From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759487Ab2BJQzG (ORCPT ); Fri, 10 Feb 2012 11:55:06 -0500 Received: from e36.co.us.ibm.com ([32.97.110.154]:52329 "EHLO e36.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751205Ab2BJQzD (ORCPT ); Fri, 10 Feb 2012 11:55:03 -0500 Date: Fri, 10 Feb 2012 08:53:38 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Ingo Molnar , "Srivatsa S. Bhat" , paul@paulmenage.org, rjw@sisk.pl, tj@kernel.org, frank.rowand@am.sony.com, pjt@google.com, tglx@linutronix.de, lizf@cn.fujitsu.com, prashanth@linux.vnet.ibm.com, vatsa@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, "akpm@linux-foundation.org" Subject: Re: [PATCH 0/4] CPU hotplug, cpusets: Fix CPU online handling related to cpusets Message-ID: <20120210165338.GI2458@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120207185411.7482.43576.stgit@srivatsabhat.in.ibm.com> <1328671335.2482.72.camel@laptop> <4F32174E.2050207@linux.vnet.ibm.com> <20120209075701.GE18387@elte.hu> <4F3386E9.7090606@linux.vnet.ibm.com> <20120209151158.GA22489@elte.hu> <1328889127.25989.14.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1328889127.25989.14.camel@laptop> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12021016-3352-0000-0000-00000288B788 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 10, 2012 at 04:52:07PM +0100, Peter Zijlstra wrote: > On Thu, 2012-02-09 at 16:11 +0100, Ingo Molnar wrote: > > > > My understanding of the code is that when a CPU is taken > > > offline, it is removed from all the cpusets and then the > > > scan_for_empty_cpusets() function is run to move tasks from > > > empty cpusets to their parent cpusets. > > > > Why is that done that way? offlining a CPU should be an > > invariant as far as cpusets are concerned. > > Can't, tasks need to run someplace. There's two choices, add a still > online cpu to the now empty cpuset or move the tasks to a parent that > still has online cpus. > > Both are destructive. OK, I will ask the stupid question... Hey, somebody has to! ;-) Would it make sense for offlining the last CPU in a cpuset to be destructive, but to allow offlining of a non-last CPU to be reversible? For example, assume that cpuset A has CPUs 0 and 1, and cpuset B has 1, 2, and 3. Then offlining any single CPU and then onlining it would restore the cpusets to their original state. Offlining both CPUs 0 and 1 would be destructive to cpuset A, so that onlining those two CPUs would leave any tasks in cpuset A in some ancestor of cpuset A, and would leave cpuset A with no assigned CPUs. However, that same operation (offlining both CPUs 0 and 1, then onlining them) would restore cpuset B to its original state, covering CPUs 1, 2, and 3. /me ducks. ;-) Thanx, Paul