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 A0557ECDFBB for ; Wed, 18 Jul 2018 15:15:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6426320850 for ; Wed, 18 Jul 2018 15:15:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6426320850 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 S1731236AbeGRPyF (ORCPT ); Wed, 18 Jul 2018 11:54:05 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:52276 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730998AbeGRPyF (ORCPT ); Wed, 18 Jul 2018 11:54:05 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6C045401EF07; Wed, 18 Jul 2018 15:15:42 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-175.bos.redhat.com [10.18.17.175]) by smtp.corp.redhat.com (Postfix) with ESMTP id 273A9111E402; Wed, 18 Jul 2018 15:15:41 +0000 (UTC) Subject: Re: [PATCH v2] locking/rwsem: Take read lock immediate if queue empty with no writer To: Peter Zijlstra , Ingo Molnar , Will Deacon Cc: linux-kernel@vger.kernel.org, Mark Ray , Joe Mario , Scott Norton References: <1531506653-5244-1-git-send-email-longman@redhat.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Wed, 18 Jul 2018 11:15:40 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.0 MIME-Version: 1.0 In-Reply-To: <1531506653-5244-1-git-send-email-longman@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.78 on 10.11.54.3 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Wed, 18 Jul 2018 15:15:42 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.5]); Wed, 18 Jul 2018 15:15:42 +0000 (UTC) for IP:'10.11.54.3' DOMAIN:'int-mx03.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 On 07/13/2018 02:30 PM, Waiman Long wrote: > 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 */ Ping! Any comment on this one? Cheers, Longman