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

[Don Zickus]

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
kdump hangs for some reason.  Having the watchdog there to 'reboot' would
be a nice safety net.

Perhaps adjusting the watchdog 'timeout' to something like 3 minutes would
be easier?

I'll wait on feedback from the watchdog community to help point me in a
good direction.

Cheers,
Don

> 
> > 
> > Of course kdump is usually executed on a panic path, so grabbing the
> > watchdog mutexes to communicate with the hardware won't work.  For now,
> > I do everything locklessly.
> > 
> > I can attempt a 'mutex_trylock' but not sure what to do in the failure
> > case?  spin up to a second?
> > 
> > This patch is more of a proof of concept right now.  Hopefully feedback
> > will help solve this problem better.
> > 
> > I have tested this with a machine using iTCO_wdt and the 'watchdog' app.
> > The extra kicked happened as expected.
> > 
> > Signed-off-by: Don Zickus <dzickus at redhat.com>
> > ---
> >  drivers/watchdog/watchdog_dev.c |   35 +++++++++++++++++++++++++++++++++++
> >  include/linux/watchdog.h        |    7 +++++++
> >  kernel/kexec.c                  |    6 ++++++
> >  3 files changed, 48 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> > index 08b48bb..6393572 100644
> > --- a/drivers/watchdog/watchdog_dev.c
> > +++ b/drivers/watchdog/watchdog_dev.c
> > @@ -84,6 +84,41 @@ out_ping:
> >  }
> >  
> >  /*
> > + *	watchdog_kick_for_kdump: kick the watchdog in the kdump path
> > + *
> > + * 	The kdump path needs time to reboot the kernel back into
> > + * 	userspace.  This window is big enough the hardware watchdog
> > + * 	may come in and reboot the box.  This hook gives the watchdog
> > + * 	one final kick to get kdump over the hump.
> > + *
> > + * 	We don't know what devices are open, so just use the legacy
> > + * 	interface for now.  We have to do this locklessly as we can
> > + * 	not wait.
> > + */
> > +
> > +void watchdog_kick_for_kdump(void)
> > +{
> > +	struct watchdog_device *wddev = old_wdd;
> > +
> > +	/* FIXME - Perhaps use a mutex_trylock? */
> > +
> > +	if (!wddev)
> > +		return;
> > +
> > +	if (test_bit(WDOG_UNREGISTERED, &wddev->status))
> > +		return;
> > +
> > +	if (!watchdog_active(wddev))
> > +		return;
> > +
> > +	if (wddev->ops->ping)
> > +		wddev->ops->ping(wddev);  /* ping the watchdog */
> > +	else
> > +		wddev->ops->start(wddev); /* restart watchdog */
> > +}
> > +EXPORT_SYMBOL_GPL(watchdog_kick_for_kdump);
> > +
> > +/*
> >   *	watchdog_start: wrapper to start the watchdog.
> >   *	@wddev: the watchdog device to start
> >   *
> > diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
> > index 2a3038e..5dff975 100644
> > --- a/include/linux/watchdog.h
> > +++ b/include/linux/watchdog.h
> > @@ -142,4 +142,11 @@ extern int watchdog_init_timeout(struct watchdog_device *wdd,
> >  extern int watchdog_register_device(struct watchdog_device *);
> >  extern void watchdog_unregister_device(struct watchdog_device *);
> >  
> > +#ifdef CONFIG_WATCHDOG_CORE
> > +/* drivers/watchdog/watchdog_dev.c */
> > +extern void watchdog_kick_for_kdump(void);
> > +#else
> > +static inline void watchdog_kick_for_kdump(void) { };
> > +#endif
> > +
> >  #endif  /* ifndef _LINUX_WATCHDOG_H */
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index bddd3d7..ced7ccd 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/vmalloc.h>
> >  #include <linux/swap.h>
> >  #include <linux/syscore_ops.h>
> > +#include <linux/watchdog.h>
> >  
> >  #include <asm/page.h>
> >  #include <asm/uaccess.h>
> > @@ -1094,6 +1095,11 @@ void crash_kexec(struct pt_regs *regs)
> >  		if (kexec_crash_image) {
> >  			struct pt_regs fixed_regs;
> >  
> > +			/*
> > +			 * Give panic path a chance to do its post processing
> > +			 */
> > +			watchdog_kick_for_kdump();
> > +
> >  			crash_setup_regs(&fixed_regs, regs);
> >  			crash_save_vmcoreinfo();
> >  			machine_crash_shutdown(&fixed_regs);
> > 
> 
> 
> -- 
> Thanks
> Dave
> 
>