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