From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751507AbdAYMQh (ORCPT ); Wed, 25 Jan 2017 07:16:37 -0500 Received: from merlin.infradead.org ([205.233.59.134]:53722 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751387AbdAYMQg (ORCPT ); Wed, 25 Jan 2017 07:16:36 -0500 Date: Wed, 25 Jan 2017 13:16:24 +0100 From: Peter Zijlstra To: Sebastian Andrzej Siewior Cc: Ingo Molnar , linux-kernel@vger.kernel.org, tglx@linutronix.de, Steven Rostedt Subject: Re: [PATCH] kernel/sched/rt: Add a rescheduling point Message-ID: <20170125121624.GM6500@twins.programming.kicks-ass.net> References: <20170124144006.29821-1-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170124144006.29821-1-bigeasy@linutronix.de> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 24, 2017 at 03:40:06PM +0100, Sebastian Andrzej Siewior wrote: > Since the change in commit fd7a4bed1835 ("sched, rt: Convert > switched_{from, to}_rt() / prio_changed_rt() to balance callbacks") we > don't reschedule a task under certain circumstances: > > Lets say taskA, SCHED_OTHER, is running on CPU0 (and it may run only on > CPU0) and holds a PI lock. This task is removed from the CPU because it > used up its time slice and another SCHED_OTHER task is running. TaskB on > CPU1 runs at RT priority and asks for the lock owned by taskA. This > results in a priority boost for taskA. TaskB goes to sleep until the > lock has been made available. TaskA is already runable (but not active) > so it receives no wake up. > The reality now is that taskA gets on the CPU once the scheduler decides > to remove the current task despite the fact that a high priority task is > enqueued and waiting. This may take a long time. > The desired behaviour is that CPU0 immediately reschedules after the > priority boost which made taskA the task with the lowest priority. > > Fixes: fd7a4bed1835 ("sched, rt: Convert switched_{from, to}_rt() / > prio_changed_rt() to balance callbacks") > Suggested-by: Peter Zijlstra > Signed-off-by: Sebastian Andrzej Siewior > --- > kernel/sched/rt.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 88254be118b0..cdba8d58dbc5 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -2198,10 +2198,10 @@ static void switched_to_rt(struct rq *rq, struct task_struct *p) > #ifdef CONFIG_SMP > if (tsk_nr_cpus_allowed(p) > 1 && rq->rt.overloaded) > queue_push_tasks(rq); > -#else > + else I killed that "else" as well, because the queue_push_tasks() can fail to actually push the task, in which case we'd still miss the preemption. > +#endif /* CONFIG_SMP */ > if (p->prio < rq->curr->prio) > resched_curr(rq); > -#endif /* CONFIG_SMP */ > } > } > > -- > 2.11.0 >