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SubjectRe: [PATCH v6 3/3] sched/rt: Check to push the task when changing its affinity
On Mon, 20 Apr 2015 16:22:48 +0800
Xunlei Pang <xlpang@126.com> wrote:


> + rq = task_rq(p);
> +
> + if (new_weight > 1 &&
> + rt_task(rq->curr) &&
> + rq->rt.rt_nr_total > 1 &&
> + !test_tsk_need_resched(rq->curr)) {
> + /*
> + * We own p->pi_lock and rq->lock. rq->lock might
> + * get released when doing direct pushing, however
> + * p->pi_lock is always held, so it's safe to assign
> + * new_mask and new_weight to p below.
> + */
> + if (!task_running(rq, p)) {
> + cpumask_copy(&p->cpus_allowed, new_mask);
> + p->nr_cpus_allowed = new_weight;
> + direct_push = 1;
> + } else if (cpumask_test_cpu(task_cpu(p), new_mask)) {
> + struct task_struct *next;
> +
> + cpumask_copy(&p->cpus_allowed, new_mask);
> + p->nr_cpus_allowed = new_weight;
> + if (!cpupri_find(&rq->rd->cpupri, p, NULL))
> + goto update;
> +
> + /*
> + * At this point, current task gets migratable most
> + * likely due to the change of its affinity, let's
> + * figure out if we can migrate it.
> + *
> + * Can we find any task with the same priority as
> + * current? To accomplish this, firstly we requeue
> + * current to the tail and peek next, then restore
> + * current to the head.

"current"? Don't you mean 'p'?


> + */
> + requeue_task_rt(rq, p, 0);
> + next = peek_next_task_rt(rq);
> + requeue_task_rt(rq, p, 1);

Actually, I'm totally confused by all this. Why are you looking at
next? The running task (current) should not be in the next_task_rt()
list to begin with. If p can not preempt current, and cpupri_find()
finds something for p to run on, then move it there, and be done with
it.

I also looked at set_cpus_allowed_ptr() and think we should probably
add a p->sched_class->pick_cpu() and do:

if (p->sched_class->pick_cpu)
dest_cpu = p->sched_class->pick_cpu(p, cpu_active_mask,
new_mask);
else
dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);

Hmm, maybe all this work should be done there, or have
do_set_cpus_allowed() return a value that states that the
sched_class->set_cpus_allowed() did all the work for us.

-- Steve

> + if (next != p && next->prio == p->prio) {
> + /*
> + * Target found, so let's reschedule to try
> + * and push current away.
> + */
> + requeue_task_rt(rq, next, 1);
> + preempt_push = 1;
> + }
> + }
> + }
> +
> +update:
> /*
> * Only update if the process changes its state from whether it
> * can migrate or not.
> */
> - if ((p->nr_cpus_allowed > 1) == (weight > 1))
> - return;
> -
> - rq = task_rq(p);
> + if ((old_weight > 1) == (new_weight > 1))
> + goto out;
>
> /*
> * The process used to be able to migrate OR it can now migrate
> */
> - if (weight <= 1) {
> + if (new_weight <= 1) {
> if (!task_current(rq, p))
> dequeue_pushable_task(rq, p);
> BUG_ON(!rq->rt.rt_nr_migratory);
> @@ -2129,6 +2184,12 @@ static void set_cpus_allowed_rt(struct task_struct *p,
> }
>
> update_rt_migration(&rq->rt);
> +
> +out:
> + if (direct_push)
> + push_rt_tasks(rq);
> + else if (preempt_push)
> + resched_curr(rq);
> }
>
> /* Assumes rq->lock is held */



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