linux-kernel - Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in .migrate_task

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Date:   Fri, 27 Sep 2019 11:40:25 -0500
From:   Scott Wood <swood@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH RT 5/8] sched/deadline: Reclaim cpuset bandwidth in
 .migrate_task_rq()

On Fri, 2019-09-27 at 10:11 +0200, Juri Lelli wrote:
> Hi Scott,
> 
> On 27/07/19 00:56, Scott Wood wrote:
> > With the changes to migrate disabling, ->set_cpus_allowed() no longer
> > gets deferred until migrate_enable().  To avoid releasing the bandwidth
> > while the task may still be executing on the old CPU, move the
> > subtraction
> > to ->migrate_task_rq().
> > 
> > Signed-off-by: Scott Wood <swood@...hat.com>
> > ---
> >  kernel/sched/deadline.c | 67 +++++++++++++++++++++++-------------------
> > -------
> >  1 file changed, 31 insertions(+), 36 deletions(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index c18be51f7608..2f18d0cf1b56 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1606,14 +1606,42 @@ static void yield_task_dl(struct rq *rq)
> >  	return cpu;
> >  }
> >  
> > +static void free_old_cpuset_bw_dl(struct rq *rq, struct task_struct *p)
> > +{
> > +	struct root_domain *src_rd = rq->rd;
> > +
> > +	/*
> > +	 * Migrating a SCHED_DEADLINE task between exclusive
> > +	 * cpusets (different root_domains) entails a bandwidth
> > +	 * update. We already made space for us in the destination
> > +	 * domain (see cpuset_can_attach()).
> > +	 */
> > +	if (!cpumask_intersects(src_rd->span, p->cpus_ptr)) {
> > +		struct dl_bw *src_dl_b;
> > +
> > +		src_dl_b = dl_bw_of(cpu_of(rq));
> > +		/*
> > +		 * We now free resources of the root_domain we are migrating
> > +		 * off. In the worst case, sched_setattr() may temporary
> > fail
> > +		 * until we complete the update.
> > +		 */
> > +		raw_spin_lock(&src_dl_b->lock);
> > +		__dl_sub(src_dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
> > +		raw_spin_unlock(&src_dl_b->lock);
> > +	}
> > +}
> > +
> >  static void migrate_task_rq_dl(struct task_struct *p, int new_cpu
> > __maybe_unused)
> >  {
> >  	struct rq *rq;
> >  
> > -	if (p->state != TASK_WAKING)
> > +	rq = task_rq(p);
> > +
> > +	if (p->state != TASK_WAKING) {
> > +		free_old_cpuset_bw_dl(rq, p);
> 
> What happens if a DEADLINE task is moved between cpusets while it was
> sleeping? Don't we miss removing from the old cpuset if the task gets
> migrated on wakeup?

In that case set_task_cpu() is called by ttwu after setting state to
TASK_WAKING.  I guess it could be annoying if the task doesn't wake up for a
long time and therefore doesn't release the bandwidth until then.

-Scott