From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 89A30ECDFB1 for ; Fri, 13 Jul 2018 18:31:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4D6B02084E for ; Fri, 13 Jul 2018 18:31:06 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4D6B02084E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731950AbeGMSqu (ORCPT ); Fri, 13 Jul 2018 14:46:50 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:43612 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1731366AbeGMSqu (ORCPT ); Fri, 13 Jul 2018 14:46:50 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 952BC81A4EA5; Fri, 13 Jul 2018 18:31:03 +0000 (UTC) Received: from llong.com (dhcp-17-175.bos.redhat.com [10.18.17.175]) by smtp.corp.redhat.com (Postfix) with ESMTP id 839C81C72F; Fri, 13 Jul 2018 18:31:01 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon Cc: linux-kernel@vger.kernel.org, Mark Ray , Joe Mario , Scott Norton , Waiman Long Subject: [PATCH v2] locking/rwsem: Take read lock immediate if queue empty with no writer Date: Fri, 13 Jul 2018 14:30:53 -0400 Message-Id: <1531506653-5244-1-git-send-email-longman@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Fri, 13 Jul 2018 18:31:03 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Fri, 13 Jul 2018 18:31:03 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'longman@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org It was discovered that a constant stream of readers might cause the count to go negative most of the time after an initial trigger by a writer even if no writer was present afterward. As a result, most of the readers would have to go through the slowpath reducing their performance. To avoid that from happening, an additional check is added to detect the special case that the reader in the critical section is the only one in the wait queue and no writer is present. When that happens, it can just have the lock and return immediately without further action. Other incoming readers won't see a waiter is present and be forced into the slowpath. The additional code is in the slowpath and so should not have an impact on rwsem performance. However, in the special case listed above, it may greatly improve performance. The issue was found in a customer site where they had an application that pounded on the pread64 syscalls heavily on an XFS filesystem. The application was run in a recent 4-socket boxes with a lot of CPUs. They saw significant spinlock contention in the rwsem_down_read_failed() call. With this patch applied, the system CPU usage went from 85% to 57%, and the spinlock contention in the pread64 syscalls was gone. v2: Add customer testing results and remove wording that may cause confusion. Signed-off-by: Waiman Long --- kernel/locking/rwsem-xadd.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 3064c50..bf0570e 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -233,8 +233,19 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, waiter.type = RWSEM_WAITING_FOR_READ; raw_spin_lock_irq(&sem->wait_lock); - if (list_empty(&sem->wait_list)) + if (list_empty(&sem->wait_list)) { + /* + * In the unlikely event that the task is the only one in + * the wait queue and a writer isn't present, it can have + * the lock and return immediately without going through + * the remaining slowpath code. + */ + if (unlikely(atomic_long_read(&sem->count) >= 0)) { + raw_spin_unlock_irq(&sem->wait_lock); + return sem; + } adjustment += RWSEM_WAITING_BIAS; + } list_add_tail(&waiter.list, &sem->wait_list); /* we're now waiting on the lock, but no longer actively locking */ -- 1.8.3.1