Re: [PATCH 08/20 v3] sched/fair: Add push task mechanism for fair

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 06, 2026 at 04:19:35PM +0100, Vincent Guittot wrote:

> +static bool __check_pushable_fair_task(struct rq *rq, struct task_struct *p)
> +{
> +	if (!task_on_rq_queued(p))
> +		return false;
> +
> +	if (p->se.sched_delayed)
> +		return false;
> +
> +	if (p->nr_cpus_allowed <= 1)
> +		return false;
> +
> +	return true;
> +}
> +
> +static bool fair_check_pushable_task(struct rq *rq, struct task_struct *p, struct task_struct *next)
> +{
> +	if (!__check_pushable_fair_task(rq, p))
> +		return false;
> +
> +	if (next == p)
> +		return false;
> +
> +	return false;
> +}

> +static void fair_remove_pushable_task(struct rq *rq, struct task_struct *p)
> +{
> +	if (sched_push_task_enabled())
> +		plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +}
> +
> +static void __fair_add_pushable_task(struct rq *rq, struct task_struct *p)
> +{
> +	plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +	plist_node_init(&p->pushable_tasks, p->prio);
> +	plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +}
> +
> +static void fair_add_pushable_prev(struct rq *rq, struct task_struct *prev,
> +				   struct task_struct *next)
> +{
> +	if (sched_push_task_enabled() && fair_check_pushable_task(rq, prev, next))
> +		__fair_add_pushable_task(rq, prev);
> +}


Can't we add the 'p->nr_cpus_allowed > 1' condition to rq->cfs_tasks and
use that, rather than adding yet another list? I mean, its used for
migration, so excluding all pinned tasks actually makes sense.

Also, I'm still not sure why we need a plist. The rq->cfs_tasks list is
MRU per set_next_task_fair() moving the current task to the front.

So reserve iterating rq->cfs_tasks, skipping blocked and current and
such, should get you much the same over-all.

That is, why can't push_task_fair() be something like:

	list_for_each_entry_reserve(p, rq->cfs_tasks, se.group_node) {

		if (!can_push_task(p,..))
			continue;

		...
	}

This would result in pushing the task that has not ran longest; which I
think makes most sense. Hmm?