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