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

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Date:	Thu, 24 Sep 2009 10:55:14 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	dipankar@...ibm.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gautham R Shenoy <ego@...ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	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 21:54 +0530, Dipankar Sarma wrote:
> You aren't, I did :)
> 
> No, for this specific case, latency isn't an issue. The issue is -
> how do we cede unused vcpus to hypervisor for better energy
> management ?
> Yes, it can be done by a hypervisor manager telling the kernel to
> offline and make a bunch of vcpus "inactive". It does have to choose
> offline (release vcpu) vs. inactive (cede but guranteed if needed).
> The problem is that long ago we exported a lot of hotplug stuff to
> userspace through the sysfs interface and we cannot do something
> inside the kernel without keeping the sysfs stuff consistent.
> This seems like a sane way to do that without undoing all the
> virtual cpu hotplug infrastructure in different supporting archs.
> 
Well, I did bring the latency into the picture. To some extent it -is- a
latency issue. Though we aren't talking milliseconds here... if the
CPU's been reallocated to another partition we are talking seconds or
minutes or more until we can get it back :-)

In any case, this sounds to me like a perfectly valid feature to have,
which s390 already does via arch specific hooks, so I fail to see why it
wouldn't be legitimate to have a common attribute for it.

I don't buy into the layering violation. It's too often a straw man and
an excuse for a lack of a proper reason.

Cheers,
Ben.

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