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 prev_time */ 119 if ((s64)(now - prev_time) < 0) 120 now = prev_time + 1; 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 + 1; 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_add_return(1, &trace_counter); 158 } 159