linux-kernel - Re: [PATCH v2 0/2] cpu: pseries: Offline state framework.

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Date:	Thu, 24 Sep 2009 09:51:07 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ibm.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	"Darrick J. Wong" <djwong@...ibm.com>
Subject: Re: [PATCH v2 0/2] cpu: pseries: Offline state framework.

On Thu, 2009-09-24 at 10:48 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2009-09-02 at 07:33 +0200, Peter Zijlstra wrote:
> > 
> > I'm still thinking this is a bad idea.
> > 
> > The OS should only know about online/offline.
> > 
> > Use the hypervisor interface to deal with the cpu once its offline.
> > 
> > That is, I think this interface you propose is a layering violation.
> > 
> I don't quite follow your logic here. This is useful for more than just
> hypervisors. For example, take the HV out of the picture for a moment
> and imagine that the HW has the ability to offline CPU in various power
> levels, with varying latencies to bring them back.

cpu-hotplug is an utter slow path, anybody saying latency and hotplug in
the same sentence doesn't seem to grasp either or both concepts.



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