From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932388AbaISV0A (ORCPT ); Fri, 19 Sep 2014 17:26:00 -0400 Received: from casper.infradead.org ([85.118.1.10]:43475 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932234AbaISVZ6 (ORCPT ); Fri, 19 Sep 2014 17:25:58 -0400 Date: Fri, 19 Sep 2014 23:25:47 +0200 From: Peter Zijlstra To: Juri Lelli Cc: mingo@redhat.com, juri.lelli@gmail.com, raistlin@linux.it, michael@amarulasolutions.com, fchecconi@gmail.com, daniel.wagner@bmw-carit.de, vincent@legout.info, luca.abeni@unitn.it, linux-kernel@vger.kernel.org, Li Zefan , cgroups@vger.kernel.org Subject: Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating tasks between exclusive cpusets Message-ID: <20140919212547.GG2832@worktop.localdomain> References: <1411118561-26323-1-git-send-email-juri.lelli@arm.com> <1411118561-26323-3-git-send-email-juri.lelli@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1411118561-26323-3-git-send-email-juri.lelli@arm.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 19, 2014 at 10:22:40AM +0100, Juri Lelli wrote: > Exclusive cpusets are the only way users can restrict SCHED_DEADLINE tasks > affinity (performing what is commonly called clustered scheduling). > Unfortunately, such thing is currently broken for two reasons: > > - No check is performed when the user tries to attach a task to > an exlusive cpuset (recall that exclusive cpusets have an > associated maximum allowed bandwidth). > > - Bandwidths of source and destination cpusets are not correctly > updated after a task is migrated between them. > > This patch fixes both things at once, as they are opposite faces > of the same coin. > > The check is performed in cpuset_can_attach(), as there aren't any > points of failure after that function. The updated is split in two > halves. We first reserve bandwidth in the destination cpuset, after > we pass the check in cpuset_can_attach(). And we then release > bandwidth from the source cpuset when the task's affinity is > actually changed. Even if there can be time windows when sched_setattr() > may erroneously fail in the source cpuset, we are fine with it, as > we can't perfom an atomic update of both cpusets at once. The thing I cannot find is if we correctly deal with updates to the cpuset. Say we first setup 2 (exclusive) sets A:cpu0 B:cpu1-3. Then assign tasks and then update the cpu masks like: B:cpu2,3, A:cpu1,2.