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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 4BC71C0044D for ; Mon, 16 Mar 2020 17:40:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 27C1620674 for ; Mon, 16 Mar 2020 17:40:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732048AbgCPRkE (ORCPT ); Mon, 16 Mar 2020 13:40:04 -0400 Received: from mx2.suse.de ([195.135.220.15]:33108 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730437AbgCPRkD (ORCPT ); Mon, 16 Mar 2020 13:40:03 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 9AB4BAF00; Mon, 16 Mar 2020 17:40:02 +0000 (UTC) Subject: Re: [PATCH 2/2] mm: swap: use smp_mb__after_atomic() to order LRU bit set To: Yang Shi , shakeelb@google.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1584124476-76534-1-git-send-email-yang.shi@linux.alibaba.com> <1584124476-76534-2-git-send-email-yang.shi@linux.alibaba.com> From: Vlastimil Babka Message-ID: <3c13c484-8fbf-3c3a-fbe1-a40434869e55@suse.cz> Date: Mon, 16 Mar 2020 18:40:01 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <1584124476-76534-2-git-send-email-yang.shi@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/13/20 7:34 PM, Yang Shi wrote: > Memory barrier is needed after setting LRU bit, but smp_mb() is too > strong. Some architectures, i.e. x86, imply memory barrier with atomic > operations, so replacing it with smp_mb__after_atomic() sounds better, > which is nop on strong ordered machines, and full memory barriers on > others. With this change the vm-calability cases would perform better > on x86, I saw total 6% improvement with this patch and previous inline > fix. > > The test data (lru-file-readtwice throughput) against v5.6-rc4: > mainline w/ inline fix w/ both (adding this) > 150MB 154MB 159MB > > Fixes: 9c4e6b1a7027 ("mm, mlock, vmscan: no more skipping pagevecs") > Cc: Shakeel Butt > Cc: Vlastimil Babka > Signed-off-by: Yang Shi According to my understanding of Documentation/memory_barriers.txt this would be correct (but it might not say much :) Acked-by: Vlastimil Babka But i have some suggestions... > --- > mm/swap.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/mm/swap.c b/mm/swap.c > index cf39d24..118bac4 100644 > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -945,20 +945,20 @@ static void __pagevec_lru_add_fn(struct page *page, struct lruvec *lruvec, > * #0: __pagevec_lru_add_fn #1: clear_page_mlock > * > * SetPageLRU() TestClearPageMlocked() > - * smp_mb() // explicit ordering // above provides strict > + * MB() // explicit ordering // above provides strict Why MB()? That would be the first appareance of 'MB()' in the whole tree. I think it's fine keeping smp_mb()... > * // ordering > * PageMlocked() PageLRU() > * > * > * if '#1' does not observe setting of PG_lru by '#0' and fails > * isolation, the explicit barrier will make sure that page_evictable > - * check will put the page in correct LRU. Without smp_mb(), SetPageLRU > + * check will put the page in correct LRU. Without MB(), SetPageLRU ... same here ... > * can be reordered after PageMlocked check and can make '#1' to fail > * the isolation of the page whose Mlocked bit is cleared (#0 is also > * looking at the same page) and the evictable page will be stranded > * in an unevictable LRU. Only here I would note that SetPageLRU() is an atomic bitop so we can use the __after_atomic() variant. And I would move the actual SetPageLRU() call from above the comment here right before the barrier. > */ > - smp_mb(); > + smp_mb__after_atomic(); Thanks. > > if (page_evictable(page)) { > lru = page_lru(page); >