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perf_events: Fix sample_period transfer on inherit

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One problem with frequency driven counters is that we cannot
predict the rate at which they trigger, therefore we have to
start them at period=1, this causes a ramp up effect. However,
if we fail to propagate the stable state on fork each new child
will have to ramp up again. This can lead to significant
artifacts in sample data.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: eranian@google.com
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1264752266.4283.2121.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2010-01-29 09:04:26 +01:00 committed by Ingo Molnar
parent 18c01f8abf
commit 75c9f3284a
1 changed files with 9 additions and 2 deletions
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11
kernel/perf_event.c
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@ -5002,8 +5002,15 @@ inherit_event(struct perf_event *parent_event,
else
child_event->state = PERF_EVENT_STATE_OFF;
if (parent_event->attr.freq)
child_event->hw.sample_period = parent_event->hw.sample_period;
if (parent_event->attr.freq) {
u64 sample_period = parent_event->hw.sample_period;
struct hw_perf_event *hwc = &child_event->hw;
hwc->sample_period = sample_period;
hwc->last_period = sample_period;
atomic64_set(&hwc->period_left, sample_period);
}
child_event->overflow_handler = parent_event->overflow_handler;