linux-kernel - Re: perf/tracepoint: another fuzzer generated lockup

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Date:	Sat, 9 Nov 2013 15:20:59 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Dave Jones <davej@...hat.com>
Subject: Re: perf/tracepoint: another fuzzer generated lockup

On Sat, Nov 09, 2013 at 03:10:39PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 08, 2013 at 11:36:58PM +0100, Frederic Weisbecker wrote:
> > [  237.627769] perf samples too long (3397569 > 2500), lowering kernel.perf_event_max_sample_rate to 50000
> > [  237.637124] INFO: NMI handler (perf_event_nmi_handler) took too long to run: 444.233 msecs
> > 
> > 444 msecs is huge.
> 
> Be glad your system lived to tell about it ;-) Calling printk() from NMI
> context is Russian roulette; I'm still waiting for the first report it
> actually locked up :-)

Haha, right. I dump with earlyprintk but that doesn't change the fact it passes through printk
machinery. Fortunately I haven't yet got burdened with that. Although... maybe printk plays a role
in the issue here...

> 
> That said, I'm not sure what kernel you're running, but there were some
> issues with time-keeping hereabouts, but more importantly that second
> timing includes the printk() call of the first -- so that's always going
> to be fucked.

It's a recent tip:master. So the delta debug printout is certainly buggy, meanwhile
these lockup only happen with Vince selftests, and they trigger a lot of these NMI-too-long
issues, or may be that's the other way round :)...

I'm trying to narrow down the issue, lets hope the lockup is not actually due to printk
itself.
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