linux-kernel - Re: [PATCH 02/11] perf: Do not get values from disabled counters in group format read

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Date:	Wed, 24 Oct 2012 14:14:07 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 02/11] perf: Do not get values from disabled counters in
 group format read

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
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