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Commit 9a45a940 authored by Anton Blanchard's avatar Anton Blanchard Committed by Benjamin Herrenschmidt
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powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events 



perf on POWER stopped working after commit e050e3f0 (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: default avatarAnton Blanchard <anton@samba.org>
Acked-by: default avatarPaul Mackerras <paulus@samba.org>
Signed-off-by: default avatarBenjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>
parent 64f8c135

arch/powerpc/kernel/perf_event.c

+7 −1
Original line number Diff line number Diff line
@@ -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);