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

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Date:	Mon, 22 Apr 2013 00:50:34 +0000
From:	"Pan, Zhenjie" <zhenjie.pan@...el.com>
To:	Don Zickus <dzickus@...hat.com>,
	Stephane Eranian <eranian@...gle.com>
CC:	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.

> 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.

Thanks
Pan Zhenjie
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