From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock Date: Thu, 23 Jan 2020 11:35:47 +0000 Message-ID: <20200123113547.GD18991@willie-the-truck> References: <20200115035920.54451-1-alex.kogan@oracle.com> <4e15fa1d-9540-3274-502a-4195a0d46f63@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4e15fa1d-9540-3274-502a-4195a0d46f63@redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Waiman Long Cc: Lihao Liang , Alex Kogan , linux@armlinux.org.uk, peterz@infradead.org, mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com, jglauber@marvell.com, dave.dice@oracle.com, steven.sistare@oracle.com, daniel.m.jordan@oracle.com List-Id: linux-arch.vger.kernel.org Hi folks, (I think Lihao is travelling at the moment, so he may be delayed in his replies) On Wed, Jan 22, 2020 at 12:24:58PM -0500, Waiman Long wrote: > On 1/22/20 6:45 AM, Lihao Liang wrote: > > On Wed, Jan 22, 2020 at 10:28 AM Alex Kogan wrote: > >> Summary > >> ------- > >> > >> Lock throughput can be increased by handing a lock to a waiter on the > >> same NUMA node as the lock holder, provided care is taken to avoid > >> starvation of waiters on other NUMA nodes. This patch introduces CNA > >> (compact NUMA-aware lock) as the slow path for qspinlock. It is > >> enabled through a configuration option (NUMA_AWARE_SPINLOCKS). > >> > > Thanks for your patches. The experimental results look promising! > > > > I understand that the new CNA qspinlock uses randomization to achieve > > long-term fairness, and provides the numa_spinlock_threshold parameter > > for users to tune. As Linux runs extremely diverse workloads, it is not > > clear how randomization affects its fairness, and how users with > > different requirements are supposed to tune this parameter. > > > > To this end, Will and I consider it beneficial to be able to answer the > > following question: > > > > With different values of numa_spinlock_threshold and > > SHUFFLE_REDUCTION_PROB_ARG, how long do threads running on different > > sockets have to wait to acquire the lock? This is particularly relevant > > in high contention situations when new threads keep arriving on the same > > socket as the lock holder. > > > > In this email, I try to provide some formal analysis to address this > > question. Let's assume the probability for the lock to stay on the > > same socket is *at least* p, which corresponds to the probability for > > the function probably(unsigned int num_bits) in the patch to return *false*, > > where SHUFFLE_REDUCTION_PROB_ARG is passed as the value of num_bits to the > > function. > > That is not strictly true from my understanding of the code. The > probably() function does not come into play if a secondary queue is > present. Also calling cna_scan_main_queue() doesn't guarantee that a > waiter in the same node can be found. So the simple mathematical > analysis isn't that applicable in this case. One will have to do an > actual simulation to find out what the actual behavior will be. It's certainly true that the analysis is based on the worst-case scenario, but I think it's still worth considering. For example, the secondary queue does not exist initially so it seems a bit odd that we only instantiate it with < 1% probability. That said, my real concern with any of this is that it makes formal modelling and analysis of the qspinlock considerably more challenging. I would /really/ like to see an update to the TLA+ model we have of the current implementation [1] and preferably also the userspace version I hacked together [2] so that we can continue to test and validate changes to the code outside of the usual kernel stress-testing. Will [1] https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/ [2] https://mirrors.edge.kernel.org/pub/linux/kernel/people/will/spinbench/ From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:48480 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726191AbgAWLfz (ORCPT ); Thu, 23 Jan 2020 06:35:55 -0500 Date: Thu, 23 Jan 2020 11:35:47 +0000 From: Will Deacon Subject: Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock Message-ID: <20200123113547.GD18991@willie-the-truck> References: <20200115035920.54451-1-alex.kogan@oracle.com> <4e15fa1d-9540-3274-502a-4195a0d46f63@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4e15fa1d-9540-3274-502a-4195a0d46f63@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Waiman Long Cc: Lihao Liang , Alex Kogan , linux@armlinux.org.uk, peterz@infradead.org, mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, bp@alien8.de, hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com, jglauber@marvell.com, dave.dice@oracle.com, steven.sistare@oracle.com, daniel.m.jordan@oracle.com Message-ID: <20200123113547.rsUjCwEJhL9QxrdS75810Z7HbQkJ2pwSP6VxRi_FSRk@z> Hi folks, (I think Lihao is travelling at the moment, so he may be delayed in his replies) On Wed, Jan 22, 2020 at 12:24:58PM -0500, Waiman Long wrote: > On 1/22/20 6:45 AM, Lihao Liang wrote: > > On Wed, Jan 22, 2020 at 10:28 AM Alex Kogan wrote: > >> Summary > >> ------- > >> > >> Lock throughput can be increased by handing a lock to a waiter on the > >> same NUMA node as the lock holder, provided care is taken to avoid > >> starvation of waiters on other NUMA nodes. This patch introduces CNA > >> (compact NUMA-aware lock) as the slow path for qspinlock. It is > >> enabled through a configuration option (NUMA_AWARE_SPINLOCKS). > >> > > Thanks for your patches. The experimental results look promising! > > > > I understand that the new CNA qspinlock uses randomization to achieve > > long-term fairness, and provides the numa_spinlock_threshold parameter > > for users to tune. As Linux runs extremely diverse workloads, it is not > > clear how randomization affects its fairness, and how users with > > different requirements are supposed to tune this parameter. > > > > To this end, Will and I consider it beneficial to be able to answer the > > following question: > > > > With different values of numa_spinlock_threshold and > > SHUFFLE_REDUCTION_PROB_ARG, how long do threads running on different > > sockets have to wait to acquire the lock? This is particularly relevant > > in high contention situations when new threads keep arriving on the same > > socket as the lock holder. > > > > In this email, I try to provide some formal analysis to address this > > question. Let's assume the probability for the lock to stay on the > > same socket is *at least* p, which corresponds to the probability for > > the function probably(unsigned int num_bits) in the patch to return *false*, > > where SHUFFLE_REDUCTION_PROB_ARG is passed as the value of num_bits to the > > function. > > That is not strictly true from my understanding of the code. The > probably() function does not come into play if a secondary queue is > present. Also calling cna_scan_main_queue() doesn't guarantee that a > waiter in the same node can be found. So the simple mathematical > analysis isn't that applicable in this case. One will have to do an > actual simulation to find out what the actual behavior will be. It's certainly true that the analysis is based on the worst-case scenario, but I think it's still worth considering. For example, the secondary queue does not exist initially so it seems a bit odd that we only instantiate it with < 1% probability. That said, my real concern with any of this is that it makes formal modelling and analysis of the qspinlock considerably more challenging. I would /really/ like to see an update to the TLA+ model we have of the current implementation [1] and preferably also the userspace version I hacked together [2] so that we can continue to test and validate changes to the code outside of the usual kernel stress-testing. Will [1] https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/ [2] https://mirrors.edge.kernel.org/pub/linux/kernel/people/will/spinbench/