cherry-pick 8b2a5dac7859dd
svn path=/dists/sid/linux-2.6/; revision=15419
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Maximilian Attems
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524cda7ce8
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553ecc0f41
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linux-2.6 (2.6.32-11) UNRELEASED; urgency=low
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[ Ben Hutchings ]
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* tracing: do not disable interrupts for trace_clock_local.
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(closes: #574368)
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-- maximilian attems <maks@debian.org> Wed, 17 Mar 2010 18:48:22 +0100
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linux-2.6 (2.6.32-10) unstable; urgency=low
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* The "Big Bang" release
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110
debian/patches/bugfix/all/tracing-do-not-disable-interrupts-for-trace_clock_lo.patch
vendored
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110
debian/patches/bugfix/all/tracing-do-not-disable-interrupts-for-trace_clock_lo.patch
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From 8b2a5dac7859dd1954095fce8b6445c3ceb36ef6 Mon Sep 17 00:00:00 2001
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From: Steven Rostedt <srostedt@redhat.com>
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Date: Wed, 11 Nov 2009 19:36:03 -0500
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Subject: [PATCH] tracing: do not disable interrupts for trace_clock_local
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Disabling interrupts in trace_clock_local takes quite a performance
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hit to the recording of traces. Using perf top we see:
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------------------------------------------------------------------------------
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PerfTop: 244 irqs/sec kernel:100.0% [1000Hz cpu-clock-msecs], (all, 4 CPUs)
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------------------------------------------------------------------------------
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samples pcnt kernel function
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_______ _____ _______________
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2842.00 - 40.4% : trace_clock_local
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1043.00 - 14.8% : rb_reserve_next_event
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784.00 - 11.1% : ring_buffer_lock_reserve
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600.00 - 8.5% : __rb_reserve_next
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579.00 - 8.2% : rb_end_commit
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440.00 - 6.3% : ring_buffer_unlock_commit
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290.00 - 4.1% : ring_buffer_producer_thread [ring_buffer_benchmark]
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155.00 - 2.2% : debug_smp_processor_id
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117.00 - 1.7% : trace_recursive_unlock
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103.00 - 1.5% : ring_buffer_event_data
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28.00 - 0.4% : do_gettimeofday
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22.00 - 0.3% : _spin_unlock_irq
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14.00 - 0.2% : native_read_tsc
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11.00 - 0.2% : getnstimeofday
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Where trace_clock_local is 40% of the tracing, and the time for recording
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a trace according to ring_buffer_benchmark is 210ns. After converting
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the interrupts to preemption disabling we have from perf top:
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------------------------------------------------------------------------------
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PerfTop: 1084 irqs/sec kernel:99.9% [1000Hz cpu-clock-msecs], (all, 4 CPUs)
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------------------------------------------------------------------------------
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samples pcnt kernel function
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_______ _____ _______________
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1277.00 - 16.8% : native_read_tsc
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1148.00 - 15.1% : rb_reserve_next_event
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896.00 - 11.8% : ring_buffer_lock_reserve
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688.00 - 9.1% : __rb_reserve_next
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664.00 - 8.8% : rb_end_commit
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563.00 - 7.4% : ring_buffer_unlock_commit
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508.00 - 6.7% : _spin_unlock_irq
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365.00 - 4.8% : debug_smp_processor_id
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321.00 - 4.2% : trace_clock_local
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303.00 - 4.0% : ring_buffer_producer_thread [ring_buffer_benchmark]
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273.00 - 3.6% : native_sched_clock
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122.00 - 1.6% : trace_recursive_unlock
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113.00 - 1.5% : sched_clock
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101.00 - 1.3% : ring_buffer_event_data
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53.00 - 0.7% : tick_nohz_stop_sched_tick
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Where trace_clock_local drops from 40% to only taking 4% of the total time.
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The trace time also goes from 210ns down to 179ns (31ns).
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I talked with Peter Zijlstra about the impact that sched_clock may have
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without having interrupts disabled, and he told me that if a timer interrupt
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comes in, sched_clock may report a wrong time.
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Balancing a seldom incorrect timestamp with a 15% performance boost, I'll
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take the performance boost.
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Acked-by: Peter Zijlstra <peterz@infradead.org>
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Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
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---
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kernel/trace/trace_clock.c | 8 +++++---
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1 files changed, 5 insertions(+), 3 deletions(-)
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diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
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index 20c5f92..878c03f 100644
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--- a/kernel/trace/trace_clock.c
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+++ b/kernel/trace/trace_clock.c
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@@ -20,6 +20,8 @@
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#include <linux/ktime.h>
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#include <linux/trace_clock.h>
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+#include "trace.h"
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+
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/*
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* trace_clock_local(): the simplest and least coherent tracing clock.
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*
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@@ -28,17 +30,17 @@
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*/
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u64 notrace trace_clock_local(void)
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{
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- unsigned long flags;
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u64 clock;
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+ int resched;
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/*
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* sched_clock() is an architecture implemented, fast, scalable,
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* lockless clock. It is not guaranteed to be coherent across
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* CPUs, nor across CPU idle events.
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*/
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- raw_local_irq_save(flags);
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+ resched = ftrace_preempt_disable();
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clock = sched_clock();
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- raw_local_irq_restore(flags);
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+ ftrace_preempt_enable(resched);
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return clock;
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}
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--
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1.7.0
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@ -0,0 +1 @@
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+ bugfix/all/tracing-do-not-disable-interrupts-for-trace_clock_lo.patch
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