linux-kernel - Re: [rfc] optimise unlock

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Date:	Wed, 9 May 2007 00:41:24 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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 Wed, May 09, 2007 at 07:30:27AM +1000, Benjamin Herrenschmidt wrote:
> 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 ?

It will wake up 1 exclusive waiter, but no limit on non exclusive waiters.
Hmm, but it won't wake up waiters behind the exclusive guy... maybe the
wake up code can check whether the waitqueue is still active after the
wakeup, and set PG_waiters again in that case?
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