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 CF138C433DF 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 AF4EF20657 for ; Sun, 16 Aug 2020 14:15:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729054AbgHPOL2 (ORCPT ); Sun, 16 Aug 2020 10:11:28 -0400 Received: from out30-130.freemail.mail.aliyun.com ([115.124.30.130]:49271 "EHLO out30-130.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726304AbgHPOL1 (ORCPT ); Sun, 16 Aug 2020 10:11:27 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R141e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e01355;MF=alex.shi@linux.alibaba.com;NM=1;PH=DS;RN=6;SR=0;TI=SMTPD_---0U5ux5MB_1597587062; Received: from IT-FVFX43SYHV2H.lan(mailfrom:alex.shi@linux.alibaba.com fp:SMTPD_---0U5ux5MB_1597587062) by smtp.aliyun-inc.com(127.0.0.1); Sun, 16 Aug 2020 22:11:03 +0800 Subject: Re: [PATCH 2/2] mm/pageblock: remove false sharing in pageblock_flags To: Matthew Wilcox Cc: Andrew Morton , Hugh Dickins , Alexander Duyck , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <1597549677-7480-1-git-send-email-alex.shi@linux.alibaba.com> <1597549677-7480-2-git-send-email-alex.shi@linux.alibaba.com> <20200816041720.GG17456@casper.infradead.org> From: Alex Shi Message-ID: <957eee62-1f46-49b6-4d5a-9671dc07c562@linux.alibaba.com> Date: Sun, 16 Aug 2020 22:10:06 +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: <20200816041720.GG17456@casper.infradead.org> Content-Type: text/plain; charset=gbk Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ÔÚ 2020/8/16 ÏÂÎç12:17, Matthew Wilcox дµÀ: > On Sun, Aug 16, 2020 at 11:47:57AM +0800, Alex Shi wrote: >> Current pageblock_flags is only 4 bits, so it has to share a char size >> in cmpxchg when get set, the false sharing cause perf drop. >> >> If we incrase the bits up to 8, false sharing would gone in cmpxchg. and >> the only cost is half char per pageblock, which is half char per 128MB >> on x86, 4 chars in 1 GB. > > I don't believe this patch has that effect, mostly because it still does > cmpxchg() on words instead of bytes. Hi Matthew, Thank a lot for comments! Sorry, I must overlook sth, would you like point out why the cmpxchg is still on words after patch 1 applied? > > But which functions would benefit? It seems to me this cmpxchg() is > only called from the set_pageblock_migratetype() morass of functions, > none of which are called in hot paths as far as I can make out. > > So are you just reasoning by analogy with the previous patch where you > have measured a performance improvement, or did you send the wrong patch, > or did I overlook a hot path that calls one of the pageblock migration > functions? > Uh, I am reading compaction.c and found the following commit introduced test_and_set_skip under a lock. It looks like the pagelock_flags setting has false sharing in cmpxchg. but I have no valid data on this yet. 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; + }