linux-kernel - Re: [PATCH 2/2] watchdog: Softlockup has regular windows where it is not armed

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Date:	Mon, 12 Dec 2011 14:53:46 -0500
From:	Don Zickus <dzickus@...hat.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Jeremy Fitzhardinge <jeremy@...source.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, jason.wessel@...driver.com
Subject: Re: [PATCH 2/2] watchdog: Softlockup has regular windows where it is
 not armed

On Mon, Dec 05, 2011 at 09:28:22PM +1100, Anton Blanchard wrote:
> 
> Hi Don,
> 
> > > There might be a reason for this two stage sync but I haven't been
> > > able to find it yet. Perhaps the unsynced versions of cpu_clock()
> > > and sched_clock_tick() are not safe to call from all contexts?
> > 
> > According to commit 8c2238eaaf0f774ca0f8d9daad7a616429bbb7f1 that was
> > the case, cpu_clock wasn't NMI-safe.  Now it is, thanks to Peter.
> 
> Thanks, that makes sense now.
> 
> > I have a couple of concerns about the patch.  I am wondering about the
> > overhead of getting the timestamp more often now as opposed to just
> > setting a boolean for later.  It makes sense to stamp it at the time
> > of the call, don't know what the cost is.
> 
> I had a similar concern since we do execute this quite a lot. The
> overhead of cpu_clock is quite low on powerpc, but not sure about the
> other architectures.

It seems like half of the users of touch_softlockup_watchdog is a slow
path (ie they are purposely spinning a long time).  The cpu_clock overhead
for those paths, we probably don't need to care about.

The other half seems to deal with long idle/suspend/kgdb paths, which may
not be that interesting in their own right, except for the fact they are
called all the time for short delays and long delays. :-/

Perhaps I can move the touch_softlockup_watchdog() calls closer to the
long path conditionals, minimize the calls a little bit.

> 
> > I am also concern about how this affects suspend/resume and kgdb. I
> > cc'd Jason above for kgdb.  I'll have to run some tests locally to
> > see what long periods of delay look like.  Oh and virt guests too.
> > You don't have any test results from that setup do you?
> 
> I haven't tested suspend resume, kgdb or virtual guests yet.

I'll try to setup a box and play with these paths to see what they look
like.

Cheers,
Don
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