kernel/trace/trace_clock.c

   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  */
  32 u64 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 EXPORT_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  */
  57 u64 notrace trace_clock(void)
  58 {
  59         return local_clock();
  60 }
  61 EXPORT_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  */
  70 u64 notrace trace_clock_jiffies(void)
  71 {
  72         return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES);
  73 }
  74 EXPORT_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. */
  86 static 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
  94 u64 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 }
 146 EXPORT_SYMBOL_GPL(trace_clock_global);
 147
 148 static 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  */
 155 u64 notrace trace_clock_counter(void)
 156 {
 157         return atomic64_inc_return(&trace_counter);
 158 }