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

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Date:	Wed, 10 Nov 2010 07:08:08 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Don Zickus <dzickus@...hat.com>, fweisbec@...il.com,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, sergey.senozhatsky@...il.com
Subject: Re: [PATCH v3] watchdog:  touch_nmi_watchdog should only touch
 local cpu not every one

On Wed, 10 Nov 2010 08:49:41 +0100 Ingo Molnar <mingo@...e.hu> 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?
> 
> Andrew, what would be your preference?
> 

I saw you were cc'ed and felt rather pleased that I wouldn't have to
decide ;)

I guess the NMI really is a cpu-local concept rather than a
machine-wide one.  And touch_nmi_watchdog() says "this CPU isn't
stuck", rather than "this CPU and all those others aren't stuck", yes?
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