1// SPDX-License-Identifier: GPL-2.0
2/*
3 * tracing clocks
4 *
5 *  Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
6 *
7 * Implements 3 trace clock variants, with differing scalability/precision
8 * tradeoffs:
9 *
10 *  -   local: CPU-local trace clock
11 *  -  medium: scalable global clock with some jitter
12 *  -  global: globally monotonic, serialized clock
13 *
14 * Tracer plugins will chose a default from these clocks.
15 */
16#include <linux/spinlock.h>
17#include <linux/irqflags.h>
18#include <linux/hardirq.h>
19#include <linux/module.h>
20#include <linux/percpu.h>
21#include <linux/sched.h>
22#include <linux/sched/clock.h>
23#include <linux/ktime.h>
24#include <linux/trace_clock.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}
47EXPORT_SYMBOL_GPL(trace_clock_local);
48
49/*
50 * trace_clock(): 'between' trace clock. Not completely serialized,
51 * but not completely incorrect when crossing CPUs either.
52 *
53 * This is based on cpu_clock(), which will allow at most ~1 jiffy of
54 * jitter between CPUs. So it's a pretty scalable clock, but there
55 * can be offsets in the trace data.
56 */
57u64 notrace trace_clock(void)
58{
59	return local_clock();
60}
61EXPORT_SYMBOL_GPL(trace_clock);
62
63/*
64 * trace_jiffy_clock(): Simply use jiffies as a clock counter.
65 * Note that this use of jiffies_64 is not completely safe on
66 * 32-bit systems. But the window is tiny, and the effect if
67 * we are affected is that we will have an obviously bogus
68 * timestamp on a trace event - i.e. not life threatening.
69 */
70u64 notrace trace_clock_jiffies(void)
71{
72	return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
73}
74EXPORT_SYMBOL_GPL(trace_clock_jiffies);
75
76/*
77 * trace_clock_global(): special globally coherent trace clock
78 *
79 * It has higher overhead than the other trace clocks but is still
80 * an order of magnitude faster than GTOD derived hardware clocks.
81 *
82 * Used by plugins that need globally coherent timestamps.
83 */
84
85/* keep prev_time and lock in the same cacheline. */
86static struct {
87	u64 prev_time;
88	arch_spinlock_t lock;
89} trace_clock_struct ____cacheline_aligned_in_smp =
90	{
91		.lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED,
92	};
93
94u64 notrace trace_clock_global(void)
95{
96	unsigned long flags;
97	int this_cpu;
98	u64 now, prev_time;
99
100	raw_local_irq_save(flags);
101
102	this_cpu = raw_smp_processor_id();
103
104	/*
105	 * The global clock "guarantees" that the events are ordered
106	 * between CPUs. But if two events on two different CPUS call
107	 * trace_clock_global at roughly the same time, it really does
108	 * not matter which one gets the earlier time. Just make sure
109	 * that the same CPU will always show a monotonic clock.
110	 *
111	 * Use a read memory barrier to get the latest written
112	 * time that was recorded.
113	 */
114	smp_rmb();
115	prev_time = READ_ONCE(trace_clock_struct.prev_time);
116	now = sched_clock_cpu(this_cpu);
117
118	/* Make sure that now is always greater than or equal to prev_time */
119	if ((s64)(now - prev_time) < 0)
120		now = prev_time;
121
122	/*
123	 * If in an NMI context then dont risk lockups and simply return
124	 * the current time.
125	 */
126	if (unlikely(in_nmi()))
127		goto out;
128
129	/* Tracing can cause strange recursion, always use a try lock */
130	if (arch_spin_trylock(&trace_clock_struct.lock)) {
131		/* Reread prev_time in case it was already updated */
132		prev_time = READ_ONCE(trace_clock_struct.prev_time);
133		if ((s64)(now - prev_time) < 0)
134			now = prev_time;
135
136		trace_clock_struct.prev_time = now;
137
138		/* The unlock acts as the wmb for the above rmb */
139		arch_spin_unlock(&trace_clock_struct.lock);
140	}
141 out:
142	raw_local_irq_restore(flags);
143
144	return now;
145}
146EXPORT_SYMBOL_GPL(trace_clock_global);
147
148static atomic64_t trace_counter;
149
150/*
151 * trace_clock_counter(): simply an atomic counter.
152 * Use the trace_counter "counter" for cases where you do not care
153 * about timings, but are interested in strict ordering.
154 */
155u64 notrace trace_clock_counter(void)
156{
157	return atomic64_add_return(1, &trace_counter);
158}
159