Re: [PATCH 1/2] sched: lockless wake-queues

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Apr 19, 2015 at 12:17:39PM -0700, Davidlohr Bueso wrote:
> +void wake_q_add(struct wake_q_head *head, struct task_struct *task)
> +{
> +	struct wake_q_node *node = &task->wake_q;
> +
> +	/*
> +	 * Atomically grab the task, if ->wake_q is !nil already it means
> +	 * its already queued (either by us or someone else) and will get the
> +	 * wakeup due to that.
> +	 *
> +	 * This cmpxchg() implies a full barrier, which pairs with the write
> +	 * barrier implied by the wakeup in wake_up_list().
> +	 */
> +	if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL))
> +		return;
> +
> +	get_task_struct(task);
> +
> +	/*
> +	 * The head is context local, there can be no concurrency.
> +	 */
> +	if (head->first == WAKE_Q_TAIL)
> +		head->first = node;
> +	else
> +		head->last->next = node;
> +
> +	head->last = node;
> +}

Do we want a sched_feat() that makes the above to an immediate wake-up
instead of the fancy thing? -- just for debuging/performance
measurements like things?