linux-kernel - Re: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework

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Date:	Wed, 16 Sep 2009 17:32:51 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	dipankar@...ibm.com
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Balbir Singh <balbir@...ibm.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Arun R Bharadwaj <arun@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	"Darrick J. Wong" <djwong@...ibm.com>
Subject: Re: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework

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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