trace_clock.c revision 5e67b51e3fb22ad43faf9589e9019ad9c6a00413 
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/*
25 * trace_clock_local(): the simplest and least coherent tracing clock.
26 *
27 * Useful for tracing that does not cross to other CPUs nor
28 * does it go through idle events.
29 */
30u64 notrace trace_clock_local(void)
31{
32	u64 clock;
33
34	/*
35	 * sched_clock() is an architecture implemented, fast, scalable,
36	 * lockless clock. It is not guaranteed to be coherent across
37	 * CPUs, nor across CPU idle events.
38	 */
39	preempt_disable_notrace();
40	clock = sched_clock();
41	preempt_enable_notrace();
42
43	return clock;
44}
45
46/*
47 * trace_clock(): 'between' trace clock. Not completely serialized,
48 * but not completely incorrect when crossing CPUs either.
49 *
50 * This is based on cpu_clock(), which will allow at most ~1 jiffy of
51 * jitter between CPUs. So it's a pretty scalable clock, but there
52 * can be offsets in the trace data.
53 */
54u64 notrace trace_clock(void)
55{
56	return local_clock();
57}
58
59
60/*
61 * trace_clock_global(): special globally coherent trace clock
62 *
63 * It has higher overhead than the other trace clocks but is still
64 * an order of magnitude faster than GTOD derived hardware clocks.
65 *
66 * Used by plugins that need globally coherent timestamps.
67 */
68
69/* keep prev_time and lock in the same cacheline. */
70static struct {
71	u64 prev_time;
72	arch_spinlock_t lock;
73} trace_clock_struct ____cacheline_aligned_in_smp =
74	{
75		.lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
76	};
77
78u64 notrace trace_clock_global(void)
79{
80	unsigned long flags;
81	int this_cpu;
82	u64 now;
83
84	local_irq_save(flags);
85
86	this_cpu = raw_smp_processor_id();
87	now = sched_clock_cpu(this_cpu);
88	/*
89	 * If in an NMI context then dont risk lockups and return the
90	 * cpu_clock() time:
91	 */
92	if (unlikely(in_nmi()))
93		goto out;
94
95	arch_spin_lock(&trace_clock_struct.lock);
96
97	/*
98	 * TODO: if this happens often then maybe we should reset
99	 * my_scd->clock to prev_time+1, to make sure
100	 * we start ticking with the local clock from now on?
101	 */
102	if ((s64)(now - trace_clock_struct.prev_time) < 0)
103		now = trace_clock_struct.prev_time + 1;
104
105	trace_clock_struct.prev_time = now;
106
107	arch_spin_unlock(&trace_clock_struct.lock);
108
109 out:
110	local_irq_restore(flags);
111
112	return now;
113}
114
115static atomic64_t trace_counter;
116
117/*
118 * trace_clock_counter(): simply an atomic counter.
119 * Use the trace_counter "counter" for cases where you do not care
120 * about timings, but are interested in strict ordering.
121 */
122u64 notrace trace_clock_counter(void)
123{
124	return atomic64_add_return(1, &trace_counter);
125}
126