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

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Date:	Mon, 04 Jun 2012 18:23:20 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	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>
Subject: Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or
 nmi

On 6/4/2012 5:40 PM, Rusty Russell wrote:
> On Mon, 04 Jun 2012 22:33:21 +0200, Peter Zijlstra <peterz@...radead.org> wrote:
>> 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?
> 
> I disagree.  Deactivating a cpu for power saving is halfway to hotplug
> anyway.  I'd rather unify the two cases, where we can specify how dead a
> CPU should be, than have individual archs and boards do random hacks.

well on PC's there really is no difference at least;
idle equals "all power removed" already there.

but I can see that on some other architectures, that lack idle that
deep, there can be a real difference.

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