Commit-ID: 8b2a5dac7859dd1954095fce8b6445c3ceb36ef6
Gitweb: http://git.kernel.org/tip/8b2a5dac7859dd1954095fce8b6445c3ceb36ef6
Author: Steven Rostedt <srostedt@xxxxxxxxxx>
AuthorDate: Wed, 11 Nov 2009 19:36:03 -0500
Committer: Steven Rostedt <rostedt@xxxxxxxxxxx>
CommitDate: Wed, 11 Nov 2009 23:38:33 -0500
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 <peterz@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
kernel/trace/trace_clock.c | 8 +++++---
1 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
index 20c5f92..878c03f 100644
--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -20,6 +20,8 @@
#include <linux/ktime.h>
#include <linux/trace_clock.h>
+#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;
}
--
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