From 8b2a5dac7859dd1954095fce8b6445c3ceb36ef6 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Wed, 11 Nov 2009 19:36:03 -0500 Subject: [PATCH] tracing: do not disable interrupts for trace_clock_local Disabling interrupts in trace_clock_local takes quite a performance hit to the recording of traces. Using perf top we see: ------------------------------------------------------------------------------ PerfTop: 244 irqs/sec kernel:100.0% [1000Hz cpu-clock-msecs], (all, 4 CPUs) ------------------------------------------------------------------------------ samples pcnt kernel function _______ _____ _______________ 2842.00 - 40.4% : trace_clock_local 1043.00 - 14.8% : rb_reserve_next_event 784.00 - 11.1% : ring_buffer_lock_reserve 600.00 - 8.5% : __rb_reserve_next 579.00 - 8.2% : rb_end_commit 440.00 - 6.3% : ring_buffer_unlock_commit 290.00 - 4.1% : ring_buffer_producer_thread [ring_buffer_benchmark] 155.00 - 2.2% : debug_smp_processor_id 117.00 - 1.7% : trace_recursive_unlock 103.00 - 1.5% : ring_buffer_event_data 28.00 - 0.4% : do_gettimeofday 22.00 - 0.3% : _spin_unlock_irq 14.00 - 0.2% : native_read_tsc 11.00 - 0.2% : getnstimeofday Where trace_clock_local is 40% of the tracing, and the time for recording a trace according to ring_buffer_benchmark is 210ns. After converting the interrupts to preemption disabling we have from perf top: ------------------------------------------------------------------------------ PerfTop: 1084 irqs/sec kernel:99.9% [1000Hz cpu-clock-msecs], (all, 4 CPUs) ------------------------------------------------------------------------------ samples pcnt kernel function _______ _____ _______________ 1277.00 - 16.8% : native_read_tsc 1148.00 - 15.1% : rb_reserve_next_event 896.00 - 11.8% : ring_buffer_lock_reserve 688.00 - 9.1% : __rb_reserve_next 664.00 - 8.8% : rb_end_commit 563.00 - 7.4% : ring_buffer_unlock_commit 508.00 - 6.7% : _spin_unlock_irq 365.00 - 4.8% : debug_smp_processor_id 321.00 - 4.2% : trace_clock_local 303.00 - 4.0% : ring_buffer_producer_thread [ring_buffer_benchmark] 273.00 - 3.6% : native_sched_clock 122.00 - 1.6% : trace_recursive_unlock 113.00 - 1.5% : sched_clock 101.00 - 1.3% : ring_buffer_event_data 53.00 - 0.7% : tick_nohz_stop_sched_tick Where trace_clock_local drops from 40% to only taking 4% of the total time. The trace time also goes from 210ns down to 179ns (31ns). I talked with Peter Zijlstra about the impact that sched_clock may have without having interrupts disabled, and he told me that if a timer interrupt comes in, sched_clock may report a wrong time. Balancing a seldom incorrect timestamp with a 15% performance boost, I'll take the performance boost. Acked-by: Peter Zijlstra Signed-off-by: Steven Rostedt --- kernel/trace/trace_clock.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c index 20c5f92e28a8..878c03f386ba 100644 --- a/kernel/trace/trace_clock.c +++ b/kernel/trace/trace_clock.c @@ -20,6 +20,8 @@ #include #include +#include "trace.h" + /* * trace_clock_local(): the simplest and least coherent tracing clock. * @@ -28,17 +30,17 @@ */ u64 notrace trace_clock_local(void) { - unsigned long flags; u64 clock; + int resched; /* * sched_clock() is an architecture implemented, fast, scalable, * lockless clock. It is not guaranteed to be coherent across * CPUs, nor across CPU idle events. */ - raw_local_irq_save(flags); + resched = ftrace_preempt_disable(); clock = sched_clock(); - raw_local_irq_restore(flags); + ftrace_preempt_enable(resched); return clock; } -- 2.34.1