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SubjectRe: [PATCH v10 5/9] sched/fair: Take into account latency priority at wakeup
On Fri, Jan 13, 2023 at 03:12:30PM +0100, Vincent Guittot wrote:
> @@ -6155,6 +6159,35 @@ static int sched_idle_cpu(int cpu)
> }
> #endif
>
> +static void set_next_buddy(struct sched_entity *se);
> +
> +static void check_preempt_from_others(struct cfs_rq *cfs, struct sched_entity *se)
> +{
> + struct sched_entity *next;
> +
> + if (se->latency_offset >= 0)
> + return;
> +
> + if (cfs->nr_running <= 1)
> + return;
> + /*
> + * When waking from another class, we don't need to check to preempt at
> + * wakeup and don't set next buddy as a candidate for being picked in
> + * priority.
> + * In case of simultaneous wakeup when current is another class, the
> + * latency sensitive tasks lost opportunity to preempt non sensitive
> + * tasks which woke up simultaneously.
> + */
> +
> + if (cfs->next)
> + next = cfs->next;
> + else
> + next = __pick_first_entity(cfs);
> +
> + if (next && wakeup_preempt_entity(next, se) == 1)
> + set_next_buddy(se);
> +}
> +
> /*
> * The enqueue_task method is called before nr_running is
> * increased. Here we update the fair scheduling stats and
> @@ -6241,14 +6274,15 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> if (!task_new)
> update_overutilized_status(rq);
>
> + if (rq->curr->sched_class != &fair_sched_class)
> + check_preempt_from_others(cfs_rq_of(&p->se), &p->se);
> +
> enqueue_throttle:
> assert_list_leaf_cfs_rq(rq);
>
> hrtick_update(rq);
> }

Hmm.. This sets a next selection when the task gets enqueued while not
running a fair task -- and looses a wakeup preemption opportunity.

Should we perhaps also do this for latency_nice == 0?, in any case I
think this can be moved to its own patch to avoid doing too much in the
one patch. It seems fairly self contained.

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