Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753753AbaGDIxW (ORCPT ); Fri, 4 Jul 2014 04:53:22 -0400 Received: from g5t1626.atlanta.hp.com ([15.192.137.9]:14157 "EHLO g5t1626.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752012AbaGDIxU (ORCPT ); Fri, 4 Jul 2014 04:53:20 -0400 Message-ID: <1404463996.2457.10.camel@buesod1.americas.hpqcorp.net> Subject: Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation From: Davidlohr Bueso To: Jason Low Cc: Dave Chinner , Peter Zijlstra , Tim Chen , Ingo Molnar , linux-kernel@vger.kernel.org, Linus Torvalds Date: Fri, 04 Jul 2014 01:53:16 -0700 In-Reply-To: <1404457579.8764.161.camel@j-VirtualBox> References: <1404413420.8764.42.camel@j-VirtualBox> <20140704010147.GY4453@dastard> <1404438366.8764.121.camel@j-VirtualBox> <1404438890.8764.125.camel@j-VirtualBox> <20140704061306.GK9508@dastard> <1404457579.8764.161.camel@j-VirtualBox> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.4 (3.6.4-3.fc18) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2014-07-04 at 00:06 -0700, Jason Low > Subject: [PATCH] rwsem: In rwsem_can_spin_on_owner(), return false if no owner Could we change the subject to something more descriptive, ie: rwsem: Allow conservative optimistic spinning for reader/writer paths Thanks, Davidlohr > Commit 4fc828e24cd9 ("locking/rwsem: Support optimistic spinning") > introduced a major performance regression for workloads such as > xfs_repair which mix read and write locking of the mmap_sem across > many threads. The result was xfs_repair ran 5x slower on 3.16-rc2 > than on 3.15 and using 20x more system CPU time. > > Perf profiles indicate in some workloads that significant time can > be spent spinning on !owner. This is because we don't set the lock > owner when readers(s) obtain the rwsem. > > In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll > return false if there is no lock owner. The rationale is that if we > just entered the slowpath, yet there is no lock owner, then there is > a possibility that a reader has the lock. To be conservative, we'll > avoid spinning in these situations. > > This patch reduced the total run time of the xfs_repair workload from > about 4 minutes 24 seconds down to approximately 1 minute 26 seconds, > back to close to the same performance as on 3.15. > > Tested-by: Dave Chinner > Signed-off-by: Jason Low > --- > kernel/locking/rwsem-xadd.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c > index dacc321..c40c7d2 100644 > --- a/kernel/locking/rwsem-xadd.c > +++ b/kernel/locking/rwsem-xadd.c > @@ -285,10 +285,10 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem) > static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) > { > struct task_struct *owner; > - bool on_cpu = true; > + bool on_cpu = false; > > if (need_resched()) > - return 0; > + return false; > > rcu_read_lock(); > owner = ACCESS_ONCE(sem->owner); > @@ -297,9 +297,9 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) > rcu_read_unlock(); > > /* > - * If sem->owner is not set, the rwsem owner may have > - * just acquired it and not set the owner yet or the rwsem > - * has been released. > + * If sem->owner is not set, yet we have just recently entered the > + * slowpath, then there is a possibility reader(s) may have the lock. > + * To be safe, avoid spinning in these situations. > */ > return on_cpu; > } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/