linux-kernel - Re: x86_pmu_start WARN

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Message-ID: <87r46xdbw4.fsf@tassilo.jf.intel.com>
Date:	Thu, 20 Feb 2014 07:47:23 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: x86_pmu_start WARN_ON.

Peter Zijlstra <peterz@...radead.org> writes:

> On Wed, Feb 19, 2014 at 05:34:49PM -0500, Vince Weaver wrote:
>> So where would the NMI counter event get disabled?  Would it never get 
>> disabled, just because it's always running and always gets the same fixed 
>> slot?  Why isn't this a problem all the time, not just with corner cases?
>
> Well it could get another counter assignment I suppose; there's no
> guarantee it always ends up in the fixed counter although that's rather
> likely.
>
>> Is somehow n_added getting confused?
>
> I'd expect worse than a warning if that happened, but who knows.
>
> You could try with the below; make sure you've got CONFIG_FUNCTION_TRACER=y.
>
> Then (assuming debugfs is mounted in /debug ; most modern distros have
> it at /sys/kernel/debug but they're wrong!):
>
> # echo 0 > /debug/tracing/tracing_on
> # echo function > /debug/tracing/current_tracer
> # echo nop > /debug/tracing/current_tracer
> # echo 0 > /debug/tracing/trace
> # echo 1 > /debug/tracing/tracing_on
>
> And make it go *BOOM*, then:
>
> # cat /debug/tracing/trace | bzip2 -9 > ~/trace.bz2

This won't work unless you apply this patch

http://lkml.org/lkml/2013/10/20/94

-Andi


-- 
ak@...ux.intel.com -- Speaking for myself only
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