From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753623Ab3LTSX3 (ORCPT ); Fri, 20 Dec 2013 13:23:29 -0500 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.225]:52607 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751039Ab3LTSX1 (ORCPT ); Fri, 20 Dec 2013 13:23:27 -0500 Date: Fri, 20 Dec 2013 13:23:23 -0500 From: Steven Rostedt To: Peter Zijlstra Cc: tglx@linutronix.de, mingo@redhat.com, oleg@redhat.com, fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, linux-kernel@vger.kernel.org, claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, juri.lelli@gmail.com, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@gmail.com, liming.wang@windriver.com, jkacur@redhat.com Subject: Re: [PATCH 09/13] sched: Add bandwidth management for sched_dl Message-ID: <20131220132323.524cf2b1@gandalf.local.home> In-Reply-To: <20131220174200.GX16438@laptop.programming.kicks-ass.net> References: <20131217122720.950475833@infradead.org> <20131217123353.180539582@infradead.org> <20131218165508.GB30183@twins.programming.kicks-ass.net> <20131220171343.GL2480@laptop.programming.kicks-ass.net> <20131220123707.44fb7192@gandalf.local.home> <20131220174200.GX16438@laptop.programming.kicks-ass.net> X-Mailer: Claws Mail 3.9.2 (GTK+ 2.24.22; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.118:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 20 Dec 2013 18:42:00 +0100 Peter Zijlstra wrote: > On Fri, Dec 20, 2013 at 12:37:07PM -0500, Steven Rostedt wrote: > > On Fri, 20 Dec 2013 18:13:43 +0100 > > Peter Zijlstra wrote: > > > > > > > @@ -4985,6 +4942,23 @@ migration_call(struct notifier_block *nf > > > unsigned long flags; > > > struct rq *rq = cpu_rq(cpu); > > > > > > + switch (action) { > > > + case CPU_DOWN_PREPARE: /* explicitly allow suspend */ > > > + { > > > + struct dl_bw *dl_b = dl_bw_of(cpu); > > > + int cpus = dl_bw_cpus(cpu); > > > + bool overflow; > > > + > > > + raw_spin_lock_irqsave(&dl_b->lock, flags); > > > + overflow = __dl_overflow(dl_b, cpus-1, 0, 0); > > > + raw_spin_unlock_irqrestore(&dl_b->lock, flags); > > > + > > > + if (overflow) > > > + return notifier_from_errno(-EBUSY); > > > > Is it possible to have a race here to create a new deadline task that > > may work with cpus but not cpus-1? That is, if a new deadline task is > > currently being created as a CPU is going offline, this check happens > > first while the creation is spinning on the dl_b->lock, and it sets > > overflow to false, then once the lock is released, the new deadline > > task makes the condition true. > > > > Should the system call have a get_online_cpus() somewhere? > > No, should be all good; the entire admission control is serialized by > that dl_b->lock, and its a raw_spin_lock (as can be seen from the above) > which already very much excludes hotplug. I'm saying what stops this? CPU 0 CPU 1 ----- ----- sched_setattr() dl_overflow() cpus = __dl_span_weight() cpu_down() raw_spin_lock() raw_spin_lock() /* blocks */ overflow = __dl_overflow(cpus-1); raw_spin_unlock(); /* gets lock */ __dl_overflow(cpus) /* all OK! */ /* cpus goes to cpus - 1 making __dl_overflow() not OK anymore */ -- Steve