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Subject[PATCH 3.11 121/128] powerpc/perf: Never program book3s PMCs with values >= 0x80000000
Date
3.11.10.14 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Anton Blanchard <anton@samba.org>

commit f56029410a13cae3652d1f34788045c40a13ffc7 upstream.

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.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
arch/powerpc/perf/core-book3s.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 8b9b9ac6005d..c05f10e95a9e 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -851,7 +851,22 @@ 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);
}

/*
--
1.9.1


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