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Subject: Re: CPU hotplug vs. cpufreq on ppc64
From: Johannes Berg <johannes@sipsolutions.net>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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On Mon, 2007-02-05 at 12:39 +1100, Benjamin Herrenschmidt wrote:

> So you mean that the policy->cpus mask is lost on unplug/replug ? Hrm...

Yes, but fixing that wouldn't fix the problem.

> I'm not sure what's the best way to restore it. Do we have a cpufreq
> backend callback on hotplug to restore things ?

Not afaict, but you can of course always use register_hotcpu_notifier.

This, however, is what cpufreq.c uses to be notified of a new CPU that
is coming online, and it then calls cpufreq_add_dev which even with a
correct policy->cpus mask wouldn't go back to adding the links back,
this is only done when CPU#0 comes online.

I think this is a more generic problem and code will need to be added to
cpufreq to
 (a) call into the cpufreq_driver asking it to update the mask on
     hotplug
 (b) after that, check the mask and re-link on hotplug.

johannes

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