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/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/sched/clock.h>
  22 #include <linux/ktime.h>
  23 #include <linux/trace_clock.h>
  24
  25 /*
  26  * trace_clock_local(): the simplest and least coherent tracing clock.
  27  *
  28  * Useful for tracing that does not cross to other CPUs nor
  29  * does it go through idle events.
  30  */
  31 u64 notrace trace_clock_local(void)
  32 {
  33         u64 clock;
  34
  35         /*
  36          * sched_clock() is an architecture implemented, fast, scalable,
  37          * lockless clock. It is not guaranteed to be coherent across
  38          * CPUs, nor across CPU idle events.
  39          */
  40         preempt_disable_notrace();
  41         clock = sched_clock();
  42         preempt_enable_notrace();
  43
  44         return clock;
  45 }
  46 EXPORT_SYMBOL_GPL(trace_clock_local);
  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  */
  56 u64 notrace trace_clock(void)
  57 {
  58         return local_clock();
  59 }
  60 EXPORT_SYMBOL_GPL(trace_clock);
  61
  62 /*
  63  * trace_jiffy_clock(): Simply use jiffies as a clock counter.
  64  * Note that this use of jiffies_64 is not completely safe on
  65  * 32-bit systems. But the window is tiny, and the effect if
  66  * we are affected is that we will have an obviously bogus
  67  * timestamp on a trace event - i.e. not life threatening.
  68  */
  69 u64 notrace trace_clock_jiffies(void)
  70 {
  71         return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
  72 }
  73 EXPORT_SYMBOL_GPL(trace_clock_jiffies);
  74
  75 /*
  76  * trace_clock_global(): special globally coherent trace clock
  77  *
  78  * It has higher overhead than the other trace clocks but is still
  79  * an order of magnitude faster than GTOD derived hardware clocks.
  80  *
  81  * Used by plugins that need globally coherent timestamps.
  82  */
  83
  84 /* keep prev_time and lock in the same cacheline. */
  85 static struct {
  86         u64 prev_time;
  87         arch_spinlock_t lock;
  88 } trace_clock_struct ____cacheline_aligned_in_smp =
  89         {
  90                 .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
  91         };
  92
  93 u64 notrace trace_clock_global(void)
  94 {
  95         unsigned long flags;
  96         int this_cpu;
  97         u64 now;
  98
  99         local_irq_save(flags);
 100
 101         this_cpu = raw_smp_processor_id();
 102         now = sched_clock_cpu(this_cpu);
 103         /*
 104          * If in an NMI context then dont risk lockups and return the
 105          * cpu_clock() time:
 106          */
 107         if (unlikely(in_nmi()))
 108                 goto out;
 109
 110         arch_spin_lock(&trace_clock_struct.lock);
 111
 112         /*
 113          * TODO: if this happens often then maybe we should reset
 114          * my_scd->clock to prev_time+1, to make sure
 115          * we start ticking with the local clock from now on?
 116          */
 117         if ((s64)(now - trace_clock_struct.prev_time) < 0)
 118                 now = trace_clock_struct.prev_time + 1;
 119
 120         trace_clock_struct.prev_time = now;
 121
 122         arch_spin_unlock(&trace_clock_struct.lock);
 123
 124  out:
 125         local_irq_restore(flags);
 126
 127         return now;
 128 }
 129 EXPORT_SYMBOL_GPL(trace_clock_global);
 130
 131 static atomic64_t trace_counter;
 132
 133 /*
 134  * trace_clock_counter(): simply an atomic counter.
 135  * Use the trace_counter "counter" for cases where you do not care
 136  * about timings, but are interested in strict ordering.
 137  */
 138 u64 notrace trace_clock_counter(void)
 139 {
 140         return atomic64_add_return(1, &trace_counter);
 141 }