. This will affect scheduling accuracy
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and will likely show up in system benchmarks.
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The clock driving sched_clock() may stop or reset to zero during system
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suspend/sleep. This does not matter to the function it serves of scheduling
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events on the system. However it may result in interesting timestamps in
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printk().
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The sched_clock() function should be callable in any context, IRQ- and
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NMI-safe and return a sane value in any context.
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Some architectures may have a limited set of time sources and lack a nice
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counter to derive a 64-bit nanosecond value, so for example on the ARM
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architecture, special helper functions have been created to provide a
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sched_clock() nanosecond base from a 16- or 32-bit counter. Sometimes the
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same counter that is also used as clock source is used for this purpose.
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On SMP systems, it is crucial for performance that sched_clock() can be called
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independently on each CPU without any synchronization performance hits.
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Some hardware (such as the x86 TSC) will cause the sched_clock() function to
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drift between the CPUs on the system. The kernel can work around this by
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enabling the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK option. This is another aspect
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that makes sched_clock() different from the ordinary clock source.
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Delay timers (some architectures only)
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--------------------------------------
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On systems with variable CPU frequency, the various kernel delay() functions
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will sometimes behave strangely. Basically these delays usually use a hard
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loop to delay a certain number of jiffy fractions using a "lpj" (loops per
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jiffy) value, calibrated on boot.
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Let's hope that your system is running on maximum frequency when this value
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is calibrated: as an effect when the frequency is geared down to half the
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full frequency, any delay() will be twice as long. Usually this does not
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hurt, as you're commonly requesting that amount of delay *or more*. But
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basically the semantics are quite unpredictable on such systems.
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Enter timer-based delays. Using these, a timer read may be used instead of
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a hard-coded loop for providing the desired delay.
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This is done by declaring a struct delay_timer and assigning the appropriate
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function pointers and rate settings for this delay timer.
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This is available on some architectures like OpenRISC or ARM.
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