Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752017AbaBKPv4 (ORCPT ); Tue, 11 Feb 2014 10:51:56 -0500 Received: from www.linutronix.de ([62.245.132.108]:42870 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751435AbaBKPvz (ORCPT ); Tue, 11 Feb 2014 10:51:55 -0500 Date: Tue, 11 Feb 2014 16:51:55 +0100 (CET) From: Thomas Gleixner To: Mel Gorman cc: Peter Zijlstra , Chris Mason , LKML , Sebastian Sewior Subject: Re: [RFC PATCH] futex: Remove requirement for lock_page in get_futex_key In-Reply-To: <20131030092611.GJ2400@suse.de> Message-ID: References: <20131029173814.GH2400@suse.de> <20131030092611.GJ2400@suse.de> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 30 Oct 2013, Mel Gorman wrote: > On Wed, Oct 30, 2013 at 09:45:31AM +0100, Thomas Gleixner wrote: > > On Tue, 29 Oct 2013, Mel Gorman wrote: > > > > > Thomas Gleixner and Peter Zijlstra discussed off-list that real-time users > > > currently have a problem with the page lock being contended for unbounded > > > periods of time during futex operations. The three of us discussed the > > > possibiltity that the page lock is unnecessary in this case because we are > > > not concerned with the usual races with reclaim and page cache updates. For > > > anonymous pages, the associated futex object is the mm_struct which does > > > not require the page lock. For inodes, we should be able to check under > > > RCU read lock if the page mapping is still valid to take a reference to > > > the inode. This just leaves one rare race that requires the page lock > > > in the slow path. This patch does not completely eliminate the page lock > > > but it should reduce contention in the majority of cases. > > > > > > Patch boots and futextest did not explode but I did no comparison > > > performance tests. Thomas, do you have details of the workload that > > > drove you to examine this problem? Alternatively, can you test it and > > > > The scenario is simple. All you need is a PSHARED futex. > > > > Task A > > get_futex_key() > > lock_page() > > > > ---> preemption > > > > Ok, so scaling numbers of threads doing something like multiple > consumers using FUTEX_WAIT and then all being woken should trigger it. > Should not be that hard to device a test if something in futextest does > not do it alreayd. > > > Now any other task trying to lock that page will have to wait until > > task A gets scheduled back in, which is an unbound time. > > > > It takes quite some time to reproduce, but I'll ask the people who > > have that workload to give it a try. > > > > Do please. I'd rather not sink time into trying to reproduce a hypothetical > problem when people who are already familiar with it can provide better > data. If it stays quiet for too long then I'll either use an existing > futextest, extend futextest or conclude that the problem was not major > in the first place if the users cannot be arsed testing a patch. Took some time, but the folks finally came around to give it a try and it fixes their problem. I did not explode either, but I doubt, that their workload can trigger any of the corner cases. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/