linux-kernel - Re: [PATCH v2] NMI: fix NMI period is not correct when cpu frequency changes issue.

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Message-ID: <20130422185929.GZ79013@redhat.com>
Date:	Mon, 22 Apr 2013 14:59:29 -0400
From:	Don Zickus <dzickus@...hat.com>
To:	"Pan, Zhenjie" <zhenjie.pan@...el.com>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"acme@...stprotocols.net" <acme@...stprotocols.net>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] NMI: fix NMI period is not correct when cpu frequency
 changes issue.

On Mon, Apr 22, 2013 at 12:50:34AM +0000, Pan, Zhenjie wrote:
> > I believe it mattered to the Chrome folks. They want the watchdog to be as
> > tight as possible so the user experience isn't a hang but a quick reboot
> > instead.  They like setting the watchdog to something like 2 seconds.
> > 
> > There was a patch a few months ago that tried to hack around this issue and I
> > suggested this approach as a better solution.  I forgot what the original
> > problem was.  Perhaps someone can jump in and explain the problem being
> > solved (other than the watchdog isn't always 10 seconds)?
> > 
> > Cheers,
> > Don
> 
> Yes, I also think the period is important sometimes.
> As I mentioned before, the case I meet is:
> When the system hang with interrupt disabled, we use NMI to detect.
> Then it will find hard lockup and cause a panic.
> Panic is very important for debug these kind of issues.
> 
> But if cpu frequency change, the period will be 2 times, 3 times even more.(if cpu can down from 2.0GHz to 200MHz, will be 10 times, it's a very big deviation)
> This make watchdog reset happen before hard lockup detect.

So you are saying with the longer hard lockup delay, the iTCO_wdt is
firing before the hard lockup detector?

Cheers,
Don
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