Re: [PATCH v2 0/2] new cpumask for hotpluggable CPUs

From: Peter Zijlstra
Date: Thu Aug 11 2011 - 14:30:52 EST

On Wed, 2011-08-10 at 13:03 -0700, Mike Turquette wrote:
> This patch series introduces a new cpumask which tracks CPUs that
> support hotplugging. The purpose of this patch series is to provide a
> simple method for kernel code to know which CPUs can be hotplugged and
> which ones cannot. Potential users of this code might be a thermal
> mitigation technique which uses hotplug to lower temperature, or a power
> capping mechanism which uses hotplug to lower power consumption.
> All the of usual cpumask helper functions are created for this new mask.
> The second patch in this series simply sets the bit for elligible CPUs
> while they are being registered. The cpumask itself is static after
> boot and should not change (like the possbile mask).

I still most strongly object to people using hotplug for these goals.

Why do you need to go through the entire dance of hotplug just to idle a
cpu? Hotplug not only idles the cpu but tears down (and rebuilds) an
insane amount of resources associated with the cpu.

Nacked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
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