From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e31.co.us.ibm.com (e31.co.us.ibm.com [32.97.110.149]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E5F071A0145 for ; Fri, 10 Apr 2015 08:40:46 +1000 (AEST) Received: from /spool/local by e31.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 9 Apr 2015 16:40:44 -0600 Received: from b03cxnp08028.gho.boulder.ibm.com (b03cxnp08028.gho.boulder.ibm.com [9.17.130.20]) by d03dlp03.boulder.ibm.com (Postfix) with ESMTP id 1D12319D8040 for ; Thu, 9 Apr 2015 16:31:48 -0600 (MDT) Received: from d03av01.boulder.ibm.com (d03av01.boulder.ibm.com [9.17.195.167]) by b03cxnp08028.gho.boulder.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id t39MeSrw40501450 for ; Thu, 9 Apr 2015 15:40:28 -0700 Received: from d03av01.boulder.ibm.com (localhost [127.0.0.1]) by d03av01.boulder.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id t39MegiR032055 for ; Thu, 9 Apr 2015 16:40:42 -0600 Date: Thu, 9 Apr 2015 15:40:40 -0700 From: Nishanth Aravamudan To: Peter Zijlstra Subject: Re: Topology updates and NUMA-level sched domains Message-ID: <20150409224040.GG53918@linux.vnet.ibm.com> References: <20150406214558.GA38501@linux.vnet.ibm.com> <20150407102147.GJ23123@twins.programming.kicks-ass.net> <20150407171410.GA62529@linux.vnet.ibm.com> <20150407194129.GT23123@twins.programming.kicks-ass.net> <552503A1.3050502@inria.fr> <20150408105212.GP21418@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 In-Reply-To: <20150408105212.GP21418@twins.programming.kicks-ass.net> Cc: Boqun Feng , Srikar Dronamraju , linux-kernel@vger.kernel.org, Ingo Molnar , Anton Blanchard , Brice Goglin , linuxppc-dev@lists.ozlabs.org, Anshuman Khandual List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On 08.04.2015 [12:52:12 +0200], Peter Zijlstra wrote: > On Wed, Apr 08, 2015 at 12:32:01PM +0200, Brice Goglin wrote: > > Le 07/04/2015 21:41, Peter Zijlstra a écrit : > > > No, that's very much not the same. Even if it were dealing with hotplug > > > it would still assume the cpu to return to the same node. > > > > > > But mostly people do not even bother to handle hotplug. > > > > > > > You said userspace assumes the cpu<->node relation is a boot-time fixed > > one, and hotplug breaks this. > > I said no such thing. Regular hotplug actually respects that relation. Wel, sort of. If you *just* hotplug a CPU out, your invariant of what CPUs are currently available on what nodes is no longer held. Similarly if you just add a CPU. And means that you could end up using cpumasks that are incorrect if you don't make them at runtime, it seems? > > How do you expect userspace to handle hotplug? > > Mostly not. Why would they? CPU hotplug is rare and mostly a case of: > don't do that then. > > Its just that some of the virt wankers are using it for resource > management which is entirely misguided. Then again, most of virt is. I guess that is a matter of opinion. -Nish