From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from viefep14-int.chello.at (viefep14-int.chello.at [62.179.121.34]) by ozlabs.org (Postfix) with ESMTP id CBFBAB7B76 for ; Thu, 24 Sep 2009 21:37:45 +1000 (EST) Subject: Re: [PATCH v2 0/2] cpu: pseries: Offline state framework. From: Peter Zijlstra To: Benjamin Herrenschmidt In-Reply-To: <1253781508.7103.437.camel@pasglop> References: <20090828095741.10641.32053.stgit@sofia.in.ibm.com> <1251869611.7547.38.camel@twins> <1253753307.7103.356.camel@pasglop> <1253778667.7695.130.camel@twins> <1253781508.7103.437.camel@pasglop> Content-Type: text/plain Date: Thu, 24 Sep 2009 13:33:07 +0200 Message-Id: <1253791987.7695.153.camel@twins> Mime-Version: 1.0 Cc: Gautham R Shenoy , Venkatesh Pallipadi , linux-kernel@vger.kernel.org, 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 Thu, 2009-09-24 at 18:38 +1000, Benjamin Herrenschmidt wrote: > On Thu, 2009-09-24 at 09:51 +0200, Peter Zijlstra wrote: > > > I don't quite follow your logic here. This is useful for more than just > > > hypervisors. For example, take the HV out of the picture for a moment > > > and imagine that the HW has the ability to offline CPU in various power > > > levels, with varying latencies to bring them back. > > > > cpu-hotplug is an utter slow path, anybody saying latency and hotplug in > > the same sentence doesn't seem to grasp either or both concepts. > > Let's forget about latency then. Let's imagine I want to set a CPU > offline to save power, vs. setting it offline -and- opening the back > door of the machine to actually physically replace it :-) If the hardware is capable of physical hotplug, then surely powering the socket down saves most power and is the preferred mode? > In any case, I don't see the added feature as being problematic, and > not such a "layering violation" as you seem to imply it is. It's a > convenient way to atomically take the CPU out -and- convey some > information about the "intent" to the hypervisor, and I really fail > to see why you have so strong objections about it. Ignorance on my part probably :-) I'm simply not seeing a use case for it, except for the virt case, which I think we should bug the virt interface with and not the cpu-hotplug interface.