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=-6.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,UNPARSEABLE_RELAY,USER_AGENT_SANE_1 autolearn=no 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 E1371C433E3 for ; Sun, 16 Aug 2020 14:15:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C111420735 for ; Sun, 16 Aug 2020 14:15:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729453AbgHPOPR (ORCPT ); Sun, 16 Aug 2020 10:15:17 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:60651 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729449AbgHPOPL (ORCPT ); Sun, 16 Aug 2020 10:15:11 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R131e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01358;MF=alex.shi@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0U5uJ-GC_1597587304; Received: from IT-FVFX43SYHV2H.lan(mailfrom:alex.shi@linux.alibaba.com fp:SMTPD_---0U5uJ-GC_1597587304) by smtp.aliyun-inc.com(127.0.0.1); Sun, 16 Aug 2020 22:15:05 +0800 Subject: Re: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags To: David Hildenbrand Cc: Andrew Morton , Hugh Dickins , Alexander Duyck , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1597549677-7480-1-git-send-email-alex.shi@linux.alibaba.com> From: Alex Shi Message-ID: <89b42892-8257-e3fc-34c9-27d9a470ed79@linux.alibaba.com> Date: Sun, 16 Aug 2020 22:14:08 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2020/8/16 下午8:16, David Hildenbrand 写道: > On 16.08.20 05:47, Alex Shi wrote: >> pageblock_flags is used as long, since every pageblock_flags is just 4 >> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags, >> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock >> flags. It would cause long waiting for sync. >> >> If we could change the pageblock_flags variable as char, we could use >> char size cmpxchg, which just sync up with 2 pageblock flags. it could >> relief much false sharing in cmpxchg. > > Do you have any performance numbers to back your claims? IOW, do we care > at all? > > Hi David, Thanks for comments! Not yet, I trace the following commit here, with this commit this hot path looks be resolved. Thanks Alex e380bebe4771548 mm, compaction: keep migration source private to a single compaction instance if (!locked) { locked = compact_trylock_irqsave(zone_lru_lock(zone), &flags, cc); - if (!locked) + + /* Allow future scanning if the lock is contended */ + if (!locked) { + clear_pageblock_skip(page); break; + } + + /* Try get exclusive access under lock */ + if (!skip_updated) { + skip_updated = true; + if (test_and_set_skip(cc, page, low_pfn)) + goto isolate_abort; + }