Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765199AbZANRXW (ORCPT ); Wed, 14 Jan 2009 12:23:22 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763998AbZANRXE (ORCPT ); Wed, 14 Jan 2009 12:23:04 -0500 Received: from casper.infradead.org ([85.118.1.10]:36717 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760827AbZANRXB (ORCPT ); Wed, 14 Jan 2009 12:23:01 -0500 Subject: Re: [PATCH -v11][RFC] mutex: implement adaptive spinning From: Peter Zijlstra To: Nick Piggin Cc: Linus Torvalds , Ingo Molnar , "Paul E. McKenney" , Gregory Haskins , Matthew Wilcox , Andi Kleen , Chris Mason , Andrew Morton , Linux Kernel Mailing List , linux-fsdevel , linux-btrfs , Thomas Gleixner , Peter Morreale , Sven Dietrich , Dmitry Adamushko , Johannes Weiner In-Reply-To: <20090114171800.GA18621@wotan.suse.de> References: <1231774622.4371.96.camel@laptop> <1231859742.442.128.camel@twins> <1231863710.7141.3.camel@twins> <1231864854.7141.8.camel@twins> <1231867314.7141.16.camel@twins> <1231952436.14825.28.camel@laptop> <20090114171800.GA18621@wotan.suse.de> Content-Type: text/plain Date: Wed, 14 Jan 2009 18:22:36 +0100 Message-Id: <1231953757.14825.33.camel@laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.24.2 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1804 Lines: 50 On Wed, 2009-01-14 at 18:18 +0100, Nick Piggin wrote: > > @@ -173,21 +237,21 @@ __mutex_lock_common(struct mutex *lock, > > spin_unlock_mutex(&lock->wait_lock, flags); > > > > debug_mutex_free_waiter(&waiter); > > + preempt_enable(); > > return -EINTR; > > } > > __set_task_state(task, state); > > > > /* didnt get the lock, go to sleep: */ > > spin_unlock_mutex(&lock->wait_lock, flags); > > - schedule(); > > + __schedule(); > > Why does this need to do a preempt-disabled schedule? After we schedule > away, the next task can do arbitrary things or reschedule itself, so if > we have not anticipated such a condition here, then I can't see what > __schedule protects. At least a comment is in order? From: http://programming.kicks-ass.net/kernel-patches/mutex-adaptive-spin/mutex-preempt.patch Subject: mutex: preemption fixes From: Peter Zijlstra Date: Wed Jan 14 15:36:26 CET 2009 The problem is that dropping the spinlock right before schedule is a voluntary preemption point and can cause a schedule, right after which we schedule again. Fix this inefficiency by keeping preemption disabled until we schedule, do this by explicitly disabling preemption and providing a schedule() variant that assumes preemption is already disabled. Signed-off-by: Peter Zijlstra > Pity to add the call overhead to schedule just for this case. Good point, seeing any way around that? > BTW. __schedule shouldn't need to be asmlinkage? TBH I've no clue, probably not, Ingo? -- 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/