Commit f5602941 authored May 29, 2014 by Anton Blanchard Committed by Benjamin Herrenschmidt Jul 11, 2014

powerpc/perf: Never program book3s PMCs with values >= 0x80000000



We are seeing a lot of PMU warnings on POWER8:

    Can't find PMC that caused IRQ

Looking closer, the active PMC is 0 at this point and we took a PMU
exception on the transition from negative to 0. Some versions of POWER8
have an issue where they edge detect and not level detect PMC overflows.

A number of places program the PMC with (0x80000000 - period_left),
where period_left can be negative. We can either fix all of these or
just ensure that period_left is always >= 1.

This patch takes the second option.

Cc: <stable@vger.kernel.org>
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

parent fb43e847

arch/powerpc/perf/core-book3s.c

+16 −1

Original line number	Diff line number	Diff line
		@@ -996,7 +996,22 @@ static void power_pmu_read(struct perf_event *event)
		} while (local64_cmpxchg(&event->hw.prev_count, prev, val) != prev);

		local64_add(delta, &event->count);
		local64_sub(delta, &event->hw.period_left);

		/*
		* A number of places program the PMC with (0x80000000 - period_left).
		* We never want period_left to be less than 1 because we will program
		* the PMC with a value >= 0x800000000 and an edge detected PMC will
		* roll around to 0 before taking an exception. We have seen this
		* on POWER8.
		*
		* To fix this, clamp the minimum value of period_left to 1.
		*/
		do {
		prev = local64_read(&event->hw.period_left);
		val = prev - delta;
		if (val < 1)
		val = 1;
		} while (local64_cmpxchg(&event->hw.period_left, prev, val) != prev);
		}

		/*