Commit de40f33e authored by Dietmar Eggemann's avatar Dietmar Eggemann Committed by Ingo Molnar
Browse files

sched/deadline: Reduce rq lock contention in dl_add_task_root_domain() 



dl_add_task_root_domain() is called during sched domain rebuild:

  rebuild_sched_domains_locked()
    partition_and_rebuild_sched_domains()
      rebuild_root_domains()
         for all top_cpuset descendants:
           update_tasks_root_domain()
             for all tasks of cpuset:
               dl_add_task_root_domain()

Change it so that only the task pi lock is taken to check if the task
has a SCHED_DEADLINE (DL) policy. In case that p is a DL task take the
rq lock as well to be able to safely de-reference root domain's DL
bandwidth structure.

Most of the tasks will have another policy (namely SCHED_NORMAL) and
can now bail without taking the rq lock.

One thing to note here: Even in case that there aren't any DL user
tasks, a slow frequency switching system with cpufreq gov schedutil has
a DL task (sugov) per frequency domain running which participates in DL
bandwidth management.

Signed-off-by: default avatarDietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Reviewed-by: default avatarQuentin Perret <qperret@google.com>
Reviewed-by: default avatarValentin Schneider <valentin.schneider@arm.com>
Reviewed-by: default avatarDaniel Bristot de Oliveira <bristot@redhat.com>
Acked-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20210119083542.19856-1-dietmar.eggemann@arm.com
parent b0d6d478

kernel/sched/deadline.c

+7 −4
Original line number Diff line number Diff line
@@ -2388,9 +2388,13 @@ void dl_add_task_root_domain(struct task_struct *p) 
	struct rq *rq;

	struct dl_bw *dl_b;



	rq = task_rq_lock(p, &rf);

	if (!dl_task(p))

		goto unlock;

	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);

	if (!dl_task(p)) {

		raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags);

		return;

	}



	rq = __task_rq_lock(p, &rf);



	dl_b = &rq->rd->dl_bw;

	raw_spin_lock(&dl_b->lock);
@@ -2399,7 +2403,6 @@ void dl_add_task_root_domain(struct task_struct *p) 


	raw_spin_unlock(&dl_b->lock);



unlock:

	task_rq_unlock(rq, p, &rf);

}