Subject: Re: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: dipankar@in.ibm.com
Cc: Gautham R Shenoy <ego@in.ibm.com>, Joel Schopp <jschopp@austin.ibm.com>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Balbir Singh <balbir@in.ibm.com>,
       Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>,
       Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
       Arun R Bharadwaj <arun@linux.vnet.ibm.com>,
       linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
       "Darrick J. Wong" <djwong@us.ibm.com>
In-Reply-To: <20090916152820.GA12571@in.ibm.com>
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	 <1253016701.5506.73.camel@laptop>  <20090916152820.GA12571@in.ibm.com>
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Date: Wed, 16 Sep 2009 17:32:51 +0200
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On Wed, 2009-09-16 at 20:58 +0530, Dipankar Sarma wrote:
> On Tue, Sep 15, 2009 at 02:11:41PM +0200, Peter Zijlstra wrote:
> > On Tue, 2009-09-15 at 17:36 +0530, Gautham R Shenoy wrote:
> > > This patchset contains the offline state driver implemented for
> > > pSeries. For pSeries, we define three available_hotplug_states. They are:
> > > 
> > >         online: The processor is online.
> > > 
> > >         offline: This is the the default behaviour when the cpu is offlined
> > > 
> > >         inactive: This cedes the vCPU to the hypervisor with a cede latency
> > > 
> > > Any feedback on the patchset will be immensely valuable.
> > 
> > I still think its a layering violation... its the hypervisor manager
> > that should be bothered in what state an off-lined cpu is in. 
> 
> The problem is that all hypervisor managers cannot figure out what sort
> of latency guest OS can tolerate under the situation. They wouldn't know
> from what context guest OS has ceded the vcpu. It has to have
> some sort of hint, which is what the guest OS provides.

I'm missing something here, hot-unplug is a slow path and should not
ever be latency critical..?

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