From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on archive.lwn.net X-Spam-Level: X-Spam-Status: No, score=-5.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by archive.lwn.net (Postfix) with ESMTP id 3275E7D581 for ; Wed, 12 Sep 2018 15:49:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727348AbeILUya (ORCPT ); Wed, 12 Sep 2018 16:54:30 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:44630 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727002AbeILUya (ORCPT ); Wed, 12 Sep 2018 16:54:30 -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 77E118011309; Wed, 12 Sep 2018 15:49:23 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-55.bos.redhat.com [10.18.17.55]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8F9664EC1; Wed, 12 Sep 2018 15:49:22 +0000 (UTC) Subject: Re: [PATCH v3 4/4] fs/dcache: Eliminate branches in nr_dentry_negative accounting To: Matthew Wilcox Cc: Alexander Viro , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, "Luis R. Rodriguez" , Kees Cook , Linus Torvalds , Jan Kara , "Paul E. McKenney" , Andrew Morton , Ingo Molnar , Miklos Szeredi , Larry Woodman , James Bottomley , "Wangkai (Kevin C)" , Michal Hocko References: <1536693506-11949-1-git-send-email-longman@redhat.com> <1536693506-11949-5-git-send-email-longman@redhat.com> <20180912023610.GB20056@bombadil.infradead.org> From: Waiman Long Organization: Red Hat Message-ID: Date: Wed, 12 Sep 2018 11:49:22 -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: <20180912023610.GB20056@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US 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]); Wed, 12 Sep 2018 15:49:23 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Wed, 12 Sep 2018 15:49:23 +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-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 09/11/2018 10:36 PM, Matthew Wilcox wrote: > On Tue, Sep 11, 2018 at 03:18:26PM -0400, Waiman Long wrote: >> Because the accounting of nr_dentry_negative depends on whether a dentry >> is a negative one or not, branch instructions are introduced to handle >> the accounting conditionally. That may potentially slow down the task >> by a noticeable amount if that introduces sizeable amount of additional >> branch mispredictions. >> >> To avoid that, the accounting code is now modified to use conditional >> move instructions instead, if supported by the architecture. > You're substituting your judgement here for the compiler's. I don't > see a reason why the compiler couldn't choose to use a cmov in order > to do this: > > if (dentry->d_flags & DCACHE_LRU_LIST) > this_cpu_inc(nr_dentry_negative); > > unless our macrology has got too clever for the compilre to see through > it. In which case, the right answer is to simplify the percpu code, > not to force the compiler to optimise the code in the way that makes > sense for your current microarchitecture. > I had actually looked at the x86 object file generated to verify that it did use cmove with the patch and use branch without. It is possible that there are other twists to make that happen with the above expression. I will need to run some experiments to figure it out. In the mean time, I am fine with dropping this patch as it is a micro-optimization that doesn't change the behavior at all. Cheers, Longman