linux-kernel - Re: 2.6.37 kernel warning in perf

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Message-ID: <AANLkTi=H9+rKjgkmfbS2jzfPjryah0cc9sajSgN8WOaQ@mail.gmail.com>
Date:	Thu, 10 Feb 2011 12:46:07 -0800
From:	Arun Sharma <arun@...rma-home.net>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: Re: 2.6.37 kernel warning in perf_events code

On Thu, Feb 10, 2011 at 12:20 PM, Arnaldo Carvalho de Melo
<acme@...hat.com> wrote:
>
>> >> perf record -g -p <pid> cs -o csw.data -- sleep 3
>
> Arun, are you shure the above line is right? I guess it should read:
>
>
> perf record -g -p <pid> -e cs -o csw.data -- sleep 3
>
> To specify the context switches soft event, right?
>

You caught a cut and paste error. I'm pretty sure I had the -e in
there when the warning triggered. I tried this command a few times,
just to verify and here's what I found:

* Under low loads, everything works fine.
* Under a heavy work load - I'm not able to reproduce the warning, but
hitting very similar symptoms:

[ perf record: Captured and wrote 2.282 MB /tmp/junk.data (~99721 samples) ]
[ perf record: Captured and wrote 1.734 MB /tmp/junk.data (~75740 samples) ]
[ perf record: Captured and wrote 0.091 MB /tmp/junk.data (~3975
samples) ]  <--- bad run

The bad run made my shell unresponsive and took around 30-40 seconds
to complete (whereas the good runs completed in less than 5 secs).
Could this be some kind of a feedback loop where what the measurement
machinery is perturbing what's being measured?

 -Arun
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