linux-kernel - Re: [RFC -v3 2/2] watchdog: update watchdog

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Message-ID: <20130723140729.GH8677@dhcp22.suse.cz>
Date:	Tue, 23 Jul 2013 16:07:29 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Don Zickus <dzickus@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC -v3 2/2] watchdog: update watchdog_tresh properly

On Tue 23-07-13 09:53:34, Don Zickus wrote:
> On Mon, Jul 22, 2013 at 04:32:46PM +0200, Michal Hocko wrote:
> > The nmi one is disabled and then reinitialized from scratch. This
> > has an unpleasant side effect that the allocation of the new event might
> > fail theoretically so the hard lockup detector would be disabled for
> > such cpus. On the other hand such a memory allocation failure is very
> > unlikely because the original event is deallocated right before.
> > It would be much nicer if we just changed perf event period but there
> > doesn't seem to be any API to do that right now.
> > It is also unfortunate that perf_event_alloc uses GFP_KERNEL allocation
> > unconditionally so we cannot use on_each_cpu() and do the same thing
> > from the per-cpu context. The update from the current CPU should be
> > safe because perf_event_disable removes the event atomically before
> > it clears the per-cpu watchdog_ev so it cannot change anything under
> > running handler feet.
> 
> I guess I don't have a problem with this.  I was hoping to have more
> shared code with the regular stop/start routines but with the pmu bit
> locking (to share pmus with oprofile), you really need to unregister
> everything to stop the lockup detector.  This makes it a little too heavy
> for a restart routine like this.

I am not sure I understand the above. Regular stop/start is about all
the machinery, I have tried to reduce the restarting to bare minimum.
Do you find the current version heavier than the full disable_all &&
enable_all?

> The only odd thing is I can't figure out which version you were using to
> apply this patch.  I can't find old_thresh (though I understand the idea
> of it).

current Linus tree (linux-next - 20130723 - has it as well AFAICS)
 
> Cheers,
> Don
-- 
Michal Hocko
SUSE Labs
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