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