linux-kernel - Re: [PATCH 0/3] cpu: idle state framework for offline CPUs.

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Date:	Fri, 7 Aug 2009 09:02:56 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Joel Schopp <jschopp@...tin.ibm.com>,
	"Brown, Len" <len.brown@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Balbir Singh <balbir@...ibm.com>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"Darrick J. Wong" <djwong@...ibm.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] cpu: idle state framework for offline CPUs.

On Thu, Aug 06, 2009 at 09:48:44PM +0800, Gautham R Shenoy wrote:
> Hi Shaohua,
> 
> On Thu, Aug 06, 2009 at 09:58:55AM +0800, Shaohua Li wrote:
> > Hi,
> > 
> > On Wed, Aug 05, 2009 at 10:25:53PM +0800, Gautham R Shenoy wrote:
> > > In this patch-series, we propose to extend the CPU-Hotplug infrastructure
> > > and allow the system administrator to choose the desired state the CPU should
> > > go to when it is offlined. We think this approach addresses the concerns about
> > > determinism as well as transparency, since CPU-Hotplug already provides
> > > notification mechanism which the userspace can listen to for any change
> > > in the configuration and correspondingly readjust any previously set
> > > cpu-affinities.
> > Peter dislikes any approach (including cpuhotplug) which breaks userspace policy,
> > even userspace can get a notification.
> 
> I think Peter's problem was more to do with the kernel offlining the CPUs
> behind the scenes, right ?
> 
> We don't do that in this patch series. The option to offline the CPUs is
> very much with the admin. The patch-series only provides the interface
> that helps the admin choose the state the CPU must reside in when it
> goes offline.
but the goal is to use cpu offline to save power, right? So we still have
Peter's problem.
 
> > > Also, approaches such as [1] can make use of this
> > > extended infrastructure instead of putting the CPU to an arbitrary C-state
> > > when it is offlined, thereby providing the system administrator a rope to hang
> > > himself with should he feel the need to do so.
> > I didn't see the reason why administrator needs to know which state offline cpu
> > should stay. Don't know about powerpc side, but in x86 side, it appears deepest
> > C-state is already preferred.
> 
> We can still provide a sane default value based on what states are
> available and what the BIOS limits us to. Thus we can still use the
> idle-state-offline patch that Venki posted sometime ago, right ?
My original concern about Venki's patch is the C-state limition, but Venki
thought if CPU has the limition, CPU should disable specific C-state, so this
isn't a problem. I had no objection about the infrastructure itself, but just
wonder why we need it.

Thanks,
Shaohua
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