linux-kernel - [PATCH] perf/x86: fix filter_events() bug with event mappings

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Date:	Sun,  6 Dec 2015 22:35:06 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	linux-kernel@...r.kernel.org
Cc:	acme@...hat.com, peterz@...radead.org, mingo@...e.hu,
	ak@...ux.intel.com, kan.liang@...el.com, jolsa@...nel.org
Subject: [PATCH] perf/x86: fix filter_events() bug with event mappings

This patch fixes a two bugs in the filter_events() function.

The first issue is that when shifting (compressing) the array
left, we need to also propagate the NULL termination,
otherwise we will have the same rightmost shifted event twice.

The second issue fixes the bug whereby if some mappings did not
exist, e.g., STALLED_CYCLES_FRONTEND, then any event after it
in the attrs array would disappear from the published list of
events. This could be verified easily on any system post
SNB (which do not publish STALLED_CYCLES_FRONTEND):

	$ ./perf stat -e cycles,ref-cycles true
 	Performance counter stats for 'true':
              1,217,348      cycles
	<not supported>      ref-cycles

The problem is that in filter_events() there is an assumption
that the argument (attrs) is organized in increasing continuous
event index related to the event_map(). But if we remove the
non-supported events by shifing the position in the array, then
the lookup x86_pmu.event_map() needs to compensate for it, otherwise
we are looking up at the wrong index. This patch corrects this problem
by compensating for the deleted events and with that ref-cycles reappears.

This problem was introduced into commit 8300daa26.

Fixes: 8300daa26 ("perf/x86: Filter out undefined events from sysfs events attribute")

Patch is relative to tip.git commit 03b6b9f.

Signed-off-by: Stephane Eranian <eranian@...gle.com>
---
 arch/x86/kernel/cpu/perf_event.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index e7e63a9..70e16c7 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1534,6 +1534,7 @@ static void __init filter_events(struct attribute **attrs)
 {
 	struct device_attribute *d;
 	struct perf_pmu_events_attr *pmu_attr;
+	int offset = 0;
 	int i, j;

 	for (i = 0; attrs[i]; i++) {
@@ -1542,14 +1543,23 @@ static void __init filter_events(struct attribute **attrs)
 		/* str trumps id */
 		if (pmu_attr->event_str)
 			continue;
-		if (x86_pmu.event_map(i))
+		if (x86_pmu.event_map(i + offset))
 			continue;

 		for (j = i; attrs[j]; j++)
 			attrs[j] = attrs[j + 1];
+		attrs[j] = NULL;

 		/* Check the shifted attr. */
 		i--;
+
+		/*
+		 * event_map() is index based, the attrs array is organized
+		 * by increasing event index. If we shift the events, then
+		 * we need to compensate for the event_map(), otherwise
+		 * we are looking up the wrong event in the map
+		 */
+		offset++;
 	}
 }

-- 
2.5.0

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