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 /* 25 * trace_clock_local(): the simplest and least coherent tracing clock. 26 * 27 * Useful for tracing that does not cross to other CPUs nor 28 * does it go through idle events. 29 */ 30 u64 notrace trace_clock_local(void) 31 { 32 u64 clock; 33 34 /* 35 * sched_clock() is an architecture implemented, fast, scalable, 36 * lockless clock. It is not guaranteed to be coherent across 37 * CPUs, nor across CPU idle events. 38 */ 39 preempt_disable_notrace(); 40 clock = sched_clock(); 41 preempt_enable_notrace(); 42 43 return clock; 44 } 45 EXPORT_SYMBOL_GPL(trace_clock_local); 46 47 /* 48 * trace_clock(): 'between' trace clock. Not completely serialized, 49 * but not completely incorrect when crossing CPUs either. 50 * 51 * This is based on cpu_clock(), which will allow at most ~1 jiffy of 52 * jitter between CPUs. So it's a pretty scalable clock, but there 53 * can be offsets in the trace data. 54 */ 55 u64 notrace trace_clock(void) 56 { 57 return local_clock(); 58 } 59 60 61 /* 62 * trace_clock_global(): special globally coherent trace clock 63 * 64 * It has higher overhead than the other trace clocks but is still 65 * an order of magnitude faster than GTOD derived hardware clocks. 66 * 67 * Used by plugins that need globally coherent timestamps. 68 */ 69 70 /* keep prev_time and lock in the same cacheline. */ 71 static struct { 72 u64 prev_time; 73 arch_spinlock_t lock; 74 } trace_clock_struct ____cacheline_aligned_in_smp = 75 { 76 .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED, 77 }; 78 79 u64 notrace trace_clock_global(void) 80 { 81 unsigned long flags; 82 int this_cpu; 83 u64 now; 84 85 local_irq_save(flags); 86 87 this_cpu = raw_smp_processor_id(); 88 now = sched_clock_cpu(this_cpu); 89 /* 90 * If in an NMI context then dont risk lockups and return the 91 * cpu_clock() time: 92 */ 93 if (unlikely(in_nmi())) 94 goto out; 95 96 arch_spin_lock(&trace_clock_struct.lock); 97 98 /* 99 * TODO: if this happens often then maybe we should reset 100 * my_scd->clock to prev_time+1, to make sure 101 * we start ticking with the local clock from now on? 102 */ 103 if ((s64)(now - trace_clock_struct.prev_time) < 0) 104 now = trace_clock_struct.prev_time + 1; 105 106 trace_clock_struct.prev_time = now; 107 108 arch_spin_unlock(&trace_clock_struct.lock); 109 110 out: 111 local_irq_restore(flags); 112 113 return now; 114 } 115 116 static atomic64_t trace_counter; 117 118 /* 119 * trace_clock_counter(): simply an atomic counter. 120 * Use the trace_counter "counter" for cases where you do not care 121 * about timings, but are interested in strict ordering. 122 */ 123 u64 notrace trace_clock_counter(void) 124 { 125 return atomic64_add_return(1, &trace_counter); 126 } 127