The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kernel/sched_clock.c

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    1 /*
    2  * sched_clock for unstable cpu clocks
    3  *
    4  *  Copyright (C) 2008 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
    5  *
    6  *  Updates and enhancements:
    7  *    Copyright (C) 2008 Red Hat, Inc. Steven Rostedt <srostedt@redhat.com>
    8  *
    9  * Based on code by:
   10  *   Ingo Molnar <mingo@redhat.com>
   11  *   Guillaume Chazarain <guichaz@gmail.com>
   12  *
   13  *
   14  * What:
   15  *
   16  * cpu_clock(i) provides a fast (execution time) high resolution
   17  * clock with bounded drift between CPUs. The value of cpu_clock(i)
   18  * is monotonic for constant i. The timestamp returned is in nanoseconds.
   19  *
   20  * ######################### BIG FAT WARNING ##########################
   21  * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
   22  * # go backwards !!                                                  #
   23  * ####################################################################
   24  *
   25  * There is no strict promise about the base, although it tends to start
   26  * at 0 on boot (but people really shouldn't rely on that).
   27  *
   28  * cpu_clock(i)       -- can be used from any context, including NMI.
   29  * sched_clock_cpu(i) -- must be used with local IRQs disabled (implied by NMI)
   30  * local_clock()      -- is cpu_clock() on the current cpu.
   31  *
   32  * How:
   33  *
   34  * The implementation either uses sched_clock() when
   35  * !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK, which means in that case the
   36  * sched_clock() is assumed to provide these properties (mostly it means
   37  * the architecture provides a globally synchronized highres time source).
   38  *
   39  * Otherwise it tries to create a semi stable clock from a mixture of other
   40  * clocks, including:
   41  *
   42  *  - GTOD (clock monotomic)
   43  *  - sched_clock()
   44  *  - explicit idle events
   45  *
   46  * We use GTOD as base and use sched_clock() deltas to improve resolution. The
   47  * deltas are filtered to provide monotonicity and keeping it within an
   48  * expected window.
   49  *
   50  * Furthermore, explicit sleep and wakeup hooks allow us to account for time
   51  * that is otherwise invisible (TSC gets stopped).
   52  *
   53  *
   54  * Notes:
   55  *
   56  * The !IRQ-safetly of sched_clock() and sched_clock_cpu() comes from things
   57  * like cpufreq interrupts that can change the base clock (TSC) multiplier
   58  * and cause funny jumps in time -- although the filtering provided by
   59  * sched_clock_cpu() should mitigate serious artifacts we cannot rely on it
   60  * in general since for !CONFIG_HAVE_UNSTABLE_SCHED_CLOCK we fully rely on
   61  * sched_clock().
   62  */
   63 #include <linux/spinlock.h>
   64 #include <linux/hardirq.h>
   65 #include <linux/module.h>
   66 #include <linux/percpu.h>
   67 #include <linux/ktime.h>
   68 #include <linux/sched.h>
   69 
   70 /*
   71  * Scheduler clock - returns current time in nanosec units.
   72  * This is default implementation.
   73  * Architectures and sub-architectures can override this.
   74  */
   75 unsigned long long __attribute__((weak)) sched_clock(void)
   76 {
   77         return (unsigned long long)(jiffies - INITIAL_JIFFIES)
   78                                         * (NSEC_PER_SEC / HZ);
   79 }
   80 EXPORT_SYMBOL_GPL(sched_clock);
   81 
   82 __read_mostly int sched_clock_running;
   83 
   84 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
   85 __read_mostly int sched_clock_stable;
   86 
   87 struct sched_clock_data {
   88         u64                     tick_raw;
   89         u64                     tick_gtod;
   90         u64                     clock;
   91 };
   92 
   93 static DEFINE_PER_CPU_SHARED_ALIGNED(struct sched_clock_data, sched_clock_data);
   94 
   95 static inline struct sched_clock_data *this_scd(void)
   96 {
   97         return &__get_cpu_var(sched_clock_data);
   98 }
   99 
  100 static inline struct sched_clock_data *cpu_sdc(int cpu)
  101 {
  102         return &per_cpu(sched_clock_data, cpu);
  103 }
  104 
  105 void sched_clock_init(void)
  106 {
  107         u64 ktime_now = ktime_to_ns(ktime_get());
  108         int cpu;
  109 
  110         for_each_possible_cpu(cpu) {
  111                 struct sched_clock_data *scd = cpu_sdc(cpu);
  112 
  113                 scd->tick_raw = 0;
  114                 scd->tick_gtod = ktime_now;
  115                 scd->clock = ktime_now;
  116         }
  117 
  118         sched_clock_running = 1;
  119 }
  120 
  121 /*
  122  * min, max except they take wrapping into account
  123  */
  124 
  125 static inline u64 wrap_min(u64 x, u64 y)
  126 {
  127         return (s64)(x - y) < 0 ? x : y;
  128 }
  129 
  130 static inline u64 wrap_max(u64 x, u64 y)
  131 {
  132         return (s64)(x - y) > 0 ? x : y;
  133 }
  134 
  135 /*
  136  * update the percpu scd from the raw @now value
  137  *
  138  *  - filter out backward motion
  139  *  - use the GTOD tick value to create a window to filter crazy TSC values
  140  */
  141 static u64 sched_clock_local(struct sched_clock_data *scd)
  142 {
  143         u64 now, clock, old_clock, min_clock, max_clock;
  144         s64 delta;
  145 
  146 again:
  147         now = sched_clock();
  148         delta = now - scd->tick_raw;
  149         if (unlikely(delta < 0))
  150                 delta = 0;
  151 
  152         old_clock = scd->clock;
  153 
  154         /*
  155          * scd->clock = clamp(scd->tick_gtod + delta,
  156          *                    max(scd->tick_gtod, scd->clock),
  157          *                    scd->tick_gtod + TICK_NSEC);
  158          */
  159 
  160         clock = scd->tick_gtod + delta;
  161         min_clock = wrap_max(scd->tick_gtod, old_clock);
  162         max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);
  163 
  164         clock = wrap_max(clock, min_clock);
  165         clock = wrap_min(clock, max_clock);
  166 
  167         if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
  168                 goto again;
  169 
  170         return clock;
  171 }
  172 
  173 static u64 sched_clock_remote(struct sched_clock_data *scd)
  174 {
  175         struct sched_clock_data *my_scd = this_scd();
  176         u64 this_clock, remote_clock;
  177         u64 *ptr, old_val, val;
  178 
  179         sched_clock_local(my_scd);
  180 again:
  181         this_clock = my_scd->clock;
  182         remote_clock = scd->clock;
  183 
  184         /*
  185          * Use the opportunity that we have both locks
  186          * taken to couple the two clocks: we take the
  187          * larger time as the latest time for both
  188          * runqueues. (this creates monotonic movement)
  189          */
  190         if (likely((s64)(remote_clock - this_clock) < 0)) {
  191                 ptr = &scd->clock;
  192                 old_val = remote_clock;
  193                 val = this_clock;
  194         } else {
  195                 /*
  196                  * Should be rare, but possible:
  197                  */
  198                 ptr = &my_scd->clock;
  199                 old_val = this_clock;
  200                 val = remote_clock;
  201         }
  202 
  203         if (cmpxchg64(ptr, old_val, val) != old_val)
  204                 goto again;
  205 
  206         return val;
  207 }
  208 
  209 /*
  210  * Similar to cpu_clock(), but requires local IRQs to be disabled.
  211  *
  212  * See cpu_clock().
  213  */
  214 u64 sched_clock_cpu(int cpu)
  215 {
  216         struct sched_clock_data *scd;
  217         u64 clock;
  218 
  219         WARN_ON_ONCE(!irqs_disabled());
  220 
  221         if (sched_clock_stable)
  222                 return sched_clock();
  223 
  224         if (unlikely(!sched_clock_running))
  225                 return 0ull;
  226 
  227         scd = cpu_sdc(cpu);
  228 
  229         if (cpu != smp_processor_id())
  230                 clock = sched_clock_remote(scd);
  231         else
  232                 clock = sched_clock_local(scd);
  233 
  234         return clock;
  235 }
  236 
  237 void sched_clock_tick(void)
  238 {
  239         struct sched_clock_data *scd;
  240         u64 now, now_gtod;
  241 
  242         if (sched_clock_stable)
  243                 return;
  244 
  245         if (unlikely(!sched_clock_running))
  246                 return;
  247 
  248         WARN_ON_ONCE(!irqs_disabled());
  249 
  250         scd = this_scd();
  251         now_gtod = ktime_to_ns(ktime_get());
  252         now = sched_clock();
  253 
  254         scd->tick_raw = now;
  255         scd->tick_gtod = now_gtod;
  256         sched_clock_local(scd);
  257 }
  258 
  259 /*
  260  * We are going deep-idle (irqs are disabled):
  261  */
  262 void sched_clock_idle_sleep_event(void)
  263 {
  264         sched_clock_cpu(smp_processor_id());
  265 }
  266 EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
  267 
  268 /*
  269  * We just idled delta nanoseconds (called with irqs disabled):
  270  */
  271 void sched_clock_idle_wakeup_event(u64 delta_ns)
  272 {
  273         if (timekeeping_suspended)
  274                 return;
  275 
  276         sched_clock_tick();
  277         touch_softlockup_watchdog();
  278 }
  279 EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
  280 
  281 /*
  282  * As outlined at the top, provides a fast, high resolution, nanosecond
  283  * time source that is monotonic per cpu argument and has bounded drift
  284  * between cpus.
  285  *
  286  * ######################### BIG FAT WARNING ##########################
  287  * # when comparing cpu_clock(i) to cpu_clock(j) for i != j, time can #
  288  * # go backwards !!                                                  #
  289  * ####################################################################
  290  */
  291 u64 cpu_clock(int cpu)
  292 {
  293         u64 clock;
  294         unsigned long flags;
  295 
  296         local_irq_save(flags);
  297         clock = sched_clock_cpu(cpu);
  298         local_irq_restore(flags);
  299 
  300         return clock;
  301 }
  302 
  303 /*
  304  * Similar to cpu_clock() for the current cpu. Time will only be observed
  305  * to be monotonic if care is taken to only compare timestampt taken on the
  306  * same CPU.
  307  *
  308  * See cpu_clock().
  309  */
  310 u64 local_clock(void)
  311 {
  312         u64 clock;
  313         unsigned long flags;
  314 
  315         local_irq_save(flags);
  316         clock = sched_clock_cpu(smp_processor_id());
  317         local_irq_restore(flags);
  318 
  319         return clock;
  320 }
  321 
  322 #else /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
  323 
  324 void sched_clock_init(void)
  325 {
  326         sched_clock_running = 1;
  327 }
  328 
  329 u64 sched_clock_cpu(int cpu)
  330 {
  331         if (unlikely(!sched_clock_running))
  332                 return 0;
  333 
  334         return sched_clock();
  335 }
  336 
  337 u64 cpu_clock(int cpu)
  338 {
  339         return sched_clock_cpu(cpu);
  340 }
  341 
  342 u64 local_clock(void)
  343 {
  344         return sched_clock_cpu(0);
  345 }
  346 
  347 #endif /* CONFIG_HAVE_UNSTABLE_SCHED_CLOCK */
  348 
  349 EXPORT_SYMBOL_GPL(cpu_clock);
  350 EXPORT_SYMBOL_GPL(local_clock);

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