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

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Date:	Mon, 11 Nov 2013 22:13:45 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: perf/tracepoint: another fuzzer generated lockup


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Nov 11, 2013 at 01:44:19PM +0100, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@...il.com> wrote:
> > 
> > > > 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.
> > 
> > I'd _very_ strongly suggest to not include the printk() overhead in the 
> > execution time delta! What that function wants to report is pure NMI 
> > execution overhead, not problem reporting overhead.
> > 
> > That way any large number reported there is always a bug somewhere, 
> > somehow.
> 
> -ENOPATCH :-)
> 
> You'll find that there's two levels of measuring NMI latency and the 
> outer will invariably include the reporting of the inner one; fixing 
> that is going to be hideously ugly.
> 
> That said, I would very strongly suggest to tear that printk() from the 
> NMI path, its just waiting to wreck someone's machine :-)

So why not just write the value somewhere and printk once at the end of 
the NMI sequence, once everything is said and done?

Thanks,

	Ingo
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