Re: [RFC PATCH v2 13/17] sched: Add core wide task selection and scheduling.

[Aaron Lu]

On Tue, Apr 23, 2019 at 04:18:18PM +0000, Vineeth Remanan Pillai wrote:
> +// XXX fairness/fwd progress conditions
> +static struct task_struct *
> +pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *max)
> +{
> +	struct task_struct *class_pick, *cookie_pick;
> +	unsigned long cookie = 0UL;
> +
> +	/*
> +	 * We must not rely on rq->core->core_cookie here, because we fail to reset
> +	 * rq->core->core_cookie on new picks, such that we can detect if we need
> +	 * to do single vs multi rq task selection.
> +	 */
> +
> +	if (max && max->core_cookie) {
> +		WARN_ON_ONCE(rq->core->core_cookie != max->core_cookie);
> +		cookie = max->core_cookie;
> +	}
> +
> +	class_pick = class->pick_task(rq);
> +	if (!cookie)
> +		return class_pick;
> +
> +	cookie_pick = sched_core_find(rq, cookie);
> +	if (!class_pick)
> +		return cookie_pick;
> +
> +	/*
> +	 * If class > max && class > cookie, it is the highest priority task on
> +	 * the core (so far) and it must be selected, otherwise we must go with
> +	 * the cookie pick in order to satisfy the constraint.
> +	 */
> +	if (cpu_prio_less(cookie_pick, class_pick) && core_prio_less(max, class_pick))

It apapears to me the cpu_prio_less(cookie_pick, class_pick) isn't
needed.

If cookie_pick is idle task, then cpu_prio_less(cookie_pick, class_pick)
is always true;
If cookie_pick is not idle task and has the same sched class as
class_pick, then class_pick is the best candidate to run accoring to
their sched class. In this case, cpu_prio_less(cookie_pick, class_pick)
shouldn't return false or it feels like a bug;
If cookie_pick is not idle task and has a different sched class as
class_pick:
 - if cookie_pick's sched class has higher priority than class_pick's
   sched class, then cookie_pick should have been selected in previous
   sched class iteration; and since its cookie matches with max,
   everything should have been finished already;
 - if cookie_pick's sched class has lower priority than class_pick's
   sched class, then cpu_prio_less(cookie_pick, class_pick) will still
   returns true.

So looks like cpu_prio_less(cookie_pick, class_pick) should always
return true and thus not needed.

> +		return class_pick;
> +
> +	return cookie_pick;
> +}