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

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Message-ID: <20190927081141.GB31660@localhost.localdomain>
Date:   Fri, 27 Sep 2019 10:11:41 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Scott Wood <swood@...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()

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?

Thanks,

Juri