linux-kernel - Re: [PATCH -v2 5/5] x86: use dmi check to treat disabled cpus as hotplug cpus.

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Date:	Wed, 13 Jan 2010 14:00:29 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"ananth@...ibm.com" <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2 5/5] x86: use dmi check to treat disabled cpus as
 hotplug cpus.

On 01/13/2010 01:46 PM, Andi Kleen wrote:
> Yinghai Lu <yinghai@...nel.org> 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.

	-hpa
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