From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757130AbYEKNy5 (ORCPT ); Sun, 11 May 2008 09:54:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753321AbYEKNyi (ORCPT ); Sun, 11 May 2008 09:54:38 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:46445 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752843AbYEKNyh (ORCPT ); Sun, 11 May 2008 09:54:37 -0400 Date: Sun, 11 May 2008 15:54:14 +0200 From: Ingo Molnar To: Matthew Wilcox Cc: Sven Wegener , Linus Torvalds , "Zhang, Yanmin" , Andi Kleen , LKML , Alexander Viro , Andrew Morton , Thomas Gleixner , "H. Peter Anvin" Subject: Re: [git pull] scheduler fixes Message-ID: <20080511135414.GA2328@elte.hu> References: <20080508120130.GA2860@elte.hu> <20080508122802.GA4880@elte.hu> <20080508144316.GA9869@elte.hu> <20080508151028.GA12109@elte.hu> <20080511110306.GP19219@parisc-linux.org> <20080511114803.GA8289@parisc-linux.org> <20080511125049.GA22513@elte.hu> <20080511125216.GA25040@elte.hu> <20080511130226.GR19219@parisc-linux.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080511130226.GR19219@parisc-linux.org> User-Agent: Mutt/1.5.17 (2007-11-01) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Matthew Wilcox wrote: > > + /* > > + * Rotate sleepers - to make sure all of them get woken in case > > + * of parallel up()s: > > + */ > > + list_move_tail(&waiter->list, &sem->wait_list); > > Seems like extra cache line dirtying for no real gain over my > solution. the gain is rather obvious: two parallel up()s (or just up()s which come close enough after each other) will wake up two tasks in parallel. With your patch, the first guy wakes up and then it wakes up the second guy. I.e. your patch serializes the wakeup chain, mine keeps it parallel. the cache line dirtying is rather secondary to any solution - the first goal for any locking primitive is to get scheduling precise: to not wake up more tasks than optimal and to not wake up less tasks than optimal. i.e. can you see any conceptual hole in the patch below? Ingo --- kernel/semaphore.c | 6 ++++++ 1 file changed, 6 insertions(+) Index: linux/kernel/semaphore.c =================================================================== --- linux.orig/kernel/semaphore.c +++ linux/kernel/semaphore.c @@ -258,5 +258,11 @@ static noinline void __sched __up(struct { struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list, struct semaphore_waiter, list); + /* + * Rotate sleepers - to make sure all of them get woken in case + * of parallel up()s: + */ + list_move_tail(&waiter->list, &sem->wait_list); + wake_up_process(waiter->task); }