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

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Message-ID: <20150420144235.GX27490@worktop.programming.kicks-ass.net>
Date:	Mon, 20 Apr 2015 16:42:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <clm@...com>, Steven Rostedt <rostedt@...dmis.org>,
	fredrik.markstrom@...driver.com, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 1/2] sched: lockless wake-queues

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?
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