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Date: Tue, 26 Apr 2011 10:04:43 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Paul Mackerras <paulus@samba.org>, Pekka Enberg <penberg@kernel.org>,
        Vegard Nossum <vegardno@ifi.uio.no>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [BUG] perf and kmemcheck : fatal combination
Message-ID: <20110426080443.GA806@elte.hu>
References: <1303747731.2747.182.camel@edumazet-laptop>
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* Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> On Mon, 2011-04-25 at 18:08 +0200, Eric Dumazet wrote:
> > Hi guys
> > 
> > Just got a panic on a kmemcheck kernel, latest linux-2.6 tree.
> > 
> > I forgot I had kmemcheck enabled, and started "perf top" just because my
> > machine was damn slow... Oh well...
> > 
> > Crash in do_nmi -> nmi_enter() -> BUG_ON(in_nmi());
> 
> Hmm,. I bet because kmemcheck triggers faults from nmi context because
> it messes about with the page protection bits a lot to track things.
> 
> Can't really think of anything except not making perf available on
> kmemcheck kernels.
> 
> ---
>  init/Kconfig |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 32745bf..94735b4 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1125,6 +1125,7 @@ config PERF_EVENTS
>  	bool "Kernel performance events and counters"
>  	default y if (PROFILING || PERF_COUNTERS)
>  	depends on HAVE_PERF_EVENTS
> +	depends on !KMEMCHECK
>  	select ANON_INODES
>  	select IRQ_WORK
>  	help

Eric, does it manage to limp along if you remove the BUG_ON()?

That risks NMI recursion but maybe it allows you to see why things are slow, 
before it crashes ;-)

Thanks,

	Ingo