powerpc/perf: power_pmu_start restores incorrect values, breaking frequency events

perf on POWER stopped working after commit e050e3f0a71b (perf: Fix
broken interrupt rate throttling). That patch exposed a bug in
the POWER perf_events code.

Since the PMCs count upwards and take an exception when the top bit
is set, we want to write 0x80000000 - left in power_pmu_start. We were
instead programming in left which effectively disables the counter
until we eventually hit 0x80000000. This could take seconds or longer.

With the patch applied I get the expected number of samples:

          SAMPLE events:       9948

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

1 files changed, 7 insertions, 1 deletions

diff --git a/arch/powerpc/kernel/perf_event.c b/arch/powerpc/kernel/perf_event.c
index 10a140f82cb8..64483fde95c6 100644
--- a/arch/powerpc/kernel/perf_event.c
+++ b/arch/powerpc/kernel/perf_event.c
@@ -865,6 +865,7 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
 {
         unsigned long flags;
         s64 left;
+        unsigned long val;
 
         if (!event->hw.idx || !event->hw.sample_period)
                 return;
@@ -880,7 +881,12 @@ static void power_pmu_start(struct perf_event *event, int ef_flags)
 
         event->hw.state = 0;
         left = local64_read(&event->hw.period_left);
-        write_pmc(event->hw.idx, left);
+
+        val = 0;
+        if (left < 0x80000000L)
+                val = 0x80000000L - left;
+
+        write_pmc(event->hw.idx, val);
 
         perf_event_update_userpage(event);
         perf_pmu_enable(event->pmu);