From: Alex Kogan <alex.kogan@oracle.com> To: Lihao Liang <lihaoliang@google.com> Cc: linux@armlinux.org.uk, Peter Zijlstra <peterz@infradead.org>, mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de, longman@redhat.com, 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, Will Deacon <will@kernel.org> Subject: Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock Date: Mon, 27 Jan 2020 11:01:33 -0500 [thread overview] Message-ID: <25401561-CD1F-4FDC-AED5-256EBE56B9F6@oracle.com> (raw) In-Reply-To: <CAC4j=Y--5UQR7Oc5n+sxAwLxd_PKi0Eb-7aiZjDTUW0FTJy8Tw@mail.gmail.com> Hi, Lihao. >>> >>>> This is particularly relevant >>>> in high contention situations when new threads keep arriving on the same >>>> socket as the lock holder. >>> In this case, the lock will stay on the same NUMA node/socket for >>> 2^numa_spinlock_threshold times, which is the worst case scenario if we >>> consider the long-term fairness. And if we have multiple nodes, it will take >>> up to 2^numa_spinlock_threshold X (nr_nodes - 1) + nr_cpus_per_node >>> lock transitions until any given thread will acquire the lock >>> (assuming 2^numa_spinlock_threshold > nr_cpus_per_node). >>> >> >> You're right that the latest version of the patch handles long-term fairness >> deterministically. >> >> As I understand it, the n-th thread in the main queue is guaranteed to >> acquire the lock after N lock handovers, where N is bounded by >> >> n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1) >> >> I'm not sure what role the variable nr_cpus_per_node plays in your analysis. >> >> Do I miss anything? >> > > If I understand correctly, there are two phases in the algorithm: > > MCS phase: when the secondary queue is empty, as explained in your emails, > the algorithm hands the lock to threads in the main queue in an FIFO order. > When probably(SHUFFLE_REDUCTION_PROB_ARG) returns false (with default > probability 1%), if the algorithm finds the first thread running on the same > socket as the lock holder in cna_scan_main_queue(), it enters the following > CNA phase Yep. When probably() returns false, we scan the main queue. If as the result of this scan the secondary queue becomes not empty, we enter what you call the CNA phase. > . > > CNA phase: when the secondary queue is not empty, the algorithm keeps > handing the lock to threads in the main queue that run on the same socket as > the lock holder. When 2^numa_spinlock_threshold is reached, it splices > the secondary queue to the front of the main queue. And we are back to the > MCS phase above. Correct. > For the n-th thread T in the main queue, the MCS phase handles threads that > arrived in the main queue before T. In high contention situations, the CNA > phase handles two kinds of threads: > > 1. Threads ahead of T that run on the same socket as the lock holder when > a transition from the MCS to CNA phase was made. Assume there are m such > threads. > > 2. Threads that keep arriving on the same socket as the lock holder. There > are at most 2^numa_spinlock_threshold of them. > > Then the number of lock handovers in the CNA phase is max(m, > 2^numa_spinlock_threshold). So the total number of lock handovers before T > acquires the lock is at most > > n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1) > > Please let me know if I misunderstand anything. I think you got it right (modulo nr_cpus_per_node instead of n, as mentioned in my other response). Regards, — Alex
WARNING: multiple messages have this Message-ID (diff)
From: Alex Kogan <alex.kogan@oracle.com> To: Lihao Liang <lihaoliang@google.com> Cc: linux-arch@vger.kernel.org, guohanjun@huawei.com, arnd@arndb.de, Peter Zijlstra <peterz@infradead.org>, dave.dice@oracle.com, jglauber@marvell.com, x86@kernel.org, will.deacon@arm.com, linux@armlinux.org.uk, steven.sistare@oracle.com, linux-kernel@vger.kernel.org, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, longman@redhat.com, tglx@linutronix.de, daniel.m.jordan@oracle.com, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v9 0/5] Add NUMA-awareness to qspinlock Date: Mon, 27 Jan 2020 11:01:33 -0500 [thread overview] Message-ID: <25401561-CD1F-4FDC-AED5-256EBE56B9F6@oracle.com> (raw) In-Reply-To: <CAC4j=Y--5UQR7Oc5n+sxAwLxd_PKi0Eb-7aiZjDTUW0FTJy8Tw@mail.gmail.com> Hi, Lihao. >>> >>>> This is particularly relevant >>>> in high contention situations when new threads keep arriving on the same >>>> socket as the lock holder. >>> In this case, the lock will stay on the same NUMA node/socket for >>> 2^numa_spinlock_threshold times, which is the worst case scenario if we >>> consider the long-term fairness. And if we have multiple nodes, it will take >>> up to 2^numa_spinlock_threshold X (nr_nodes - 1) + nr_cpus_per_node >>> lock transitions until any given thread will acquire the lock >>> (assuming 2^numa_spinlock_threshold > nr_cpus_per_node). >>> >> >> You're right that the latest version of the patch handles long-term fairness >> deterministically. >> >> As I understand it, the n-th thread in the main queue is guaranteed to >> acquire the lock after N lock handovers, where N is bounded by >> >> n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1) >> >> I'm not sure what role the variable nr_cpus_per_node plays in your analysis. >> >> Do I miss anything? >> > > If I understand correctly, there are two phases in the algorithm: > > MCS phase: when the secondary queue is empty, as explained in your emails, > the algorithm hands the lock to threads in the main queue in an FIFO order. > When probably(SHUFFLE_REDUCTION_PROB_ARG) returns false (with default > probability 1%), if the algorithm finds the first thread running on the same > socket as the lock holder in cna_scan_main_queue(), it enters the following > CNA phase Yep. When probably() returns false, we scan the main queue. If as the result of this scan the secondary queue becomes not empty, we enter what you call the CNA phase. > . > > CNA phase: when the secondary queue is not empty, the algorithm keeps > handing the lock to threads in the main queue that run on the same socket as > the lock holder. When 2^numa_spinlock_threshold is reached, it splices > the secondary queue to the front of the main queue. And we are back to the > MCS phase above. Correct. > For the n-th thread T in the main queue, the MCS phase handles threads that > arrived in the main queue before T. In high contention situations, the CNA > phase handles two kinds of threads: > > 1. Threads ahead of T that run on the same socket as the lock holder when > a transition from the MCS to CNA phase was made. Assume there are m such > threads. > > 2. Threads that keep arriving on the same socket as the lock holder. There > are at most 2^numa_spinlock_threshold of them. > > Then the number of lock handovers in the CNA phase is max(m, > 2^numa_spinlock_threshold). So the total number of lock handovers before T > acquires the lock is at most > > n - 1 + 2^numa_spinlock_threshold * (nr_nodes - 1) > > Please let me know if I misunderstand anything. I think you got it right (modulo nr_cpus_per_node instead of n, as mentioned in my other response). Regards, — Alex _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2020-01-27 16:02 UTC|newest] Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-01-15 3:59 [PATCH v9 0/5] Add NUMA-awareness to qspinlock Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-15 3:59 ` [PATCH v9 1/5] locking/qspinlock: Rename mcs lock/unlock macros and make them more generic Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-15 3:59 ` [PATCH v9 2/5] locking/qspinlock: Refactor the qspinlock slow path Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-15 3:59 ` [PATCH v9 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-23 9:26 ` Peter Zijlstra 2020-01-23 9:26 ` Peter Zijlstra 2020-01-23 9:26 ` Peter Zijlstra 2020-01-23 10:06 ` Peter Zijlstra 2020-01-23 10:06 ` Peter Zijlstra 2020-01-23 10:06 ` Peter Zijlstra 2020-01-23 10:16 ` Peter Zijlstra 2020-01-23 10:16 ` Peter Zijlstra 2020-01-23 10:16 ` Peter Zijlstra 2020-01-23 11:22 ` Will Deacon 2020-01-23 11:22 ` Will Deacon 2020-01-23 13:17 ` Peter Zijlstra 2020-01-23 13:17 ` Peter Zijlstra 2020-01-23 13:17 ` Peter Zijlstra 2020-01-23 14:15 ` Waiman Long 2020-01-23 14:15 ` Waiman Long 2020-01-23 15:29 ` Peter Zijlstra 2020-01-23 15:29 ` Peter Zijlstra 2020-01-23 15:29 ` Peter Zijlstra 2020-01-15 3:59 ` [PATCH v9 4/5] locking/qspinlock: Introduce starvation avoidance into CNA Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-01-23 19:55 ` Waiman Long 2020-01-23 19:55 ` Waiman Long 2020-01-23 20:39 ` Waiman Long 2020-01-23 20:39 ` Waiman Long 2020-01-23 23:39 ` Alex Kogan 2020-01-23 23:39 ` Alex Kogan 2020-01-15 3:59 ` [PATCH v9 5/5] locking/qspinlock: Introduce the shuffle reduction optimization " Alex Kogan 2020-01-15 3:59 ` Alex Kogan 2020-03-02 1:14 ` [locking/qspinlock] 7b6da71157: unixbench.score 8.4% improvement kernel test robot 2020-03-02 1:14 ` kernel test robot 2020-03-02 1:14 ` kernel test robot 2020-01-22 11:45 ` [PATCH v9 0/5] Add NUMA-awareness to qspinlock Lihao Liang 2020-01-22 11:45 ` Lihao Liang 2020-01-22 17:24 ` Waiman Long 2020-01-22 17:24 ` Waiman Long 2020-01-23 11:35 ` Will Deacon 2020-01-23 11:35 ` Will Deacon 2020-01-23 15:25 ` Waiman Long 2020-01-23 15:25 ` Waiman Long 2020-01-23 19:08 ` Waiman Long 2020-01-23 19:08 ` Waiman Long 2020-01-22 19:29 ` Alex Kogan 2020-01-22 19:29 ` Alex Kogan 2020-01-26 0:32 ` Lihao Liang 2020-01-26 0:32 ` Lihao Liang 2020-01-26 1:58 ` Lihao Liang 2020-01-26 1:58 ` Lihao Liang 2020-01-26 1:58 ` Lihao Liang 2020-01-27 16:01 ` Alex Kogan [this message] 2020-01-27 16:01 ` Alex Kogan 2020-01-29 1:39 ` Lihao Liang 2020-01-29 1:39 ` Lihao Liang 2020-01-27 6:16 ` Alex Kogan 2020-01-27 6:16 ` Alex Kogan 2020-01-24 22:24 ` Paul E. McKenney 2020-01-24 22:24 ` Paul E. McKenney [not found] ` <6AAE7FC6-F5DE-4067-8BC4-77F27948CD09@oracle.com> 2020-01-25 0:57 ` Paul E. McKenney 2020-01-25 0:57 ` Paul E. McKenney 2020-01-25 1:59 ` Waiman Long 2020-01-25 1:59 ` Waiman Long [not found] ` <adb4fb09-f374-4d64-096b-ba9ad8b35fd5@redhat.com> 2020-01-25 4:58 ` Paul E. McKenney 2020-01-25 4:58 ` Paul E. McKenney 2020-01-25 19:41 ` Waiman Long 2020-01-25 19:41 ` Waiman Long 2020-01-26 15:35 ` Paul E. McKenney 2020-01-26 15:35 ` Paul E. McKenney 2020-01-26 22:42 ` Paul E. McKenney 2020-01-26 22:42 ` Paul E. McKenney 2020-01-26 23:32 ` Paul E. McKenney 2020-01-26 23:32 ` Paul E. McKenney 2020-01-27 6:04 ` Alex Kogan 2020-01-27 6:04 ` Alex Kogan 2020-01-27 14:11 ` Waiman Long 2020-01-27 14:11 ` Waiman Long 2020-01-27 15:09 ` Paul E. McKenney 2020-01-27 15:09 ` Paul E. McKenney [not found] ` <9b3a3f16-5405-b6d1-d023-b85f4aab46dd@redhat.com> 2020-01-27 17:17 ` Waiman Long 2020-01-27 17:17 ` Waiman Long
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