From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753011AbcEQAqR (ORCPT ); Mon, 16 May 2016 20:46:17 -0400 Received: from g1t6225.austin.hp.com ([15.73.96.126]:52347 "EHLO g1t6225.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751625AbcEQAqP (ORCPT ); Mon, 16 May 2016 20:46:15 -0400 From: Jason Low To: Peter Zijlstra , Ingo Molnar Cc: linux-kernel@vger.kernel.org, Andrew Morton , Richard Henderson , Linus Torvalds , Ivan Kokshaysky , Matt Turner , Tony Luck , Fenghua Yu , Martin Schwidefsky , Terry Rudd , Heiko Carstens , Thomas Gleixner , Arnd Bergmann , Christoph Lameter , Davidlohr Bueso , Waiman Long , Tim Chen , Peter Hurley , Jason Low , Jason Low Subject: [RFC][PATCH 1/7] locking/rwsem: Optimize write lock by reducing operations in slowpath Date: Mon, 16 May 2016 17:38:00 -0700 Message-Id: <1463445486-16078-2-git-send-email-jason.low2@hpe.com> X-Mailer: git-send-email 1.9.1 In-Reply-To: <1463445486-16078-1-git-send-email-jason.low2@hpe.com> References: <1463445486-16078-1-git-send-email-jason.low2@hpe.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org When acquiring the rwsem write lock in the slowpath, we first try to set count to RWSEM_WAITING_BIAS. When that is successful, we then atomically add the RWSEM_WAITING_BIAS in cases where there are other tasks on the wait list. This causes write lock operations to often issue multiple atomic operations. We can instead make the list_is_singular() check first, and then set the count accordingly, so that we issue at most 1 atomic operation when acquiring the write lock and reduce unnecessary cacheline contention. Signed-off-by: Jason Low Acked-by: Waiman Long Acked-by: Davidlohr Bueso --- kernel/locking/rwsem-xadd.c | 25 ++++++++++++++++++------- 1 file changed, 18 insertions(+), 7 deletions(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 09e30c6..296d421 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -255,17 +255,28 @@ struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem) } EXPORT_SYMBOL(rwsem_down_read_failed); +/* + * This function must be called with the sem->wait_lock held to prevent + * race conditions between checking the rwsem wait list and setting the + * sem->count accordingly. + */ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem) { /* - * Try acquiring the write lock. Check count first in order - * to reduce unnecessary expensive cmpxchg() operations. + * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS. */ - if (count == RWSEM_WAITING_BIAS && - cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, - RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) { - if (!list_is_singular(&sem->wait_list)) - rwsem_atomic_update(RWSEM_WAITING_BIAS, sem); + if (count != RWSEM_WAITING_BIAS) + return false; + + /* + * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there + * are other tasks on the wait list, we need to add on WAITING_BIAS. + */ + count = list_is_singular(&sem->wait_list) ? + RWSEM_ACTIVE_WRITE_BIAS : + RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS; + + if (cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count) == RWSEM_WAITING_BIAS) { rwsem_set_owner(sem); return true; } -- 2.1.4