From: Jiri Olsa <jolsa@redhat.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: linux-kernel@vger.kernel.org,
Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
Ingo Molnar <mingo@elte.hu>, Paul Mackerras <paulus@samba.org>,
Corey Ashford <cjashfor@linux.vnet.ibm.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Namhyung Kim <namhyung@kernel.org>
Subject: Re: [PATCH 02/11] perf: Do not get values from disabled counters in group format read
Date: Wed, 24 Oct 2012 14:14:07 +0200 [thread overview]
Message-ID: <20121024121406.GA5582@krava.brq.redhat.com> (raw)
In-Reply-To: <1351080078.13456.60.camel@twins>
On Wed, Oct 24, 2012 at 02:01:18PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-10-23 at 18:50 +0200, Jiri Olsa wrote:
> > On Tue, Oct 23, 2012 at 06:13:09PM +0200, Peter Zijlstra wrote:
> > > On Sat, 2012-10-20 at 16:33 +0200, Jiri Olsa wrote:
> > > > It's possible some of the counters in the group could be
> > > > disabled when sampling member of the event group is reading
> > > > the rest via PERF_SAMPLE_READ sample type processing. Disabled
> > > > counters could then produce wrong numbers.
> > > >
> > > > Fixing that by reading only enabled counters for PERF_SAMPLE_READ
> > > > sample type processing.
> > > >
> > >
> > > However did you run into this?
> >
> > yep, with perf record -a
> >
> > hm, I just checked and we enable/disable event groups atomicaly..
> > I haven't checked that before because it seemed obvious :-/
> >
> > So, I'm not sure now about the exact code path that triggered it
> > in my test.. however you could always disable child event from
> > group and hit this issue, but thats not what happened in perf.
> >
> > might be some other bug... I'll check
>
> Right, so I don't object to the patch per-se, I was just curious how you
> ran into it, because ISTR what you just said, we enable all this stuff
> together.
>
> Also, why would disabled counters give strange values? They'd simply
> return the same old value time after time, right?
well, x86_pmu_read calls x86_perf_event_update, which expects the event
is active.. if it's not it'll update the count from whatever left in
event.hw.idx counter.. could be uninitialized or used by others..
I can easily reproduce this one, so let's see.. ;)
jirka
next prev parent reply other threads:[~2012-10-24 12:14 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-20 14:33 [PATCH 00/11] perf tool: Add PERF_SAMPLE_READ sample read support Jiri Olsa
2012-10-20 14:33 ` [PATCH 01/11] perf: Add PERF_EVENT_IOC_ID ioctl to return event ID Jiri Olsa
2012-10-20 14:33 ` [PATCH 02/11] perf: Do not get values from disabled counters in group format read Jiri Olsa
2012-10-23 16:13 ` Peter Zijlstra
2012-10-23 16:50 ` Jiri Olsa
2012-10-24 12:01 ` Peter Zijlstra
2012-10-24 12:14 ` Jiri Olsa [this message]
2012-10-24 12:32 ` Peter Zijlstra
2012-10-24 16:03 ` Jiri Olsa
2012-10-20 14:33 ` [PATCH 03/11] perf tool: Use PERF_EVENT_IOC_ID perf ioctl to read event id Jiri Olsa
2012-10-22 7:57 ` Namhyung Kim
2012-10-22 8:40 ` Jiri Olsa
2012-10-20 14:33 ` [PATCH 04/11] perf tool: Add support for parsing PERF_SAMPLE_READ sample type Jiri Olsa
2012-10-22 8:56 ` Namhyung Kim
2012-10-20 14:33 ` [PATCH 05/11] perf tool: Fix event ID retrieval for group format read case Jiri Olsa
2012-10-20 14:33 ` [PATCH 06/11] perf tool: Add perf_evlist__id2sid function to get event ID related data Jiri Olsa
2012-10-20 14:33 ` [PATCH 07/11] perf tool: Add PERF_SAMPLE_READ sample related processing Jiri Olsa
2012-10-20 14:33 ` [PATCH 08/11] perf tool: Add 'S' event/group modifier to read sample value Jiri Olsa
2012-10-20 14:33 ` [PATCH 09/11] perf test: Add parse events tests for leader sampling Jiri Olsa
2012-10-22 8:58 ` Namhyung Kim
2012-10-22 9:12 ` Jiri Olsa
2012-10-20 14:33 ` [PATCH 10/11] perf tool: Display period values for all group members Jiri Olsa
2012-10-20 14:33 ` [PATCH 11/11] perf record: Fix mmap error output condition Jiri Olsa
2012-10-30 12:11 ` [tip:perf/core] " tip-bot for Jiri Olsa
2012-10-21 16:38 ` [PATCH 00/11] perf tool: Add PERF_SAMPLE_READ sample read support Ingo Molnar
2012-10-22 8:09 ` Jiri Olsa
2012-10-22 8:51 ` Namhyung Kim
2012-10-22 9:15 ` Jiri Olsa
2012-10-22 7:32 ` Namhyung Kim
2012-10-22 7:53 ` Jiri Olsa
2012-10-22 8:53 ` Namhyung Kim
2012-10-22 9:06 ` Jiri Olsa
2012-10-26 1:29 ` Namhyung Kim
2012-10-26 9:14 ` Peter Zijlstra
2012-10-26 10:23 ` Jiri Olsa
2012-10-26 15:39 ` Namhyung Kim
2012-10-26 16:14 ` David Ahern
2012-10-26 16:25 ` Namhyung Kim
2012-10-26 16:47 ` David Ahern
2012-10-26 17:00 ` Arnaldo Carvalho de Melo
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