From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Date: Fri, 18 Oct 2019 17:55:01 -0400 Message-ID: <3b8afb53-18e8-bd1d-9def-e1e7bdc73087@redhat.com> References: <20191016042903.61081-1-alex.kogan@oracle.com> <20191016042903.61081-4-alex.kogan@oracle.com> <6ce50aeb-6b87-5d1c-9011-4329e8dadfec@redhat.com> <1B59E517-D418-46DF-BC58-174BAFC5EC23@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <1B59E517-D418-46DF-BC58-174BAFC5EC23@oracle.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Alex Kogan Cc: linux@armlinux.org.uk, Peter Zijlstra , Ingo Molnar , Will Deacon , Arnd Bergmann , linux-arch@vger.kernel.org, linux-arm-kernel , linux-kernel@vger.kernel.org, Thomas Gleixner , Borislav Petkov , hpa@zytor.com, x86@kernel.org, Hanjun Guo , Jan Glauber , Steven Sistare , Daniel Jordan , dave.dice@oracle.com, Rahul Yadav List-Id: linux-arch.vger.kernel.org On 10/18/19 5:37 PM, Alex Kogan wrote: >> On Oct 18, 2019, at 12:03 PM, Waiman Long wrote: >> >> On 10/16/19 12:29 AM, Alex Kogan wrote: >>> +static inline void cna_pass_lock(struct mcs_spinlock *node, >>> + struct mcs_spinlock *next) >>> +{ >>> + struct cna_node *cn = (struct cna_node *)node; >>> + struct mcs_spinlock *next_holder = next, *tail_2nd; >>> + u32 val = 1; >>> + >>> + u32 scan = cn->pre_scan_result; >>> + >>> + /* >>> + * check if a successor from the same numa node has not been found in >>> + * pre-scan, and if so, try to find it in post-scan starting from the >>> + * node where pre-scan stopped (stored in @pre_scan_result) >>> + */ >>> + if (scan > 0) >>> + scan = cna_scan_main_queue(node, decode_tail(scan)); >>> + >>> + if (!scan) { /* if found a successor from the same numa node */ >>> + next_holder = node->next; >>> + /* >>> + * make sure @val gets 1 if current holder's @locked is 0 as >>> + * we have to store a non-zero value in successor's @locked >>> + * to pass the lock >>> + */ >>> + val = node->locked + (node->locked == 0); >> node->locked can be 0 when the cpu enters into an empty MCS queue. We >> could unconditionally set node->locked to 1 for this case in qspinlock.c >> or with your above code. > Right, I was doing that in the first two versions of the series. It adds > unnecessary store into @locked for non-CNA variants, and even if it does not > have any real performance implications, I think Peter did not like that (or, > at least, the comment I had to explain why we needed that store). > >> Perhaps, a comment about when node->locked will >> be 0. > Yeah, I was tinkering with this comment. Here is how it read in v3: > /* > * We unlock a successor by passing a non-zero value, > * so set @val to 1 iff @locked is 0, which will happen > * if we acquired the MCS lock when its queue was empty > */ > > I can change back to something like that if it is better. That looks OK. >> It may be easier to understand if you just do >> >> val = node->locked ? node->locked : 1; > You’re right, that’s another possibility. > However, it adds yet another if-statement on the critical path, which I was > trying to avoid that. Have you compared the generated assembly code if one is better than the other? I am OK with whatever one generates a better code, but often time cmove is used for ?: statements. If the same code is generated, I will prefer an easier to understand statement. Cheers, Longman From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:40610 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2394243AbfJRVzF (ORCPT ); Fri, 18 Oct 2019 17:55:05 -0400 Subject: Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock References: <20191016042903.61081-1-alex.kogan@oracle.com> <20191016042903.61081-4-alex.kogan@oracle.com> <6ce50aeb-6b87-5d1c-9011-4329e8dadfec@redhat.com> <1B59E517-D418-46DF-BC58-174BAFC5EC23@oracle.com> From: Waiman Long Message-ID: <3b8afb53-18e8-bd1d-9def-e1e7bdc73087@redhat.com> Date: Fri, 18 Oct 2019 17:55:01 -0400 MIME-Version: 1.0 In-Reply-To: <1B59E517-D418-46DF-BC58-174BAFC5EC23@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-arch-owner@vger.kernel.org List-ID: To: Alex Kogan Cc: linux@armlinux.org.uk, Peter Zijlstra , Ingo Molnar , Will Deacon , Arnd Bergmann , linux-arch@vger.kernel.org, linux-arm-kernel , linux-kernel@vger.kernel.org, Thomas Gleixner , Borislav Petkov , hpa@zytor.com, x86@kernel.org, Hanjun Guo , Jan Glauber , Steven Sistare , Daniel Jordan , dave.dice@oracle.com, Rahul Yadav Message-ID: <20191018215501.f2TODiww0qj_rWgyiFMRjzhltXvOTywRLCuBO8ixbFo@z> On 10/18/19 5:37 PM, Alex Kogan wrote: >> On Oct 18, 2019, at 12:03 PM, Waiman Long wrote: >> >> On 10/16/19 12:29 AM, Alex Kogan wrote: >>> +static inline void cna_pass_lock(struct mcs_spinlock *node, >>> + struct mcs_spinlock *next) >>> +{ >>> + struct cna_node *cn = (struct cna_node *)node; >>> + struct mcs_spinlock *next_holder = next, *tail_2nd; >>> + u32 val = 1; >>> + >>> + u32 scan = cn->pre_scan_result; >>> + >>> + /* >>> + * check if a successor from the same numa node has not been found in >>> + * pre-scan, and if so, try to find it in post-scan starting from the >>> + * node where pre-scan stopped (stored in @pre_scan_result) >>> + */ >>> + if (scan > 0) >>> + scan = cna_scan_main_queue(node, decode_tail(scan)); >>> + >>> + if (!scan) { /* if found a successor from the same numa node */ >>> + next_holder = node->next; >>> + /* >>> + * make sure @val gets 1 if current holder's @locked is 0 as >>> + * we have to store a non-zero value in successor's @locked >>> + * to pass the lock >>> + */ >>> + val = node->locked + (node->locked == 0); >> node->locked can be 0 when the cpu enters into an empty MCS queue. We >> could unconditionally set node->locked to 1 for this case in qspinlock.c >> or with your above code. > Right, I was doing that in the first two versions of the series. It adds > unnecessary store into @locked for non-CNA variants, and even if it does not > have any real performance implications, I think Peter did not like that (or, > at least, the comment I had to explain why we needed that store). > >> Perhaps, a comment about when node->locked will >> be 0. > Yeah, I was tinkering with this comment. Here is how it read in v3: > /* > * We unlock a successor by passing a non-zero value, > * so set @val to 1 iff @locked is 0, which will happen > * if we acquired the MCS lock when its queue was empty > */ > > I can change back to something like that if it is better. That looks OK. >> It may be easier to understand if you just do >> >> val = node->locked ? node->locked : 1; > You’re right, that’s another possibility. > However, it adds yet another if-statement on the critical path, which I was > trying to avoid that. Have you compared the generated assembly code if one is better than the other? I am OK with whatever one generates a better code, but often time cmove is used for ?: statements. If the same code is generated, I will prefer an easier to understand statement. Cheers, Longman