From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait() Date: Thu, 6 Jul 2017 10:29:16 -0700 Message-ID: <20170706172916.GK2393@linux.vnet.ibm.com> References: <20170629235918.GA6445@linux.vnet.ibm.com> <20170705232955.GA15992@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6DD0033F01@AcuExch.aculab.com> <20170706160555.xc63yydk77gmttae@hirez.programming.kicks-ass.net> <20170706162024.GD2393@linux.vnet.ibm.com> <20170706165036.v4u5rbz56si4emw5@hirez.programming.kicks-ass.net> <20170706170850.GI15574@arm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170706170850.GI15574@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Will Deacon Cc: Peter Zijlstra , David Laight , "linux-kernel@vger.kernel.org" , "netfilter-devel@vger.kernel.org" , "netdev@vger.kernel.org" , "oleg@redhat.com" , "akpm@linux-foundation.org" , "mingo@redhat.com" , "dave@stgolabs.net" , "manfred@colorfullife.com" , "tj@kernel.org" , "arnd@arndb.de" , "linux-arch@vger.kernel.org" , "stern@rowland.harvard.edu" , "parri.andrea@gmail.com" , "torvalds@linux-foundation.org" List-Id: linux-arch.vger.kernel.org On Thu, Jul 06, 2017 at 06:08:50PM +0100, Will Deacon wrote: > On Thu, Jul 06, 2017 at 06:50:36PM +0200, Peter Zijlstra wrote: > > On Thu, Jul 06, 2017 at 09:20:24AM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 06, 2017 at 06:05:55PM +0200, Peter Zijlstra wrote: > > > > On Thu, Jul 06, 2017 at 02:12:24PM +0000, David Laight wrote: > > > > > From: Paul E. McKenney > > > > > > [ . . . ] > > > > > > > Now on the one hand I feel like Oleg that it would be a shame to loose > > > > the optimization, OTOH this thing is really really tricky to use, > > > > and has lead to a number of bugs already. > > > > > > I do agree, it is a bit sad to see these optimizations go. So, should > > > this make mainline, I will be tagging the commits that spin_unlock_wait() > > > so that they can be easily reverted should someone come up with good > > > semantics and a compelling use case with compelling performance benefits. > > > > Ha!, but what would constitute 'good semantics' ? > > > > The current thing is something along the lines of: > > > > "Waits for the currently observed critical section > > to complete with ACQUIRE ordering such that it will observe > > whatever state was left by said critical section." > > > > With the 'obvious' benefit of limited interference on those actually > > wanting to acquire the lock, and a shorter wait time on our side too, > > since we only need to wait for completion of the current section, and > > not for however many contender are before us. > > > > Not sure I have an actual (micro) benchmark that shows a difference > > though. > > > > > > > > Is this all good enough to retain the thing, I dunno. Like I said, I'm > > conflicted on the whole thing. On the one hand its a nice optimization, > > on the other hand I don't want to have to keep fixing these bugs. > > As I've said, I'd be keen to see us drop this and bring it back if/when we > get a compelling use-case along with performance numbers. At that point, > we'd be in a better position to define the semantics anyway, knowing what > exactly is expected by the use-case. Hear, hear!!! ;-) Thanx, Paul From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:56003 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751813AbdGFR3Y (ORCPT ); Thu, 6 Jul 2017 13:29:24 -0400 Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v66HSgZ1090156 for ; Thu, 6 Jul 2017 13:29:23 -0400 Received: from e16.ny.us.ibm.com (e16.ny.us.ibm.com [129.33.205.206]) by mx0b-001b2d01.pphosted.com with ESMTP id 2bhqu8cv78-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 06 Jul 2017 13:29:23 -0400 Received: from localhost by e16.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 6 Jul 2017 13:29:23 -0400 Date: Thu, 6 Jul 2017 10:29:16 -0700 From: "Paul E. McKenney" Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait() Reply-To: paulmck@linux.vnet.ibm.com References: <20170629235918.GA6445@linux.vnet.ibm.com> <20170705232955.GA15992@linux.vnet.ibm.com> <063D6719AE5E284EB5DD2968C1650D6DD0033F01@AcuExch.aculab.com> <20170706160555.xc63yydk77gmttae@hirez.programming.kicks-ass.net> <20170706162024.GD2393@linux.vnet.ibm.com> <20170706165036.v4u5rbz56si4emw5@hirez.programming.kicks-ass.net> <20170706170850.GI15574@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170706170850.GI15574@arm.com> Message-ID: <20170706172916.GK2393@linux.vnet.ibm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Will Deacon Cc: Peter Zijlstra , David Laight , "linux-kernel@vger.kernel.org" , "netfilter-devel@vger.kernel.org" , "netdev@vger.kernel.org" , "oleg@redhat.com" , "akpm@linux-foundation.org" , "mingo@redhat.com" , "dave@stgolabs.net" , "manfred@colorfullife.com" , "tj@kernel.org" , "arnd@arndb.de" , "linux-arch@vger.kernel.org" , "stern@rowland.harvard.edu" , "parri.andrea@gmail.com" , "torvalds@linux-foundation.org" Message-ID: <20170706172916.GblxW9t25FVIY55oIPoLfYb2kh-i_EBg-C1Yw8eGS10@z> On Thu, Jul 06, 2017 at 06:08:50PM +0100, Will Deacon wrote: > On Thu, Jul 06, 2017 at 06:50:36PM +0200, Peter Zijlstra wrote: > > On Thu, Jul 06, 2017 at 09:20:24AM -0700, Paul E. McKenney wrote: > > > On Thu, Jul 06, 2017 at 06:05:55PM +0200, Peter Zijlstra wrote: > > > > On Thu, Jul 06, 2017 at 02:12:24PM +0000, David Laight wrote: > > > > > From: Paul E. McKenney > > > > > > [ . . . ] > > > > > > > Now on the one hand I feel like Oleg that it would be a shame to loose > > > > the optimization, OTOH this thing is really really tricky to use, > > > > and has lead to a number of bugs already. > > > > > > I do agree, it is a bit sad to see these optimizations go. So, should > > > this make mainline, I will be tagging the commits that spin_unlock_wait() > > > so that they can be easily reverted should someone come up with good > > > semantics and a compelling use case with compelling performance benefits. > > > > Ha!, but what would constitute 'good semantics' ? > > > > The current thing is something along the lines of: > > > > "Waits for the currently observed critical section > > to complete with ACQUIRE ordering such that it will observe > > whatever state was left by said critical section." > > > > With the 'obvious' benefit of limited interference on those actually > > wanting to acquire the lock, and a shorter wait time on our side too, > > since we only need to wait for completion of the current section, and > > not for however many contender are before us. > > > > Not sure I have an actual (micro) benchmark that shows a difference > > though. > > > > > > > > Is this all good enough to retain the thing, I dunno. Like I said, I'm > > conflicted on the whole thing. On the one hand its a nice optimization, > > on the other hand I don't want to have to keep fixing these bugs. > > As I've said, I'd be keen to see us drop this and bring it back if/when we > get a compelling use-case along with performance numbers. At that point, > we'd be in a better position to define the semantics anyway, knowing what > exactly is expected by the use-case. Hear, hear!!! ;-) Thanx, Paul