Re: [PATCH v10 2/5] sched: CGroup tagging interface for core scheduling

[Peter Zijlstra]

On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> +static void sched_core_update_cookie(struct task_struct *p, unsigned long cookie,
> +				     enum sched_core_cookie_type cookie_type)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +
> +	if (!p)
> +		return;
> +
> +	rq = task_rq_lock(p, &rf);
> +
> +	switch (cookie_type) {
> +	case sched_core_task_cookie_type:
> +		p->core_task_cookie = cookie;
> +		break;
> +	case sched_core_group_cookie_type:
> +		p->core_group_cookie = cookie;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +	}
> +
> +	/* Set p->core_cookie, which is the overall cookie */
> +	__sched_core_update_cookie(p);
> +
> +	if (sched_core_enqueued(p)) {
> +		sched_core_dequeue(rq, p);
> +		if (!p->core_cookie) {
> +			task_rq_unlock(rq, p, &rf);
> +			return;
> +		}
> +	}
> +
> +	if (sched_core_enabled(rq) &&
> +	    p->core_cookie && task_on_rq_queued(p))
> +		sched_core_enqueue(task_rq(p), p);
> +
> +	/*
> +	 * If task is currently running or waking, it may not be compatible
> +	 * anymore after the cookie change, so enter the scheduler on its CPU
> +	 * to schedule it away.
> +	 */
> +	if (task_running(rq, p) || p->state == TASK_WAKING)
> +		resched_curr(rq);

I'm not immediately seeing the need for that WAKING test. Since we're
holding it's rq->lock, the only place that task can be WAKING is on the
wake_list. And if it's there, it needs to acquire rq->lock to get
enqueued, and rq->lock again to get scheduled.

What am I missing?

> +
> +	task_rq_unlock(rq, p, &rf);
> +}