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perf_event: Fix broken calc_timer_values() 

We detected a serious issue with PERF_SAMPLE_READ and
timing information when events were being multiplexing.

Samples would have time_running > time_enabled. That
was easy to reproduce with a libpfm4 example (ran 3
times to cause multiplexing on Core 2):

 $ syst_smpl -e uops_retired:freq=1 &
 $ syst_smpl -e uops_retired:freq=1 &
 $ syst_smpl -e uops_retired:freq=1 &
 IIP:0x0000000040062d ... PERIOD:2355332948 ENA=40144625315 RUN=60014875184
 syst_smpl: WARNING: time_running > time_enabled
	63277537998 uops_retired:freq=1 , scaled

The bug was not present in kernel up to (and including) 3.0. It turns
out the bug was introduced by the following commit:

commit c479429591

    events: Move lockless timer calculation into helper function

The parameters of the function got reversed yet the call sites
were not updated to reflect the change. That lead to time_running
and time_enabled being swapped. That had no effect when there was
no multiplexing because in that case time_running = time_enabled
but it would show up in any other scenario.

Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20110829124112.GA4828@quad
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Eric B Munson 2011-06-23 16:34:38 -04:00 committed by Ingo Molnar
parent a8d757ef07
commit 7f310a5d4e
1 changed files with 2 additions and 2 deletions
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4
kernel/events/core.c
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@ -3396,8 +3396,8 @@ static int perf_event_index(struct perf_event *event)
}
static void calc_timer_values(struct perf_event *event,
u64 *running,
u64 *enabled)
u64 *enabled,
u64 *running)
{
u64 now, ctx_time;