linux/kernel/trace/trace_clock.c

// SPDX-License-Identifier: GPL-2.0
/*
 * tracing clocks
 *
 *  Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <[email protected]>
 *
 * Implements 3 trace clock variants, with differing scalability/precision
 * tradeoffs:
 *
 *  -   local: CPU-local trace clock
 *  -  medium: scalable global clock with some jitter
 *  -  global: globally monotonic, serialized clock
 *
 * Tracer plugins will chose a default from these clocks.
 */
#include <linux/spinlock.h>
#include <linux/irqflags.h>
#include <linux/hardirq.h>
#include <linux/module.h>
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/ktime.h>
#include <linux/trace_clock.h>

/*
 * trace_clock_local(): the simplest and least coherent tracing clock.
 *
 * Useful for tracing that does not cross to other CPUs nor
 * does it go through idle events.
 */
u64 notrace trace_clock_local(void)
{ … }
EXPORT_SYMBOL_GPL(…);

/*
 * trace_clock(): 'between' trace clock. Not completely serialized,
 * but not completely incorrect when crossing CPUs either.
 *
 * This is based on cpu_clock(), which will allow at most ~1 jiffy of
 * jitter between CPUs. So it's a pretty scalable clock, but there
 * can be offsets in the trace data.
 */
u64 notrace trace_clock(void)
{ … }
EXPORT_SYMBOL_GPL(…);

/*
 * trace_jiffy_clock(): Simply use jiffies as a clock counter.
 * Note that this use of jiffies_64 is not completely safe on
 * 32-bit systems. But the window is tiny, and the effect if
 * we are affected is that we will have an obviously bogus
 * timestamp on a trace event - i.e. not life threatening.
 */
u64 notrace trace_clock_jiffies(void)
{ … }
EXPORT_SYMBOL_GPL(…);

/*
 * trace_clock_global(): special globally coherent trace clock
 *
 * It has higher overhead than the other trace clocks but is still
 * an order of magnitude faster than GTOD derived hardware clocks.
 *
 * Used by plugins that need globally coherent timestamps.
 */

/* keep prev_time and lock in the same cacheline. */
static struct { … } trace_clock_struct ____cacheline_aligned_in_smp = …;

u64 notrace trace_clock_global(void)
{ … }
EXPORT_SYMBOL_GPL(…);

static atomic64_t trace_counter;

/*
 * trace_clock_counter(): simply an atomic counter.
 * Use the trace_counter "counter" for cases where you do not care
 * about timings, but are interested in strict ordering.
 */
u64 notrace trace_clock_counter(void)
{ … }