linux-kernel - Re: [PATCH] watchdog: add a parameter for stop wdt before register

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Date:	Wed, 15 Jan 2014 09:11:42 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>, wim@...ana.be,
	dzickus@...hat.com, bhe@...hat.com, linux-watchdog@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] watchdog: add a parameter for stop wdt before register

On 01/14/14 at 11:24am, Vivek Goyal wrote:
> On Tue, Jan 14, 2014 at 12:16:39PM +0000, One Thousand Gnomes wrote:
> > On Tue, 14 Jan 2014 16:23:23 +0800
> > Dave Young <dyoung@...hat.com> wrote:
> > 
> > > In kdump kernel watchdog could interrupt vmcore capturing because we
> > > have no way to disable/stop it while crashing happens.
> > 
> > Lots of watchdogs cannot be stopped.
> > 
> > > Add a module parameter stop_before_register so watchdog can be stopped
> > > before register in driver loading path. Thus we can try to load the
> > > watchdog driver as early as possible in kdump kernel to ensure vmcore
> > > capturing.
> > 
> > If you want to kdump then don't start the watchdog. The goal of the
> > watchdog is to make sure the system never gets stuck. Adding conditions
> > and special cases simply increases the odds of something bad not
> > triggering the watchdog.
> > 
> > If you have a system that can stop the watchdog then providing no way out
> > is not set you can open it and stop it.
> > 
> > I don't see the need for any kernel change here
> > 
> > - if it can't be stopped you lost
> > - if "nowayout" is set then by design you lost
> > - if it can be stopped, you can open and stop it
> > 
> > Now whether in the !nowayout case the watchdog core should catch whatever
> > hooks/notifiers are available and stop any watchdogs it can on a
> > kexec/kdump is a more interesting question and probably needs to default
> > to not doing so but with the option to force otherwise for debugging work.
> 
> Hi All,
> 
> I thought this problem was resolved (atleast conceptually) last time
> when Don Zickus brought it up.
> 
> He mentioned that it was concluded that keep watchdog interval long
> enough, say 60 seconds and keep on kicking it fast enough, say every
> 10-20 seconds. That would ensure that after the crash, there is atleast
> 60 - 20 = 40 seconds left before watchdog expires. And in that duration
> we should try to boot into second kernel load watchdog driver early enough
> from initramfs which can start kicking watchdog again.

Some drivers did stop the watchdog while module loading such as iTCO_wdt.
so we can load them as early as possible and not necessary to kick them
again. Thus I wrote this patch to add the stop to generic code so more drivers
can be covered.

But as Alan said I begin to feel that this is not a good design. the iTCO_wdt
nowayout become useless because of this stop_before_register..

For wdts which can not be stopped we can still continue working on kicking
them early in initramfs.

> 
> I am wondering what happened to this idea. Dave, did we try to implement/
> experiment with this?

No, we are just begin with iTCO_wdt and is trying to add iTCO_wdt firstly.
Other devices has not been investigated.

Thanks
Dave
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