linux-kernel - Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi

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Message-ID: <1338842001.28282.135.camel@twins>
Date:	Mon, 04 Jun 2012 22:33:21 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	H Peter Anvin <hpa@...or.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Tony Luck <tony.luck@...el.com>,
	Asit K Mallick <asit.k.mallick@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	x86 <x86@...nel.org>, linux-pm <linux-pm@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or
 nmi

On Mon, 2012-06-04 at 22:11 +0200, Thomas Gleixner wrote:

> I understand what you are trying to do, though I completely disagree
> with the solution.
> 
> The main problem of the current hotplug code is that it is an all or
> nothing approach. You have to tear down the whole thing completely
> instead of just taking it out of the usable set of cpus.
> 
> I'm working on a proper state machine driven online/offline sequence,
> where you can put the cpu into an intermediate state which avoids
> bringing it down completely. This is enough to get the full
> powersaving benefits w/o having to go through all the synchronization
> states of a full online/offline. That will shorten the onlining time
> of an previously offlined cpu to almost nothing.
> 
> I really want to avoid adding more bandaids to the hotplug code before
> we have sorted out the existing horror.

Its far worse.. you shouldn't _ever_ care about hotplug latency unless
you've got absolutely braindead hardware. We all now ARM has been
particularly creative here, but is Intel now trying to trump ARM at
stupid?
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