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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 8B72CC433DF for ; Thu, 15 Oct 2020 09:08:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2DF7D2222E for ; Thu, 15 Oct 2020 09:08:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388688AbgJOJIq (ORCPT ); Thu, 15 Oct 2020 05:08:46 -0400 Received: from mx2.suse.de ([195.135.220.15]:50568 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726329AbgJOJIp (ORCPT ); Thu, 15 Oct 2020 05:08:45 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 2E5BCACC2; Thu, 15 Oct 2020 09:08:44 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id CE38A1E1338; Thu, 15 Oct 2020 11:08:43 +0200 (CEST) Date: Thu, 15 Oct 2020 11:08:43 +0200 From: Jan Kara To: NeilBrown Cc: Jan Kara , kernel test robot , Linus Torvalds , Andrew Morton , Christoph Hellwig , Trond Myklebust , Chuck Lever , LKML , lkp@lists.01.org, lkp@intel.com, ying.huang@intel.com, feng.tang@intel.com, zhengjun.xing@intel.com Subject: Re: [mm/writeback] 8d92890bd6: will-it-scale.per_process_ops -15.3% regression Message-ID: <20201015090843.GA7037@quack2.suse.cz> References: <20201014084706.GB11647@shao2-debian> <20201014101904.GA11144@quack2.suse.cz> <87pn5kfply.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pn5kfply.fsf@notabene.neil.brown.name> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 15-10-20 08:46:01, NeilBrown wrote: > On Wed, Oct 14 2020, Jan Kara wrote: > > > On Wed 14-10-20 16:47:06, kernel test robot wrote: > >> Greeting, > >> > >> FYI, we noticed a -15.3% regression of will-it-scale.per_process_ops due > >> to commit: > >> > >> commit: 8d92890bd6b8502d6aee4b37430ae6444ade7a8c ("mm/writeback: discard > >> NR_UNSTABLE_NFS, use NR_WRITEBACK instead") > >> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master > > > > Thanks for report but it doesn't quite make sense to me. If we omit > > reporting & NFS changes in that commit (which is code not excercised by > > this benchmark), what remains are changes like: > > > > nr_pages += node_page_state(pgdat, NR_FILE_DIRTY); > > - nr_pages += node_page_state(pgdat, NR_UNSTABLE_NFS); > > nr_pages += node_page_state(pgdat, NR_WRITEBACK); > > ... > > - nr_reclaimable = global_node_page_state(NR_FILE_DIRTY) + > > - global_node_page_state(NR_UNSTABLE_NFS); > > + nr_reclaimable = global_node_page_state(NR_FILE_DIRTY); > > ... > > - gdtc->dirty = global_node_page_state(NR_FILE_DIRTY) + > > - global_node_page_state(NR_UNSTABLE_NFS); > > + gdtc->dirty = global_node_page_state(NR_FILE_DIRTY); > > > > So if there's any negative performance impact of these changes, they're > > likely due to code alignment changes or something like that... So I don't > > think there's much to do here since optimal code alignment is highly specific > > to a particular CPU etc. > > I agree, it seems odd. > > Removing NR_UNSTABLE_NFS from enum node_stat_item would renumber all the > following value and would, I think, change NR_DIRTIED from 32 to 31. > Might that move something to a different cache line and change some > contention? Interesting theory, it could be possible. > That would be easy enough to test: just re-add NR_UNSTABLE_NFS. Yeah, easy enough to test. Patch for this is attached. 0-day people, can you check whether applying this patch changes anything in your perf numbers? > I have no experience reading will-it-scale results, but 15% does seem > like a lot. Well, will-it-scale is a micro-benchmark that usually runs in extremely parallel loads so 15% can be caused by fairly obscure issues like different code alignment of a hot loop, slightly different cache line sharing, or so... Honza -- Jan Kara SUSE Labs, CR