linux/kernel/trace/trace_clock.c
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   1/*
   2 * tracing clocks
   3 *
   4 *  Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
   5 *
   6 * Implements 3 trace clock variants, with differing scalability/precision
   7 * tradeoffs:
   8 *
   9 *  -   local: CPU-local trace clock
  10 *  -  medium: scalable global clock with some jitter
  11 *  -  global: globally monotonic, serialized clock
  12 *
  13 * Tracer plugins will chose a default from these clocks.
  14 */
  15#include <linux/spinlock.h>
  16#include <linux/irqflags.h>
  17#include <linux/hardirq.h>
  18#include <linux/module.h>
  19#include <linux/percpu.h>
  20#include <linux/sched.h>
  21#include <linux/ktime.h>
  22#include <linux/trace_clock.h>
  23
  24#include "trace.h"
  25
  26/*
  27 * trace_clock_local(): the simplest and least coherent tracing clock.
  28 *
  29 * Useful for tracing that does not cross to other CPUs nor
  30 * does it go through idle events.
  31 */
  32u64 notrace trace_clock_local(void)
  33{
  34        u64 clock;
  35
  36        /*
  37         * sched_clock() is an architecture implemented, fast, scalable,
  38         * lockless clock. It is not guaranteed to be coherent across
  39         * CPUs, nor across CPU idle events.
  40         */
  41        preempt_disable_notrace();
  42        clock = sched_clock();
  43        preempt_enable_notrace();
  44
  45        return clock;
  46}
  47
  48/*
  49 * trace_clock(): 'between' trace clock. Not completely serialized,
  50 * but not completely incorrect when crossing CPUs either.
  51 *
  52 * This is based on cpu_clock(), which will allow at most ~1 jiffy of
  53 * jitter between CPUs. So it's a pretty scalable clock, but there
  54 * can be offsets in the trace data.
  55 */
  56u64 notrace trace_clock(void)
  57{
  58        return local_clock();
  59}
  60
  61
  62/*
  63 * trace_clock_global(): special globally coherent trace clock
  64 *
  65 * It has higher overhead than the other trace clocks but is still
  66 * an order of magnitude faster than GTOD derived hardware clocks.
  67 *
  68 * Used by plugins that need globally coherent timestamps.
  69 */
  70
  71/* keep prev_time and lock in the same cacheline. */
  72static struct {
  73        u64 prev_time;
  74        arch_spinlock_t lock;
  75} trace_clock_struct ____cacheline_aligned_in_smp =
  76        {
  77                .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
  78        };
  79
  80u64 notrace trace_clock_global(void)
  81{
  82        unsigned long flags;
  83        int this_cpu;
  84        u64 now;
  85
  86        local_irq_save(flags);
  87
  88        this_cpu = raw_smp_processor_id();
  89        now = cpu_clock(this_cpu);
  90        /*
  91         * If in an NMI context then dont risk lockups and return the
  92         * cpu_clock() time:
  93         */
  94        if (unlikely(in_nmi()))
  95                goto out;
  96
  97        arch_spin_lock(&trace_clock_struct.lock);
  98
  99        /*
 100         * TODO: if this happens often then maybe we should reset
 101         * my_scd->clock to prev_time+1, to make sure
 102         * we start ticking with the local clock from now on?
 103         */
 104        if ((s64)(now - trace_clock_struct.prev_time) < 0)
 105                now = trace_clock_struct.prev_time + 1;
 106
 107        trace_clock_struct.prev_time = now;
 108
 109        arch_spin_unlock(&trace_clock_struct.lock);
 110
 111 out:
 112        local_irq_restore(flags);
 113
 114        return now;
 115}
 116
 117static atomic64_t trace_counter;
 118
 119/*
 120 * trace_clock_counter(): simply an atomic counter.
 121 * Use the trace_counter "counter" for cases where you do not care
 122 * about timings, but are interested in strict ordering.
 123 */
 124u64 notrace trace_clock_counter(void)
 125{
 126        return atomic64_add_return(1, &trace_counter);
 127}
 128
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