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

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Message-ID: <20101110191432.GB30227@elte.hu>
Date:	Wed, 10 Nov 2010 20:14:32 +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:

> On Wed, Nov 10, 2010 at 07:07:11PM +0100, Ingo Molnar wrote:
> > 
> > * 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.
> 
> You mean inside the panic() routine? like a ratelimit?

A ratelimit, but some really serious one - like only one crash displayed per 10 
minutes, or so - to give the user time to make a picture of the first crash, if it's 
still visible on the screen or so.

Thanks,

	Ingo
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