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Subject[PATCH 15/15] perf_counter: minimize context time updates
Push the update_context_time() calls up the stack so that we get less
invokations and thereby a less noisy output:

before:

# ./perfstat -e 1:0 -e 1:1 -e 1:1 -e 1:1 -l ls > /dev/null
Performance counter stats for 'ls':

10.163691 cpu clock ticks (msecs) (scaled from 98.94%)
10.215360 task clock ticks (msecs) (scaled from 98.18%)
10.185549 task clock ticks (msecs) (scaled from 98.53%)
10.183581 task clock ticks (msecs) (scaled from 98.71%)

Wall-clock time elapsed: 11.912858 msecs

after:

# ./perfstat -e 1:0 -e 1:1 -e 1:1 -e 1:1 -l ls > /dev/null
Performance counter stats for 'ls':

9.316630 cpu clock ticks (msecs)
9.280789 task clock ticks (msecs)
9.280789 task clock ticks (msecs)
9.280789 task clock ticks (msecs)

Wall-clock time elapsed: 9.574872 msecs

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
kernel/perf_counter.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
Index: linux-2.6/kernel/perf_counter.c
===================================================================
--- linux-2.6.orig/kernel/perf_counter.c
+++ linux-2.6/kernel/perf_counter.c
@@ -319,6 +319,8 @@ static void __perf_counter_disable(void

spin_lock_irqsave(&ctx->lock, flags);

+ update_context_time(ctx);
+
/*
* If the counter is on, turn it off.
* If it is in error state, leave it in error state.
@@ -797,6 +799,8 @@ void perf_counter_task_sched_out(struct
if (likely(!cpuctx->task_ctx))
return;

+ update_context_time(ctx);
+
regs = task_pt_regs(task);
perf_swcounter_event(PERF_COUNT_CONTEXT_SWITCHES, 1, 1, regs);
__perf_counter_sched_out(ctx, cpuctx);
@@ -2336,7 +2340,6 @@ static void task_clock_perf_counter_upda
u64 prev, now;
s64 delta;

- update_context_time(counter->ctx);
now = counter->ctx->time;

prev = atomic64_xchg(&counter->hw.prev_count, now);
@@ -2349,7 +2352,6 @@ static int task_clock_perf_counter_enabl
struct hw_perf_counter *hwc = &counter->hw;
u64 now;

- update_context_time(counter->ctx);
now = counter->ctx->time;

atomic64_set(&hwc->prev_count, now);
@@ -2372,6 +2374,7 @@ static void task_clock_perf_counter_disa

static void task_clock_perf_counter_read(struct perf_counter *counter)
{
+ update_context_time(counter->ctx);
task_clock_perf_counter_update(counter);
}

--



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