Root/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/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

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