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 EXPORT_SYMBOL_GPL(trace_clock); 60 61 /* 62 * trace_jiffy_clock(): Simply use jiffies as a clock counter. 63 * Note that this use of jiffies_64 is not completely safe on 64 * 32-bit systems. But the window is tiny, and the effect if 65 * we are affected is that we will have an obviously bogus 66 * timestamp on a trace event - i.e. not life threatening. 67 */ 68 u64 notrace trace_clock_jiffies(void) 69 { 70 return jiffies_64_to_clock_t(jiffies_64 - INITIAL_JIFFIES); 71 } 72 EXPORT_SYMBOL_GPL(trace_clock_jiffies); 73 74 /* 75 * trace_clock_global(): special globally coherent trace clock 76 * 77 * It has higher overhead than the other trace clocks but is still 78 * an order of magnitude faster than GTOD derived hardware clocks. 79 * 80 * Used by plugins that need globally coherent timestamps. 81 */ 82 83 /* keep prev_time and lock in the same cacheline. */ 84 static struct { 85 u64 prev_time; 86 arch_spinlock_t lock; 87 } trace_clock_struct ____cacheline_aligned_in_smp = 88 { 89 .lock = (arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED, 90 }; 91 92 u64 notrace trace_clock_global(void) 93 { 94 unsigned long flags; 95 int this_cpu; 96 u64 now; 97 98 local_irq_save(flags); 99 100 this_cpu = raw_smp_processor_id(); 101 now = sched_clock_cpu(this_cpu); 102 /* 103 * If in an NMI context then dont risk lockups and return the 104 * cpu_clock() time: 105 */ 106 if (unlikely(in_nmi())) 107 goto out; 108 109 arch_spin_lock(&trace_clock_struct.lock); 110 111 /* 112 * TODO: if this happens often then maybe we should reset 113 * my_scd->clock to prev_time+1, to make sure 114 * we start ticking with the local clock from now on? 115 */ 116 if ((s64)(now - trace_clock_struct.prev_time) < 0) 117 now = trace_clock_struct.prev_time + 1; 118 119 trace_clock_struct.prev_time = now; 120 121 arch_spin_unlock(&trace_clock_struct.lock); 122 123 out: 124 local_irq_restore(flags); 125 126 return now; 127 } 128 EXPORT_SYMBOL_GPL(trace_clock_global); 129 130 static atomic64_t trace_counter; 131 132 /* 133 * trace_clock_counter(): simply an atomic counter. 134 * Use the trace_counter "counter" for cases where you do not care 135 * about timings, but are interested in strict ordering. 136 */ 137 u64 notrace trace_clock_counter(void) 138 { 139 return atomic64_add_return(1, &trace_counter); 140 } 141