linux-kernel - Re: [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock

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Date:	Thu, 22 May 2014 12:56:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue
 lookups in unlock_page fastpath v7

On Thu, May 22, 2014 at 11:40:51AM +0100, Mel Gorman wrote:
> +void __wake_up_page_bit(wait_queue_head_t *wqh, struct page *page, void *word, int bit)
> +{
> +	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(word, bit);
> +	unsigned long flags;
> +
> +	/* If there is no PG_waiters bit, always take the slow path */

That comment is misleading, this is actually a fast path for
!PG_waiters.

> +	if (!__PG_WAITERS && waitqueue_active(wq)) {
> +		__wake_up(wq, TASK_NORMAL, 1, &key);
> +		return;
> +	}
> +
> +	/*
> +	 * Unlike __wake_up_bit it is necessary to check waitqueue_active to be
> +	 * checked under the wqh->lock to avoid races with parallel additions
> +	 * to the waitqueue. Otherwise races could result in lost wakeups
> +	 */
> +	spin_lock_irqsave(&wqh->lock, flags);
> +	if (waitqueue_active(wqh))
> +		__wake_up_common(wqh, TASK_NORMAL, 1, 0, &key);
> +	else
> +		ClearPageWaiters(page);
> +	spin_unlock_irqrestore(&wqh->lock, flags);
> +}

So I think you missed one Clear opportunity here that was in my original
proposal, possibly because you also frobbed PG_writeback in.

If you do:

	spin_lock_irqsave(&wqh->lock, flags);
	if (!waitqueue_active(wqh) || !__wake_up_common(wqh, TASK_NORMAL, 1, 0, &key))
		ClearPageWaiters(page);
	spin_unlock_irqrestore(&wqh->lock, flags);

With the below change to __wake_up_common(), we'll also clear the bit
when there's no waiters of @page, even if there's waiters for another
page.

I suppose the one thing to say for the big open coded loop is that its
much easier to read than this scattered stuff.

---
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 0ffa20ae657b..213c5bfe6b56 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -61,18 +61,23 @@ EXPORT_SYMBOL(remove_wait_queue);
  * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
  * zero in this (rare) case, and we handle it by continuing to scan the queue.
  */
-static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
+static bool __wake_up_common(wait_queue_head_t *q, unsigned int mode,
 			int nr_exclusive, int wake_flags, void *key)
 {
 	wait_queue_t *curr, *next;
+	bool woke = false;
 
 	list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
 		unsigned flags = curr->flags;
 
-		if (curr->func(curr, mode, wake_flags, key) &&
-				(flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
-			break;
+		if (curr->func(curr, mode, wake_flags, key)) {
+			woke = true;
+			if ((flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
+				break;
+		}
 	}
+
+	return woke;
 }
 
 /**



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