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>

1 files changed, 16 insertions, 1 deletions

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index bae697cd5925..6b0641c3f03f 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -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);
 }
 
 /*