Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756126Ab0DFNbO (ORCPT ); Tue, 6 Apr 2010 09:31:14 -0400 Received: from mx1.redhat.com ([209.132.183.28]:24077 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755908Ab0DFNbH (ORCPT ); Tue, 6 Apr 2010 09:31:07 -0400 Message-ID: <4BBB377B.2020204@redhat.com> Date: Tue, 06 Apr 2010 16:30:35 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Darren Hart CC: linux-kernel@vger.kernel.org, Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Eric Dumazet , "Peter W. Morreale" , Rik van Riel , Steven Rostedt , Gregory Haskins , Sven-Thorsten Dietrich , Chris Mason , John Cooper , Chris Wright Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive spinning References: <1270499039-23728-1-git-send-email-dvhltc@us.ibm.com> <4BBA5305.7010002@redhat.com> <4BBA5C00.4090703@us.ibm.com> <4BBA6279.20802@redhat.com> <4BBA6F0B.2060008@us.ibm.com> In-Reply-To: <4BBA6F0B.2060008@us.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1989 Lines: 48 On 04/06/2010 02:15 AM, Darren Hart wrote: > Avi Kivity wrote: > >>>> An interesting (but perhaps difficult to achieve) optimization >>>> would be to spin in userspace. >>> >>> I couldn't think of a lightweight way to determine when the owner >>> has been scheduled out in userspace. Kernel assistance is required. >>> You could do this on the schedule() side of things, but I figured >>> I'd get some strong pushback if I tried to add a hook into >>> descheduling that flipped a bit in the futex value stating the owner >>> was about to deschedule(). Still, that might be something to explore. >> >> In the futex value it's hopeless (since a thread can hold many locks), > > It can, but there is a futex value per lock. If the task_struct had a > list of held futex locks (as it does for pi futex locks) the > deschedule() path could walk that and mark the FUTEX_OWNER_SLEEPING bit. > You don't want the context switch path to walk a list whose length is user controlled. >> but I don't think it's unreasonable to set a bit in the thread local >> storage area. The futex format would then need to be extended to >> contain a pointer to this bit. > > This appears to be 1 bit per task instead of 1 bit per lock. Yes. O(1) on context switch instead of O(n). > Also, the value is thread-specific... so how would a potential waiter > be able to determine if the owner of a particular lock was running or > not with this method? ... maybe I'm missing some core bit about > TLS... are you talking about pthread_key_create() and > pthread_getspecific() ? The lock would need to contain a pointer to the owning task. Could be set with cmpxchg{8,16}b on x86. -- error compiling committee.c: too many arguments to function -- 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/