1 /* 2 * tracing clocks 3 * 4 * Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <mingo@redhat.com> 5 * 6 * Implements 3 trace clock variants, with differing scalability/precision 7 * tradeoffs: 8 * 9 * - local: CPU-local trace clock 10 * - medium: scalable global clock with some jitter 11 * - global: globally monotonic, serialized clock 12 * 13 * Tracer plugins will chose a default from these clocks. 14 */ 15 #include <linux/spinlock.h> 16 #include <linux/irqflags.h> 17 #include <linux/hardirq.h> 18 #include <linux/module.h> 19 #include <linux/percpu.h> 20 #include <linux/sched.h> 21 #include <linux/ktime.h> 22 #include <linux/trace_clock.h> 23 24 #include "trace.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 int resched; 36 37 /* 38 * sched_clock() is an architecture implemented, fast, scalable, 39 * lockless clock. It is not guaranteed to be coherent across 40 * CPUs, nor across CPU idle events. 41 */ 42 resched = ftrace_preempt_disable(); 43 clock = sched_clock(); 44 ftrace_preempt_enable(resched); 45 46 return clock; 47 } 48 49 /* 50 * trace_clock(): 'inbetween' 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 cpu_clock(raw_smp_processor_id()); 60 } 61 62 63 /* 64 * trace_clock_global(): special globally coherent trace clock 65 * 66 * It has higher overhead than the other trace clocks but is still 67 * an order of magnitude faster than GTOD derived hardware clocks. 68 * 69 * Used by plugins that need globally coherent timestamps. 70 */ 71 72 /* keep prev_time and lock in the same cacheline. */ 73 static struct { 74 u64 prev_time; 75 arch_spinlock_t lock; 76 } trace_clock_struct ____cacheline_aligned_in_smp = 77 { 78 .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED, 79 }; 80 81 u64 notrace trace_clock_global(void) 82 { 83 unsigned long flags; 84 int this_cpu; 85 u64 now; 86 87 local_irq_save(flags); 88 89 this_cpu = raw_smp_processor_id(); 90 now = cpu_clock(this_cpu); 91 /* 92 * If in an NMI context then dont risk lockups and return the 93 * cpu_clock() time: 94 */ 95 if (unlikely(in_nmi())) 96 goto out; 97 98 arch_spin_lock(&trace_clock_struct.lock); 99 100 /* 101 * TODO: if this happens often then maybe we should reset 102 * my_scd->clock to prev_time+1, to make sure 103 * we start ticking with the local clock from now on? 104 */ 105 if ((s64)(now - trace_clock_struct.prev_time) < 0) 106 now = trace_clock_struct.prev_time + 1; 107 108 trace_clock_struct.prev_time = now; 109 110 arch_spin_unlock(&trace_clock_struct.lock); 111 112 out: 113 local_irq_restore(flags); 114 115 return now; 116 } 117