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

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Date:	Mon, 5 Dec 2011 21:28:22 +1100
From:	Anton Blanchard <anton@...ba.org>
To:	Don Zickus <dzickus@...hat.com>
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


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.

> 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.

Anton
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