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

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Message-ID: <20101110180711.GB22410@elte.hu>
Date:	Wed, 10 Nov 2010 19:07:11 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Don Zickus <dzickus@...hat.com>
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

* Don Zickus <dzickus@...hat.com> wrote:

> > 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).

Say the kernel crashes on a CPU and keeps spewing new oopses, while write-holding 
tasklist_lock.

Any other CPU that delivers a signal from IRQ context, trying to take the 
tasklist_lock, will loop indefinitely until that crashing CPU releases the lock.

In that case the 'secondary' NMI warnings from all other CPUs (eventually every CPU 
gets stuck in such a scenario) will start spewing NMI lockup messages.

Dunno. Maybe we should do your change - but also have an option to 'shut up' the 
kernel after the first hard oops [not warning]. That would silence the secondary NMI 
watchdog messages as well.

Thanks,

	Ingo
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