Linux Kernel  3.7.1
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trace_clock.c
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1 /*
2  * tracing clocks
3  *
4  * Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <[email protected]>
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  */
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  */
42  clock = sched_clock();
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  */
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. */
72 static struct {
75 } trace_clock_struct ____cacheline_aligned_in_smp =
76  {
78  };
79 
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 
117 static 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  */
125 {
126  return atomic64_add_return(1, &trace_counter);
127 }