perf on POWER stopped working after commit e050e3f0a71b (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:
# taskset -c 0 yes > /dev/null &
# perf record -C 0 -a sleep 10
# perf report -D | grep SAMPLE | tail -1
SAMPLE events: 9948
Signed-off-by: Anton Blanchard <an...@samba.org>
---
Cc: <sta...@kernel.org>
Index: linux-build/arch/powerpc/kernel/perf_event.c
===================================================================
--- linux-build.orig/arch/powerpc/kernel/perf_event.c 2012-02-16
15:07:57.465384699 +1100
+++ linux-build/arch/powerpc/kernel/perf_event.c 2012-02-16
15:11:48.449579581 +1100
@@ -865,6 +865,7 @@ static void power_pmu_start(struct perf_
{
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->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);
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