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,URIBL_BLOCKED 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 33493C433F5 for ; Thu, 30 Aug 2018 13:08:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CDF1820658 for ; Thu, 30 Aug 2018 13:08:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org CDF1820658 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 S1728888AbeH3RK6 (ORCPT ); Thu, 30 Aug 2018 13:10:58 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:55198 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728686AbeH3RK6 (ORCPT ); Thu, 30 Aug 2018 13:10:58 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0343840200BA; Thu, 30 Aug 2018 13:08:49 +0000 (UTC) Received: from llong.com (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id B99202166B41; Thu, 30 Aug 2018 13:08:45 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon Cc: linux-kernel@vger.kernel.org, Joe Mario , Davidlohr Bueso , Waiman Long Subject: [PATCH RESEND v3] locking/rwsem: Exit read lock slowpath if queue empty & no writer Date: Thu, 30 Aug 2018 09:08:33 -0400 Message-Id: <1535634513-8961-1-git-send-email-longman@redhat.com> X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Thu, 30 Aug 2018 13:08:54 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Thu, 30 Aug 2018 13:08:54 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.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 with occassional writers pounding on a rwsem may cause many of the readers to enter the slowpath unnecessarily thus increasing latency and lowering performance. In the current code, a reader entering the slowpath critical section will unconditionally set the WAITING_BIAS, if not set yet, and clear its active count even if no one is in the wait queue and no writer is present. This causes some incoming readers to observe the presence of waiters in the wait queue and hence have to go into the slowpath themselves. With sufficient numbers of readers and a relatively short lock hold time, the WAITING_BIAS may be repeatedly turned on and off and a substantial portion of the readers will go into the slowpath sustaining a rather long queue in the wait queue spinlock and repeated WAITING_BIAS on/off cycle until the logjam is broken opportunistically. To avoid this situation 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 exit the slowpath and return immediately as its active count has already been set in the lock. Other incoming readers won't observe the presence of waiters and so will not be forced into the slowpath. 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 down from 85% to 57%, and the spinlock contention in the pread64 syscalls was gone. v3: Revise the commit log and comment again. v2: Add customer testing results and remove wording that may cause confusion. Signed-off-by: Waiman Long Reviewed-by: Davidlohr Bueso Acked-by: Will Deacon --- 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..01fcb80 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 case the wait queue is empty and the lock isn't owned + * by a writer, this reader can exit the slowpath and return + * immediately as its RWSEM_ACTIVE_READ_BIAS has already + * been set in the count. + */ + if (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