linux-kernel - Re: [rfc] optimise unlock

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Date:	Wed, 09 May 2007 07:30:27 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [rfc] optimise unlock_page

On Tue, 2007-05-08 at 13:40 +0200, Nick Piggin wrote:
> This patch trades a page flag for a significant improvement in the unlock_page
> fastpath. Various problems in the previous version were spotted by Hugh and
> Ben (and fixed in this one).
> 
> Comments?
> 
> --
> 
> Speed up unlock_page by introducing a new page flag to signal that there are
> page waitqueue waiters for PG_locked. This means a memory barrier and a random
> waitqueue hash cacheline load can be avoided in the fastpath when there is no
> contention.

I'm not 100% familiar with the exclusive vs. non exclusive wait thingy
but wake_up_page() does __wake_up_bit() which calls __wake_up() with
nr_exclusive set to 1. Doesn't that mean that only one waiter will be
woken up ?

If that's the case, then we lose because we'll have clear PG_waiters but
only wake up one of them.

Waking them all would fix it but at the risk of causing other
problems... Maybe PG_waiters need to actually be a counter but if that
is the case, then it complicates things even more.

Any smart idea ?

Ben.

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