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

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Message-ID: <20130418145413.GA1980@roeck-us.net>
Date:	Thu, 18 Apr 2013 07:54:13 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Don Zickus <dzickus@...hat.com>
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 09:52:57AM -0400, Don Zickus wrote:
> 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?
> 
Usually it is configured in /etc/watchdog.conf if the watchdog package
is installed. The standard ping interval is "interval", the timeout is
"watchdog-timeout". See "man watchdog.conf" for details.

Minimum and maximum values for a given watchdog driver are not exported
to user space, so you would have to look into the driver sources to find
out what they are.

Guenter
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