Commit f5602941 authored by Anton Blanchard's avatar Anton Blanchard Committed by Benjamin Herrenschmidt
Browse files

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: <>
Signed-off-by: default avatarAnton Blanchard <>
Signed-off-by: default avatarBenjamin Herrenschmidt <>
parent fb43e847
......@@ -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);
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