linux-kernel - Re: [rfc] optimise unlock

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Date:	Wed, 9 May 2007 00:50:12 +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 12:41:24AM +0200, Nick Piggin wrote:
> 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?

Hm, I don't know if we can do that without a race either...

OTOH, waking all non exclusive waiters may not be a really bad idea.

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