From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating tasks between exclusive cpusets Date: Fri, 19 Sep 2014 23:25:47 +0200 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 Return-path: Content-Disposition: inline In-Reply-To: <1411118561-26323-3-git-send-email-juri.lelli-5wv7dgnIgG8@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Juri Lelli Cc: mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, juri.lelli-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, raistlin-k2GhghHVRtY@public.gmane.org, michael-dyjBcgdgk7Pe9wHmmfpqLFaTQe2KTcn/@public.gmane.org, fchecconi-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, daniel.wagner-98C5kh4wR6ohFhg+JK9F0w@public.gmane.org, vincent-9z8vmPu0pS/iB9QmIjCX8w@public.gmane.org, luca.abeni-3IIOeSMMxS4@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Li Zefan , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.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.