Re: [PATCH -v2 5/5] x86: use dmi check to treat disabled cpus ashotplug cpus.

From: H. Peter Anvin
Date: Wed Jan 13 2010 - 17:02:16 EST

On 01/13/2010 01:46 PM, Andi Kleen wrote:
> Yinghai Lu <yinghai@xxxxxxxxxx> writes:
>> so you want to treat those disabled cpus in the those system as hotplug cpus or not?
> Why not? It's not that a hotplug cpu is particularly expensive.
> It's just a bunch of memory and not even very much of it.
> White and blacklists just to save a small amount of memory
> seem like a bad idea.

There are configurations in which percpu memory is in the megabytes.
This is exactly why we need high water mark allocation of percpu memory:
for configurations where there are possible hotpluggable CPU sockets
(which may be virtual, and a lot larger number than necessary) we
shouldn't need to allocate memory for a processor which has never been
added and is net unlikely to ever be added. The only alternative is to
go to great length to keep the
(percpu memory) x (possible cpus - actual cpus) product as small as
possible, which is shortchanging the utility of the percpu memory system.

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