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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

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.