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

From: Gautham R Shenoy <[email-masked]>
Date: Thu, 6 Aug 2009 19:18:44 +0530

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.

> 
> > 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 ?

> Thanks,
> Shaohua

-- 
Thanks and Regards
gautham