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Date: Fri, 18 Mar 2011 11:23:53 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Don Zickus <dzickus@redhat.com>
Cc: x86@kernel.org, Peter Zijlstra <peterz@infradead.org>,
        jwjstone@fastmail.fm, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/2 v2] watchdog, nmi:  Allow hardlockup to panic by
 default
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On Fri, 18 Mar 2011 13:19:32 -0400
Don Zickus <dzickus@redhat.com> wrote:

> On Thu, Mar 17, 2011 at 06:50:13PM -0700, Andrew Morton wrote:
> > On Mon,  7 Mar 2011 16:37:39 -0500 Don Zickus <dzickus@redhat.com> wrote:
> > 
> > > Add a Kconfig option to allow users to set the hardlockup to panic
> > > by default.  Also add in a 'nmi_watchdog=nopanic' to override this.
> > > 
> > 
> > Changelog forgot to tell us "why".
> 
> Yeah, sorry about that.
> 
> When a cpu is considered stuck, instead of limping along and just printing
> a warning, it is sometimes preferred to just panic, let kdump capture the
> vmcore and reboot.  This gets the machine back into a stable state quickly
> while saving the info that got it into a stuck state to begin with.

Ah, makes sense, thanks.  I updated the changelog.

> 
> > 
> > >  			Format: [state][,regs][,debounce][,die]
> > >  
> > >  	nmi_watchdog=	[KNL,BUGS=X86] Debugging features for SMP kernels
> > > -			Format: [panic,][num]
> > > +			Format: [panic,][nopanic,][num]
> > 
> > It would be better to support panic=[0|1], if that can be simply done
> > in a back-compatible fashion.
> 
> I am open to the idea, just can't figure the best way to implement that in
> a backwards compatible way.

It's not worth busting a gut over ;)

> Personally I was wondering if there were situations where you would _not_
> want it to panic.  If the cpu is stuck spinning after 60 seconds, the odds
> of it freeing itself is low and you are probably stuck rebooting anyway.
> 
> 
> > 
> > >  static int __init hardlockup_panic_setup(char *str)
> > >  {
> > >  	if (!strncmp(str, "panic", 5))
> > >  		hardlockup_panic = 1;
> > > +	else if (!strncmp(str, "nopanic", 5))
> > 
> > s/5/7/
> 
> doh.
> 
> I can send a refreshed patch with the above changes.

I fixed that up.