This change “perf/x86: Fix filter_events() bug with event mappings” in Linux kernel is authored by Stephane Eranian <eranian [at] google.com> on Mon Dec 7 20:33:25 2015 +0100.
perf/x86: Fix filter_events() bug with event mappings
This patch fixes a bug in the filter_events() function.
The patch 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 in /sys/devices/cpu/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easily on any system post SNB (which do not publish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 indexes 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 the wrong index. This patch corrects this problem
by compensating for the deleted events and with that ref-cycles
reappears (here shown on Haswell):
$ perf stat -e ref-cycles,cycles true
Performance counter stats for 'true':
4,525,910 ref-cycles
1,064,920 cycles
0.002943888 seconds time elapsed
Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: jolsa@kernel.org
Cc: kan.liang@intel.com
Fixes: 8300daa26755 ("perf/x86: Filter out undefined events from sysfs events attribute")
Link: http://lkml.kernel.org/r/1449516805-6637-1-git-send-email-eranian@google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 61b87ca.
There are 11 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.
arch/x86/kernel/cpu/perf_event.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index e7e63a9..1b443db 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++) {
@@ -1542,7 +1543,7 @@ 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++)
@@ -1550,6 +1551,14 @@ static void __init filter_events(struct attribute **attrs)
/* 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++;
}
}
The commit for this change in Linux stable tree is 61b87ca (patch).