trace_clock.c revision 25985edcedea6396277003854657b5f3cb31a628 
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/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 */
32u64 notrace trace_clock_local(void)
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	 */
41	preempt_disable_notrace();
42	clock = sched_clock();
43	preempt_enable_notrace();
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 */
56u64 notrace trace_clock(void)
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. */
72static struct {
73	u64 prev_time;
74	arch_spinlock_t lock;
75} trace_clock_struct ____cacheline_aligned_in_smp =
76	{
77		.lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
78	};
79
80u64 notrace trace_clock_global(void)
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