|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
|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.
|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 running. It
|may stop during system suspend.
|The clock source shall have as high resolution as possible, and the frequency
|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 in between 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 basically is 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 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 desirable: instead the number is taken as close as
|possible to a nanosecond value using only the arithmetic operations
|multiply 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 determine the
|mult factor from a fixed shift, and clocksource_register_hz() and
|clocksource_register_khz() which will help out assigning both shift and mult
|factors using the frequency of the clock source 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
|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 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
|Clock events are the conceptual reverse of clock sources: they take a
|desired time specification value and calculate the values to poke into
|hardware timer registers.
|Clock events are 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 has to be able to
|fire interrupts, so as to trigger events on the system timeline. On an SMP
|system, it is ideal (and customary) to have one such event driving timer per
|CPU core, so that each core can trigger events independently of any other
|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
|arithmetic, 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.
|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
|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 requires some of the same basic
|characteristics as the clock source, i.e. it should 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
|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() functions
|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 semantics 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 appropriate
|function pointers and rate settings for this delay timer.
|This is available on some architectures like OpenRISC or ARM.