LXR linux/kernel/trace/trace

   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