Date:	Tue, 9 Apr 2013 11:14:23 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Dave Young <dyoung@...hat.com>, linux-watchdog@...r.kernel.org,
	kexec@...ts.infradead.org, wim@...ana.be,
	LKML <linux-kernel@...r.kernel.org>, vgoyal@...hat.com
Subject: Re: [RFC PATCH] watchdog: Add hook for kicking in kdump path

On Tue, Apr 09, 2013 at 07:52:28AM -0700, Guenter Roeck wrote:
> On Tue, Apr 09, 2013 at 10:44:31AM -0400, Don Zickus wrote:
> > On Mon, Apr 08, 2013 at 08:15:09AM -0700, Guenter Roeck wrote:
> > > On Mon, Apr 08, 2013 at 08:48:58AM -0400, Don Zickus wrote:
> > > > On Mon, Apr 08, 2013 at 01:46:58PM +0800, Dave Young wrote:
> > > > > On 04/06/2013 04:16 AM, Don Zickus wrote:
> > > > > > 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.
> > > > > 
> > > > > For kdump kernel some devices need to reset, this might increase the
> > > > > boot time, it's not so reliable for the 10-30s for us to kicking the
> > > > > watchdog.
> > > > > 
> > > > > Could we have another option to disable/stop the watchdog while panic
> > > > > happens? Ie. add a kernel cmdline panic_stop_wd=<0|1> for 1st kernel, if
> > > > > it's set to 1, then just stop the watchdog or we can kick the watchdog
> > > > > like what you do in this patch. Of course stopping watchdog should be
> > > > > lockless as well..
> > > > 
> > > > Hmm, I can look into that.  But I am not sure all watchdogs have the
> > > > ability to stop once started.  I was also worried about the case where
> > > 
> > > Correct.
> > > 
> > > > kdump hangs for some reason.  Having the watchdog there to 'reboot' would
> > > > be a nice safety net.
> > > > 
> > > Absolutely agree. After all, the reason for the kdump is most likely that
> > > something went really wrong, meaning there is some likelyhood for the hang
> > > to occur. Turning off the watchdog in this condition does not seem to be
> > > a good idea.
> > > 
> > > > Perhaps adjusting the watchdog 'timeout' to something like 3 minutes would
> > > > be easier?
> > > > 
> > > Not all watchdogs support such large timeouts, unfortunately. Maybe it would
> > > make sense to implement infrastructure support for a softdog on top of the
> > > hardware watchdog. Several drivers implement that outside the infrastructure
> > > already.
> > 
> > Hi Guenter,
> > 
> > I am not familar with a softdog.  Can you give me an example of how it
> > works?
> > 
> Just look for the use of mod_timer in the watchdog directory.

So looking at the mod_timer logic in various drivers, it seems regardless
if the /dev/watchdog device is opened or not, if it is running, it will
automagically kick the watchdog.

This seems that we can avoid pulling in userspace pieces for this.  Just
load the driver and the hardware starts getting kicked.

Is that true?  And if so, do all drivers detect if the hardware is already
running during their init?  Or is it based on the first device open?

Cheers,
Don
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