From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 1C252B7043 for ; Thu, 24 Sep 2009 10:57:36 +1000 (EST) Subject: Re: [PATCH v3 0/3] cpu: pseries: Cpu offline states framework From: Benjamin Herrenschmidt To: dipankar@in.ibm.com In-Reply-To: <20090916162459.GB12571@in.ibm.com> References: <20090915120629.20523.79019.stgit@sofia.in.ibm.com> <1253016701.5506.73.camel@laptop> <20090916152820.GA12571@in.ibm.com> <1253115171.7180.1.camel@laptop> <20090916162459.GB12571@in.ibm.com> Content-Type: text/plain Date: Thu, 24 Sep 2009 10:55:14 +1000 Message-Id: <1253753715.7103.361.camel@pasglop> Mime-Version: 1.0 Cc: Peter Zijlstra , Gautham R Shenoy , linux-kernel@vger.kernel.org, Venkatesh Pallipadi , Arun R Bharadwaj , linuxppc-dev@lists.ozlabs.org, "Darrick J. Wong" List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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.