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}
45EXPORT_SYMBOL_GPL(trace_clock_local);
46
47/*
48 * trace_clock(): 'between' trace clock. Not completely serialized,
49 * but not completely incorrect when crossing CPUs either.
50 *
51 * This is based on cpu_clock(), which will allow at most ~1 jiffy of
52 * jitter between CPUs. So it's a pretty scalable clock, but there
53 * can be offsets in the trace data.
54 */
55u64 notrace trace_clock(void)
56{
57	return local_clock();
58}
59
60/*
61 * trace_jiffy_clock(): Simply use jiffies as a clock counter.
62 * Note that this use of jiffies_64 is not completely safe on
63 * 32-bit systems. But the window is tiny, and the effect if
64 * we are affected is that we will have an obviously bogus
65 * timestamp on a trace event - i.e. not life threatening.
66 */
67u64 notrace trace_clock_jiffies(void)
68{
69	return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
70}
71
72/*
73 * trace_clock_global(): special globally coherent trace clock
74 *
75 * It has higher overhead than the other trace clocks but is still
76 * an order of magnitude faster than GTOD derived hardware clocks.
77 *
78 * Used by plugins that need globally coherent timestamps.
79 */
80
81/* keep prev_time and lock in the same cacheline. */
82static struct {
83	u64 prev_time;
84	arch_spinlock_t lock;
85} trace_clock_struct ____cacheline_aligned_in_smp =
86	{
87		.lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
88	};
89
90u64 notrace trace_clock_global(void)
91{
92	unsigned long flags;
93	int this_cpu;
94	u64 now;
95
96	local_irq_save(flags);
97
98	this_cpu = raw_smp_processor_id();
99	now = sched_clock_cpu(this_cpu);
100	/*
101	 * If in an NMI context then dont risk lockups and return the
102	 * cpu_clock() time:
103	 */
104	if (unlikely(in_nmi()))
105		goto out;
106
107	arch_spin_lock(&trace_clock_struct.lock);
108
109	/*
110	 * TODO: if this happens often then maybe we should reset
111	 * my_scd->clock to prev_time+1, to make sure
112	 * we start ticking with the local clock from now on?
113	 */
114	if ((s64)(now - trace_clock_struct.prev_time) < 0)
115		now = trace_clock_struct.prev_time + 1;
116
117	trace_clock_struct.prev_time = now;
118
119	arch_spin_unlock(&trace_clock_struct.lock);
120
121 out:
122	local_irq_restore(flags);
123
124	return now;
125}
126
127static atomic64_t trace_counter;
128
129/*
130 * trace_clock_counter(): simply an atomic counter.
131 * Use the trace_counter "counter" for cases where you do not care
132 * about timings, but are interested in strict ordering.
133 */
134u64 notrace trace_clock_counter(void)
135{
136	return atomic64_add_return(1, &trace_counter);
137}
138