Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756217AbbGFPyL (ORCPT ); Mon, 6 Jul 2015 11:54:11 -0400 Received: from g2t2354.austin.hp.com ([15.217.128.53]:58409 "EHLO g2t2354.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753238AbbGFPnY (ORCPT ); Mon, 6 Jul 2015 11:43:24 -0400 From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Arnd Bergmann , Thomas Gleixner Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Will Deacon , Scott J Norton , Douglas Hatch , Waiman Long Subject: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency Date: Mon, 6 Jul 2015 11:43:04 -0400 Message-Id: <1436197386-58635-3-git-send-email-Waiman.Long@hp.com> X-Mailer: git-send-email 1.7.1 In-Reply-To: <1436197386-58635-1-git-send-email-Waiman.Long@hp.com> References: <1436197386-58635-1-git-send-email-Waiman.Long@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2207 Lines: 53 Currently, a reader will check first to make sure that the writer mode byte is cleared before incrementing the reader count. That waiting is not really necessary. It increases the latency in the reader/writer to reader transition and reduces readers performance. This patch eliminates that waiting. It also has the side effect of reducing the chance of writer lock stealing and improving the fairness of the lock. Using a locking microbenchmark, a 10-threads 5M locking loop of mostly readers (RW ratio = 10,000:1) has the following performance numbers in a Haswell-EX box: Kernel Locking Rate (Kops/s) ------ --------------------- 4.1.1 15,063,081 Patched 4.1.1 17,241,552 Signed-off-by: Waiman Long --- kernel/locking/qrwlock.c | 12 ++++-------- 1 files changed, 4 insertions(+), 8 deletions(-) diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c index 81bae99..ecd2d19 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -88,15 +88,11 @@ void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) arch_spin_lock(&lock->lock); /* - * At the head of the wait queue now, wait until the writer state - * goes to 0 and then try to increment the reader count and get - * the lock. It is possible that an incoming writer may steal the - * lock in the interim, so it is necessary to check the writer byte - * to make sure that the write lock isn't taken. + * At the head of the wait queue now, increment the reader count + * and wait until the writer, if it has the lock, has gone away. + * At ths stage, it is not possible for a writer to remain in the + * waiting state (_QW_WAITING). So there won't be any deadlock. */ - while (atomic_read(&lock->cnts) & _QW_WMASK) - cpu_relax_lowlatency(); - cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS; rspin_until_writer_unlock(lock, cnts); -- 1.7.1 -- 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/