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 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 48 /* 49 * trace_clock(): 'inbetween' trace clock. Not completely serialized, 50 * but not completely incorrect when crossing CPUs either. 51 * 52 * This is based on cpu_clock(), which will allow at most ~1 jiffy of 53 * jitter between CPUs. So it's a pretty scalable clock, but there 54 * can be offsets in the trace data. 55 */ 56 u64 notrace trace_clock(void) 57 { 58 return local_clock(); 59 } 60 61 62 /* 63 * trace_clock_global(): special globally coherent trace clock 64 * 65 * It has higher overhead than the other trace clocks but is still 66 * an order of magnitude faster than GTOD derived hardware clocks. 67 * 68 * Used by plugins that need globally coherent timestamps. 69 */ 70 71 /* keep prev_time and lock in the same cacheline. */ 72 static struct { 73 u64 prev_time; 74 arch_spinlock_t lock; 75 } trace_clock_struct ____cacheline_aligned_in_smp = 76 { 77 .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED, 78 }; 79 80 u64 notrace trace_clock_global(void) 81 { 82 unsigned long flags; 83 int this_cpu; 84 u64 now; 85 86 local_irq_save(flags); 87 88 this_cpu = raw_smp_processor_id(); 89 now = cpu_clock(this_cpu); 90 /* 91 * If in an NMI context then dont risk lockups and return the 92 * cpu_clock() time: 93 */ 94 if (unlikely(in_nmi())) 95 goto out; 96 97 arch_spin_lock(&trace_clock_struct.lock); 98 99 /* 100 * TODO: if this happens often then maybe we should reset 101 * my_scd->clock to prev_time+1, to make sure 102 * we start ticking with the local clock from now on? 103 */ 104 if ((s64)(now - trace_clock_struct.prev_time) < 0) 105 now = trace_clock_struct.prev_time + 1; 106 107 trace_clock_struct.prev_time = now; 108 109 arch_spin_unlock(&trace_clock_struct.lock); 110 111 out: 112 local_irq_restore(flags); 113 114 return now; 115 } 116