linux-kernel - Re: [PATCH 4/4] watchdog: configure nmi watchdog period based on watchdog

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Date:	Tue, 17 May 2011 10:03:34 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcin Slusarz <marcin.slusarz@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 4/4] watchdog: configure nmi watchdog period based on
 watchdog_thresh

On Tue, May 17, 2011 at 09:16:42AM +0200, Ingo Molnar wrote:
> Hm, our tolerance for the two thresholds is not just human but technical: hard 
> lockup warnings should indeed be triggered after just a few seconds, soft 
> lockups can have false positives under extreme conditions.
> 
> So we generally want a higher threshold for soft lockups than for hard lockups.
> 
> So how about we couple the thresholds with a factor: we make the soft threshold 
> twice the amount of time the hard threshold is? Then we could change the 
> upstream default as well i think: lets change the NMI timeout to 10 seconds 
> (and thus have the soft threshold at 20 seconds). Is 20 seconds short enough 
> for most users to not hit reset?

Making softlockup twice as long as hardlockup seems to make sense.
Setting the hardlockup to 10 seconds can be ok, but then you get into
power savings issues.  For example, I have the timers setup to trigger 5
times a period (I know it probably should be 2 times), so at 10 seconds
that means the timers are firing every 2 seconds.  That shows up on
powertop :-(.  Though I was flirting with the idea of trying to slow down
or stop the timer when the cpu goes into deeper c-states.  But that is a
different problem.

> 
> We might want to change another aspect of the NMI watchdog: right now it tries 
> to abort the offending task - which is really nasty if there was a spuriously 
> long irqs-off section somewhere in the kernel. How about we just print a 
> warning instead?

I dont understand this.  IIRC NMI watchdog will either printk or panic on
a hardlockup.  What do you mean by 'aborting' the task?

Cheers,
Don

> 
> Thanks,
> 
> 	Ingo
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