Messages in this thread Patch in this message | | | Subject | [PATCH 5/4] perf_events: Fix sample_period transfer on inherit | From | Peter Zijlstra <> | Date | Fri, 29 Jan 2010 09:04:26 +0100 |
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Subject: perf_events: Fix sample_period transfer on inherit From: Peter Zijlstra <a.p.zijlstra@chello.nl> Date: Thu Jan 28 12:34:07 CET 2010
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> --- kernel/perf_event.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
Index: linux-2.6/kernel/perf_event.c =================================================================== --- linux-2.6.orig/kernel/perf_event.c +++ linux-2.6/kernel/perf_event.c @@ -5011,8 +5011,15 @@ inherit_event(struct perf_event *parent_ 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;
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