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=-8.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS 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 3790FC433E7 for ; Wed, 14 Oct 2020 21:46:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D5D71221FF for ; Wed, 14 Oct 2020 21:46:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388086AbgJNVqN (ORCPT ); Wed, 14 Oct 2020 17:46:13 -0400 Received: from mx2.suse.de ([195.135.220.15]:52206 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726680AbgJNVqN (ORCPT ); Wed, 14 Oct 2020 17:46:13 -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 A2AB2AF49; Wed, 14 Oct 2020 21:46:11 +0000 (UTC) From: NeilBrown To: Jan Kara , kernel test robot Date: Thu, 15 Oct 2020 08:46:01 +1100 Cc: Linus Torvalds , Andrew Morton , Jan Kara , 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 In-Reply-To: <20201014101904.GA11144@quack2.suse.cz> References: <20201014084706.GB11647@shao2-debian> <20201014101904.GA11144@quack2.suse.cz> Message-ID: <87pn5kfply.fsf@notabene.neil.brown.name> MIME-Version: 1.0 Content-Type: multipart/signed; boundary="=-=-="; micalg=pgp-sha256; protocol="application/pgp-signature" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-=-= Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Wed, Oct 14 2020, Jan Kara wrote: > On Wed 14-10-20 16:47:06, kernel test robot wrote: >> Greeting, >>=20 >> FYI, we noticed a -15.3% regression of will-it-scale.per_process_ops due >> to commit: >>=20 >> 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 +=3D node_page_state(pgdat, NR_FILE_DIRTY); > - nr_pages +=3D node_page_state(pgdat, NR_UNSTABLE_NFS); > nr_pages +=3D node_page_state(pgdat, NR_WRITEBACK); > ... > - nr_reclaimable =3D global_node_page_state(NR_FILE_DIRTY) + > - global_node_page_state(NR_UNSTABL= E_NFS); > + nr_reclaimable =3D global_node_page_state(NR_FILE_DIRTY); > ... > - gdtc->dirty =3D global_node_page_state(NR_FILE_DIRTY) + > - global_node_page_state(NR_UNSTABLE_NFS); > + gdtc->dirty =3D 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 spec= ific > 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? That would be easy enough to test: just re-add NR_UNSTABLE_NFS. I have no experience reading will-it-scale results, but 15% does seem like a lot. NeilBrown --=-=-= Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQJCBAEBCAAsFiEEG8Yp69OQ2HB7X0l6Oeye3VZigbkFAl+HcZoOHG5laWxiQHN1 c2UuZGUACgkQOeye3VZigbktnQ//Wz7j0MLuDVxAX0s3N1XVhdNL1RjAH1dvVmsa c4OeECJNbY9O4OubfR7krntaLWPj8rzOGaQy/13/WR1FxvJWlsmv/NYcPoUxZLAx pe+Srawhz0KsZ2peL0kGdYXtg/Es7UV/2IrKIOv75WxGZvpj9q58e0H4wBpRIVwK avg6W0OX4Lb+0M98llSL38f0oXrZCDhA55VEBeRTgclEKZp9O49eUg5LPGxLhZfe PMFyUDYaL7Rq+jP4kMNvpikbJabi/jyMbYA/AnuL8mf6Hxiu9+SN4OX9CKUXH7pt CbqDnXhQNWIk+Wl31YP2Qm8qlLbIynY3RDh/Se7S2m/o4ZrT2dbpAG67G5LMhv2h tM9dBr1JBjOpuB0IW97prQ7zeR39RS3o7nbrqQLz/NyGmKfZJuIoXj/3gUj0Ilc8 0q1U0/MwpANNMjwqw6z58c/Ds9Z1qTrPvKfIF7fmFNw03prRHHUDu9BA2eT8SGBY WxfPN/8SoxOLg6hcADt1GSm6l8g/vuFaFx5ac/Y6CA1Fp+kanUXTES08pTM71FeW FxSsLPqtTUhWPsY3nKV0kDx9FACaDnW38J96FKwYB1h/5nLXa7LCwYsaDjjF+ax7 lmF8hWr39BD4dLs0GbSoQ/B+KP5+9lMSPK0LFSmf7B/nWhkmGnvEHgpan7m/j4tE HQLQrcU= =Pelj -----END PGP SIGNATURE----- --=-=-=--