From 813312110bede27bffd082c25cd31730bd567beb Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Date: Tue, 7 Aug 2012 15:07:19 +0000
Subject: [PATCH] powerpc/perf: Use pmc_overflow() to detect rolled back events
For certain speculative events on Power7, 'perf stat' reports far higher
event count than 'perf record' for the same event.
As described in following commit, a performance monitor exception is raised
even when the the performance events are rolled back.
commit 0837e3242c73566fc1c0196b4ec61779c25ffc93
Author: Anton Blanchard <anton@samba.org>
Date: Wed Mar 9 14:38:42 2011 +1100
perf_event_interrupt() records an event only when an overflow occurs. But
this check for overflow is a simple 'if (val < 0)'.
Because the events are rolled back, this check for overflow fails and the
event is not recorded. perf_event_interrupt() later uses pmc_overflow() to
detect the overflow and resets the counters and the events are lost completely.
To properly detect the overflow of rolled back events, use pmc_overflow()
even when recording events.
To reproduce:
$ cat strcpy.c
#include <stdio.h>
#include <string.h>
main()
{
char buf[256];
alarm(5);
while(1)
strcpy(buf, "string1");
}
$ perf record -e r20014 ./strcpy
$ perf report -n > report.1
$ perf stat -e r20014 > report.2
# Compare report.1 and report.2
Reported-by: Maynard Johnson <mpjohn@us.ibm.com>
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
arch/powerpc/perf/core-book3s.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 77b49ddda9d3..7cd2dbd6e4c4 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -1431,7 +1431,7 @@ static void perf_event_interrupt(struct pt_regs *regs)
if (!event->hw.idx || is_limited_pmc(event->hw.idx))
continue;
val = read_pmc(event->hw.idx);
- if ((int)val < 0) {
+ if (pmc_overflow(val)) {
/* event has overflowed */
found = 1;
record_and_restart(event, val, regs);