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powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

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perf on POWER stopped working after commit e050e3f0a7 (perf: Fix
broken interrupt rate throttling). That patch exposed a bug in
the POWER perf_events code.

Since the PMCs count upwards and take an exception when the top bit
is set, we want to write 0x80000000 - left in power_pmu_start. We were
instead programming in left which effectively disables the counter
until we eventually hit 0x80000000. This could take seconds or longer.

With the patch applied I get the expected number of samples:

          SAMPLE events:       9948

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>
This commit is contained in:
Anton Blanchard 2012-02-15 18:48:22 +00:00 committed by Benjamin Herrenschmidt
parent 64f8c13561
commit 9a45a9407c
1 changed files with 7 additions and 1 deletions
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8
arch/powerpc/kernel/perf_event.c
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@ -865,6 +865,7 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
{
unsigned long flags;
s64 left;
unsigned long val;
if (!event->hw.idx || !event->hw.sample_period)
return;
@ -880,7 +881,12 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
event->hw.state = 0;
left = local64_read(&event->hw.period_left);
write_pmc(event->hw.idx, left);
val = 0;
if (left < 0x80000000L)
val = 0x80000000L - left;
write_pmc(event->hw.idx, val);
perf_event_update_userpage(event);
perf_pmu_enable(event->pmu);