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

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Message-ID: <1366285369.19383.19.camel@laptop>
Date:	Thu, 18 Apr 2013 13:42:49 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Pan, Zhenjie" <zhenjie.pan@...el.com>
Cc:	"'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>,
	"'dzickus@...hat.com'" <dzickus@...hat.com>,
	"'tglx@...utronix.de'" <tglx@...utronix.de>,
	"Liu, Chuansheng" <chuansheng.liu@...el.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v2] NMI: fix NMI period is not correct when cpu
 frequency changes issue.

On Tue, 2013-04-16 at 06:57 +0000, Pan, Zhenjie wrote:
> Watchdog use performance monitor of cpu clock cycle to generate NMI to detect hard lockup.
> But when cpu's frequency changes, the event period will also change.
> It's not as expected as the configration.
> For example, set the NMI event handler period is 10 seconds when the cpu is 2.0GHz.
> If the cpu changes to 800MHz, the period will be 10*(2000/800)=25 seconds.
> So it may make hard lockup detect not work if the watchdog timeout is not long enough.
> Now, set a notifier to listen to the cpu frequency change.
> And dynamic re-config the NMI event to make the event period correct.
> 


Urgh,. does this really matter.. all we really want is for that NMI to
hit eventually in the not too distant future. Does the frequency really
matter _that_ much?

Also, can't we simply pick an event that's invariant to the cpufreq
nonsense? Something like CPU_CLK_UNHALTED.REF -- or better the
fixed_ctr2 which nobody ever uses anyway.

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