1 /* 2 * tracing clocks 3 * 4 * Copyright (C) 2009 Red Hat, Inc., Ingo Molnar <[email protected]> 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/hardirq.h> 17 #include <linux/module.h> 18 #include <linux/percpu.h> 19 #include <linux/sched.h> 20 #include <linux/ktime.h> 21 22 /* 23 * trace_clock_local(): the simplest and least coherent tracing clock. 24 * 25 * Useful for tracing that does not cross to other CPUs nor 26 * does it go through idle events. 27 */ 28 u64 notrace trace_clock_local(void) 29 { 30 unsigned long flags; 31 u64 clock; 32 33 /* 34 * sched_clock() is an architecture implemented, fast, scalable, 35 * lockless clock. It is not guaranteed to be coherent across 36 * CPUs, nor across CPU idle events. 37 */ 38 raw_local_irq_save(flags); 39 clock = sched_clock(); 40 raw_local_irq_restore(flags); 41 42 return clock; 43 } 44 45 /* 46 * trace_clock(): 'inbetween' trace clock. Not completely serialized, 47 * but not completely incorrect when crossing CPUs either. 48 * 49 * This is based on cpu_clock(), which will allow at most ~1 jiffy of 50 * jitter between CPUs. So it's a pretty scalable clock, but there 51 * can be offsets in the trace data. 52 */ 53 u64 notrace trace_clock(void) 54 { 55 return cpu_clock(raw_smp_processor_id()); 56 } 57 58 59 /* 60 * trace_clock_global(): special globally coherent trace clock 61 * 62 * It has higher overhead than the other trace clocks but is still 63 * an order of magnitude faster than GTOD derived hardware clocks. 64 * 65 * Used by plugins that need globally coherent timestamps. 66 */ 67 68 static u64 prev_trace_clock_time; 69 70 static raw_spinlock_t trace_clock_lock ____cacheline_aligned_in_smp = 71 (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED; 72 73 u64 notrace trace_clock_global(void) 74 { 75 unsigned long flags; 76 int this_cpu; 77 u64 now; 78 79 raw_local_irq_save(flags); 80 81 this_cpu = raw_smp_processor_id(); 82 now = cpu_clock(this_cpu); 83 /* 84 * If in an NMI context then dont risk lockups and return the 85 * cpu_clock() time: 86 */ 87 if (unlikely(in_nmi())) 88 goto out; 89 90 __raw_spin_lock(&trace_clock_lock); 91 92 /* 93 * TODO: if this happens often then maybe we should reset 94 * my_scd->clock to prev_trace_clock_time+1, to make sure 95 * we start ticking with the local clock from now on? 96 */ 97 if ((s64)(now - prev_trace_clock_time) < 0) 98 now = prev_trace_clock_time + 1; 99 100 prev_trace_clock_time = now; 101 102 __raw_spin_unlock(&trace_clock_lock); 103 104 out: 105 raw_local_irq_restore(flags); 106 107 return now; 108 } 109