linux-kernel - Re: [PATCH v3] watchdog: touch_nmi_watchdog should only touch local cpu not every one

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Message-ID: <20101110160516.GZ4823@redhat.com>
Date:	Wed, 10 Nov 2010 11:05:16 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	fweisbec@...il.com, Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, akpm@...ux-foundation.org,
	sergey.senozhatsky@...il.com
Subject: Re: [PATCH v3] watchdog:  touch_nmi_watchdog should only touch local
 cpu not every one

On Wed, Nov 10, 2010 at 08:49:41AM +0100, Ingo Molnar wrote:
> 
> * Don Zickus <dzickus@...hat.com> wrote:
> 
> > I ran into a scenario where while one cpu was stuck and should have panic'd 
> > because of the NMI watchdog, it didn't.  The reason was another cpu was spewing 
> > stack dumps on to the console.  Upon investigation, I noticed that when writing to 
> > the console and also when dumping the stack, the watchdog is touched.
> > 
> > This causes all the cpus to reset their NMI watchdog flags and the 'stuck' cpu 
> > just spins forever.
> 
> Hm, the flip side is that if a CPU is stuck spewing backtraces, we will now make all 
> the other CPUs a lot more noisy - which might only 'lock up' because this CPU is 
> stuck spewing oopses, right?

When you say the other CPUs will be a lot more noisy, is that because they
are busy processing backtraces for the first cpu to spew?  I guess I don't
understand how the other CPUs could have their interrupts off the whole
time while the first cpu is spewing a backtrace (just trying to educate
myself).

Cheers,
Don
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