kernel/trace/trace_clock.c

   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/hardirq.h>
  17 #include <linux/module.h>
  18 #include <linux/percpu.h>
  19 #include <linux/sched.h>
  20 #include <linux/ktime.h>
  21 #include <linux/trace_clock.h>
  22
  23 /*
  24  * trace_clock_local(): the simplest and least coherent tracing clock.
  25  *
  26  * Useful for tracing that does not cross to other CPUs nor
  27  * does it go through idle events.
  28  */
  29 u64 notrace trace_clock_local(void)
  30 {
  31         unsigned long flags;
  32         u64 clock;
  33
  34         /*
  35          * sched_clock() is an architecture implemented, fast, scalable,
  36          * lockless clock. It is not guaranteed to be coherent across
  37          * CPUs, nor across CPU idle events.
  38          */
  39         raw_local_irq_save(flags);
  40         clock = sched_clock();
  41         raw_local_irq_restore(flags);
  42
  43         return clock;
  44 }
  45
  46 /*
  47  * trace_clock(): 'inbetween' trace clock. Not completely serialized,
  48  * but not completely incorrect when crossing CPUs either.
  49  *
  50  * This is based on cpu_clock(), which will allow at most ~1 jiffy of
  51  * jitter between CPUs. So it's a pretty scalable clock, but there
  52  * can be offsets in the trace data.
  53  */
  54 u64 notrace trace_clock(void)
  55 {
  56         return cpu_clock(raw_smp_processor_id());
  57 }
  58
  59
  60 /*
  61  * trace_clock_global(): special globally coherent trace clock
  62  *
  63  * It has higher overhead than the other trace clocks but is still
  64  * an order of magnitude faster than GTOD derived hardware clocks.
  65  *
  66  * Used by plugins that need globally coherent timestamps.
  67  */
  68
  69 static u64 prev_trace_clock_time;
  70
  71 static raw_spinlock_t trace_clock_lock ____cacheline_aligned_in_smp =
  72         (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
  73
  74 u64 notrace trace_clock_global(void)
  75 {
  76         unsigned long flags;
  77         int this_cpu;
  78         u64 now;
  79
  80         raw_local_irq_save(flags);
  81
  82         this_cpu = raw_smp_processor_id();
  83         now = cpu_clock(this_cpu);
  84         /*
  85          * If in an NMI context then dont risk lockups and return the
  86          * cpu_clock() time:
  87          */
  88         if (unlikely(in_nmi()))
  89                 goto out;
  90
  91         __raw_spin_lock(&trace_clock_lock);
  92
  93         /*
  94          * TODO: if this happens often then maybe we should reset
  95          * my_scd->clock to prev_trace_clock_time+1, to make sure
  96          * we start ticking with the local clock from now on?
  97          */
  98         if ((s64)(now - prev_trace_clock_time) < 0)
  99                 now = prev_trace_clock_time + 1;
 100
 101         prev_trace_clock_time = now;
 102
 103         __raw_spin_unlock(&trace_clock_lock);
 104
 105  out:
 106         raw_local_irq_restore(flags);
 107
 108         return now;
 109 }