From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 9 May 2007 00:50:12 +0200 From: Nick Piggin Subject: Re: [rfc] optimise unlock_page Message-ID: <20070508225012.GF20174@wotan.suse.de> References: <20070508113709.GA19294@wotan.suse.de> <20070508114003.GB19294@wotan.suse.de> <1178659827.14928.85.camel@localhost.localdomain> <20070508224124.GD20174@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070508224124.GD20174@wotan.suse.de> Sender: owner-linux-mm@kvack.org Return-Path: To: Benjamin Herrenschmidt Cc: linux-arch@vger.kernel.org, Andrew Morton , Linux Kernel Mailing List , Linux Memory Management List List-ID: 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org