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

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Date:	Thu, 15 May 2014 15:29:45 +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>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue
 lookups in unlock_page fastpath v4

On Thu, May 15, 2014 at 03:20:58PM +0200, Peter Zijlstra wrote:
> void __unlock_page(struct page *page)
> {
> 	struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(&page->flags, PG_locked);
> 	struct wait_queue_head_t *wqh = page_waitqueue(page);
> 	wait_queue_t *curr;

	if (!PG_waiters && !waitqueue_active(wqh))
		return;

> 	spin_lock_irq(&wqh->lock);
> 	list_for_each_entry(curr, &wqh->task_list, task_list) {
> 		unsigned int flags = curr->flags;
> 
> 		if (curr->func(curr, TASK_NORMAL, 0, &key))
> 			goto unlock;
> 	}
> 	ClearPageWaiters(page);
> unlock:
> 	spin_unlock_irq(&wqh->lock);
> }
> 
> Yes, the __unlock_page() will have the unconditional wqh->lock, but it
> should also call __unlock_page() a lot less, and it doesn't have that
> horrid timeout.
> 
> Now, the above is clearly sub-optimal when !extended_page_flags, but I
> suppose we could have two versions of __unlock_page() for that.

Or I suppose the above would fix it too.

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