From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751722AbbINTTo (ORCPT ); Mon, 14 Sep 2015 15:19:44 -0400 Received: from g2t4618.austin.hp.com ([15.73.212.83]:36080 "EHLO g2t4618.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751438AbbINTTn (ORCPT ); Mon, 14 Sep 2015 15:19:43 -0400 Message-ID: <55F71DCD.9050101@hpe.com> Date: Mon, 14 Sep 2015 15:19:41 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: Ingo Molnar , Thomas Gleixner , "H. Peter Anvin" , x86@kernel.org, linux-kernel@vger.kernel.org, Scott J Norton , Douglas Hatch , Davidlohr Bueso Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt References: <1441996658-62854-1-git-send-email-Waiman.Long@hpe.com> <1441996658-62854-6-git-send-email-Waiman.Long@hpe.com> <20150914140456.GU18489@twins.programming.kicks-ass.net> In-Reply-To: <20150914140456.GU18489@twins.programming.kicks-ass.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/14/2015 10:04 AM, Peter Zijlstra wrote: > On Fri, Sep 11, 2015 at 02:37:37PM -0400, Waiman Long wrote: >> This patch allows one attempt for the lock waiter to steal the lock >> when entering the PV slowpath. This helps to reduce the performance >> penalty caused by lock waiter preemption while not having much of >> the downsides of a real unfair lock. > >> @@ -416,7 +414,8 @@ queue: >> * does not imply a full barrier. >> * >> */ > If it really were once, like the Changelog says it is, then you could > have simply added: > > if (pv_try_steal_lock(...)) > goto release; My previous mail has clarified where the lock stealing happen. Will add the necessary comment to the patch. > here, and not wrecked pv_wait_head() like you did. Note that if you do > it like this, you also do not need to play games with the hash, because > you'll never get into that situation. > >> - pv_wait_head(lock, node); >> + if (pv_wait_head_and_lock(lock, node, tail)) >> + goto release; >> while ((val = smp_load_acquire(&lock->val.counter))& _Q_LOCKED_PENDING_MASK) >> cpu_relax(); >> Because we need to use atomic op to get the lock, we can't use the native logic to do the acquire. I know it is kind of hacky, but I don't have a good alternative here. Cheers, Longman