1 =========================================================== 2 Clock sources, Clock events, sched_clock() and delay timers 3 =========================================================== 4 5 This document tries to briefly explain some basic kernel timekeeping 6 abstractions. It partly pertains to the drivers usually found in 7 drivers/clocksource in the kernel tree, but the code may be spread out 8 across the kernel. 9 10 If you grep through the kernel source you will find a number of architecture- 11 specific implementations of clock sources, clockevents and several likewise 12 architecture-specific overrides of the sched_clock() function and some 13 delay timers. 14 15 To provide timekeeping for your platform, the clock source provides 16 the basic timeline, whereas clock events shoot interrupts on certain points 17 on this timeline, providing facilities such as high-resolution timers. 18 sched_clock() is used for scheduling and timestamping, and delay timers 19 provide an accurate delay source using hardware counters. 20 21 22 Clock sources 23 ------------- 24 25 The purpose of the clock source is to provide a timeline for the system that 26 tells you where you are in time. For example issuing the command 'date' on 27 a Linux system will eventually read the clock source to determine exactly 28 what time it is. 29 30 Typically the clock source is a monotonic, atomic counter which will provide 31 n bits which count from 0 to (2^n)-1 and then wraps around to 0 and start over. 32 It will ideally NEVER stop ticking as long as the system is running. It 33 may stop during system suspend. 34 35 The clock source shall have as high resolution as possible, and the frequency 36 shall be as stable and correct as possible as compared to a real-world wall 37 clock. It should not move unpredictably back and forth in time or miss a few 38 cycles here and there. 39 40 It must be immune to the kind of effects that occur in hardware where e.g. 41 the counter register is read in two phases on the bus lowest 16 bits first 42 and the higher 16 bits in a second bus cycle with the counter bits 43 potentially being updated in between leading to the risk of very strange 44 values from the counter. 45 46 When the wall-clock accuracy of the clock source isn't satisfactory, there 47 are various quirks and layers in the timekeeping code for e.g. synchronizing 48 the user-visible time to RTC clocks in the system or against networked time 49 servers using NTP, but all they do basically is update an offset against 50 the clock source, which provides the fundamental timeline for the system. 51 These measures does not affect the clock source per se, they only adapt the 52 system to the shortcomings of it. 53 54 The clock source struct shall provide means to translate the provided counter 55 into a nanosecond value as an unsigned long long (unsigned 64 bit) number. 56 Since this operation may be invoked very often, doing this in a strict 57 mathematical sense is not desirable: instead the number is taken as close as 58 possible to a nanosecond value using only the arithmetic operations 59 multiply and shift, so in clocksource_cyc2ns() you find: 60 61 ns ~= (clocksource * mult) >> shift 62 63 You will find a number of helper functions in the clock source code intended 64 to aid in providing these mult and shift values, such as 65 clocksource_khz2mult(), clocksource_hz2mult() that help determine the 66 mult factor from a fixed shift, and clocksource_register_hz() and 67 clocksource_register_khz() which will help out assigning both shift and mult 68 factors using the frequency of the clock source as the only input. 69 70 For real simple clock sources accessed from a single I/O memory location 71 there is nowadays even clocksource_mmio_init() which will take a memory 72 location, bit width, a parameter telling whether the counter in the 73 register counts up or down, and the timer clock rate, and then conjure all 74 necessary parameters. 75 76 Since a 32-bit counter at say 100 MHz will wrap around to zero after some 43 77 seconds, the code handling the clock source will have to compensate for this. 78 That is the reason why the clock source struct also contains a 'mask' 79 member telling how many bits of the source are valid. This way the timekeeping 80 code knows when the counter will wrap around and can insert the necessary 81 compensation code on both sides of the wrap point so that the system timeline 82 remains monotonic. 83 84 85 Clock events 86 ------------ 87 88 Clock events are the conceptual reverse of clock sources: they take a 89 desired time specification value and calculate the values to poke into 90 hardware timer registers. 91 92 Clock events are orthogonal to clock sources. The same hardware 93 and register range may be used for the clock event, but it is essentially 94 a different thing. The hardware driving clock events has to be able to 95 fire interrupts, so as to trigger events on the system timeline. On an SMP 96 system, it is ideal (and customary) to have one such event driving timer per 97 CPU core, so that each core can trigger events independently of any other 98 core. 99 100 You will notice that the clock event device code is based on the same basic 101 idea about translating counters to nanoseconds using mult and shift 102 arithmetic, and you find the same family of helper functions again for 103 assigning these values. The clock event driver does not need a 'mask' 104 attribute however: the system will not try to plan events beyond the time 105 horizon of the clock event. 106 107 108 sched_clock() 109 ------------- 110 111 In addition to the clock sources and clock events there is a special weak 112 function in the kernel called sched_clock(). This function shall return the 113 number of nanoseconds since the system was started. An architecture may or 114 may not provide an implementation of sched_clock() on its own. If a local 115 implementation is not provided, the system jiffy counter will be used as 116 sched_clock(). 117 118 As the name suggests, sched_clock() is used for scheduling the system, 119 determining the absolute timeslice for a certain process in the CFS scheduler 120 for example. It is also used for printk timestamps when you have selected to 121 include time information in printk for things like bootcharts. 122 123 Compared to clock sources, sched_clock() has to be very fast: it is called 124 much more often, especially by the scheduler. If you have to do trade-offs 125 between accuracy compared to the clock source, you may sacrifice accuracy 126 for speed in sched_clock(). It however requires some of the same basic 127 characteristics as the clock source, i.e. it should be monotonic. 128 129 The sched_clock() function may wrap only on unsigned long long boundaries, 130 i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps 131 after circa 585 years. (For most practical systems this means "never".) 132 133 If an architecture does not provide its own implementation of this function, 134 it will fall back to using jiffies, making its maximum resolution 1/HZ of the 135 jiffy frequency for the architecture. This will affect scheduling accuracy 136 and will likely show up in system benchmarks. 137 138 The clock driving sched_clock() may stop or reset to zero during system 139 suspend/sleep. This does not matter to the function it serves of scheduling 140 events on the system. However it may result in interesting timestamps in 141 printk(). 142 143 The sched_clock() function should be callable in any context, IRQ- and 144 NMI-safe and return a sane value in any context. 145 146 Some architectures may have a limited set of time sources and lack a nice 147 counter to derive a 64-bit nanosecond value, so for example on the ARM 148 architecture, special helper functions have been created to provide a 149 sched_clock() nanosecond base from a 16- or 32-bit counter. Sometimes the 150 same counter that is also used as clock source is used for this purpose. 151 152 On SMP systems, it is crucial for performance that sched_clock() can be called 153 independently on each CPU without any synchronization performance hits. 154 Some hardware (such as the x86 TSC) will cause the sched_clock() function to 155 drift between the CPUs on the system. The kernel can work around this by 156 enabling the CONFIG_HAVE_UNSTABLE_SCHED_CLOCK option. This is another aspect 157 that makes sched_clock() different from the ordinary clock source. 158 159 160 Delay timers (some architectures only) 161 -------------------------------------- 162 163 On systems with variable CPU frequency, the various kernel delay() functions 164 will sometimes behave strangely. Basically these delays usually use a hard 165 loop to delay a certain number of jiffy fractions using a "lpj" (loops per 166 jiffy) value, calibrated on boot. 167 168 Let's hope that your system is running on maximum frequency when this value 169 is calibrated: as an effect when the frequency is geared down to half the 170 full frequency, any delay() will be twice as long. Usually this does not 171 hurt, as you're commonly requesting that amount of delay *or more*. But 172 basically the semantics are quite unpredictable on such systems. 173 174 Enter timer-based delays. Using these, a timer read may be used instead of 175 a hard-coded loop for providing the desired delay. 176 177 This is done by declaring a struct delay_timer and assigning the appropriate 178 function pointers and rate settings for this delay timer. 179 180 This is available on some architectures like OpenRISC or ARM.
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