[PATCH] powerpc: perf: Fix frequency calculation for overflowing counters

Anton Blanchard anton at samba.org
Mon Jan 17 16:17:42 EST 2011

When profiling a benchmark that is almost 100% userspace, I noticed some
wildly inaccurate profiles that showed almost all time spent in the kernel.
Closer examination shows we were programming a tiny number of cycles into
the PMU after each overflow (about ~200 away from the next overflow). This
gets us stuck in a loop which we eventually break out of by throttling the
PMU (there are regular throttle/unthrottle events in the log).

It looks like we aren't setting event->hw.last_period to something same
and the frequency to period calculations in perf are going haywire. With
the following patch we find the correct period after a few interrupts and
stay there. I also see no more throttle events.

Signed-off-by: Anton Blanchard <anton at samba.org>

diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c
index 5674807..ab6f6be 100644
--- a/arch/powerpc/kernel/perf_event.c
+++ b/arch/powerpc/kernel/perf_event.c
@@ -1212,6 +1212,7 @@ static void record_and_restart(struct perf_event *event, unsigned long val,
 			if (left <= 0)
 				left = period;
 			record = 1;
+			event->hw.last_period = event->hw.sample_period;
 		if (left < 0x80000000LL)
 			val = 0x80000000LL - left;

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