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

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Message-ID: <4FCE254A.6050504@linux.intel.com>
Date:	Tue, 05 Jun 2012 08:27:06 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	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 6/5/2012 7:17 AM, Alan Stern wrote:
> On Mon, 4 Jun 2012, Arjan van de Ven wrote:
> 
>>> 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.
> 
> This doesn't sound right at all.  Len Brown has often told us that on
> Intel chips, power can't be removed from a package until _all_ the
> cores in the package are idle.

but "cpu hotplug" does not change that.. in fact, cpu hotplug is
implemented as a C state...


and to be specific; power will get removed from the cores one at a time.
you just cannot remove the power from the memory controller until the
last one is off.

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