Re: [RFC PATCH] watchdog: Add hook for kicking in kdump path

[Guenter Roeck <linux@roeck-us.net>]

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.

Guenter