linux-kernel - Re: [PATCH v3] watchdog: Add hook for kicking in kdump path

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Message-ID: <20130418135257.GL79013@redhat.com>
Date:	Thu, 18 Apr 2013 09:52:57 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-watchdog@...r.kernel.org, kexec@...ts.infradead.org,
	wim@...ana.be, LKML <linux-kernel@...r.kernel.org>,
	vgoyal@...hat.com, dyoung@...hat.com
Subject: Re: [PATCH v3] watchdog: Add hook for kicking in kdump path

On Thu, Apr 18, 2013 at 06:49:04AM -0700, Guenter Roeck wrote:
> On Thu, Apr 18, 2013 at 09:00:09AM -0400, Don Zickus wrote:
> > On Wed, Apr 17, 2013 at 02:49:59PM -0700, Eric W. Biederman wrote:
> > > Don Zickus <dzickus@...hat.com> writes:
> > > 
> > > > A common problem with kdump is that during the boot up of the
> > > > second kernel, the hardware watchdog times out and reboots the
> > > > machine before a vmcore can be captured.
> > > >
> > > > Instead of tellling customers to disable their hardware watchdog
> > > > timers, I hacked up a hook to put in the kdump path that provides
> > > > one last kick before jumping into the second kernel.
> > > >
> > > > The assumption is the watchdog timeout is at least 10-30 seconds
> > > > long, enough to get the second kernel to userspace to kick the watchdog
> > > > again, if needed.
> > > 
> > > Why not double the watchdog timeout? and/or pet the watchdog a little
> > > more frequently.
> > 
> > I am not sure if the watchdog timeouts can be doubled.  I think Guenter
> > was saying some have a max of a couple seconds?? Petting a little more
> > frequently might be an option.  Guenter can that be done with a softdog
> > option?
> > 
> Most watchdog driver permit at least a minute. Some are more limited.
> Worst I have seen is the BookE watchdog timer (non-Freescale version)
> which has a maximum of three seconds. But that is broken anyway.
> 
> Most hardware watchdogs implement a softdog on top of the hardware watchdog
> if the hardware needs to be pinged faster than every 60 seconds.
> 
> So, yes, for the most common case you should actually be able to live with a,
> say, 30-60 second timeout which is pinged at least every 5-10 seconds. I thought
> that somehow did not work in your case. Maybe a misunderstanding ?

No, that will probably work.  It is my misunderstanding.  Is there a
common way to check the timeout length and the ping frequency?

Cheers,
Don
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