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,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, 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 8A176C4332F for ; Mon, 20 Sep 2021 21:41:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B19261213 for ; Mon, 20 Sep 2021 21:41:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238430AbhITVnJ (ORCPT ); Mon, 20 Sep 2021 17:43:09 -0400 Received: from mga01.intel.com ([192.55.52.88]:46277 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232816AbhITVlI (ORCPT ); Mon, 20 Sep 2021 17:41:08 -0400 X-IronPort-AV: E=McAfee;i="6200,9189,10113"; a="245650238" X-IronPort-AV: E=Sophos;i="5.85,309,1624345200"; d="scan'208";a="245650238" Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Sep 2021 14:37:32 -0700 X-IronPort-AV: E=Sophos;i="5.85,309,1624345200"; d="scan'208";a="511542810" Received: from jkeelapa-mobl.amr.corp.intel.com (HELO [10.251.20.76]) ([10.251.20.76]) by fmsmga008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 20 Sep 2021 14:37:31 -0700 Subject: Re: [PATCH 1/2] mm/migrate: optimize hotplug-time demotion order updates To: "Huang, Ying" , Dave Hansen Cc: linux-kernel@vger.kernel.org, oliver.sang@intel.com, mhocko@suse.com, weixugc@google.com, osalvador@suse.de, rientjes@google.com, dan.j.williams@intel.com, david@redhat.com, gthelen@google.com, yang.shi@linux.alibaba.com, akpm@linux-foundation.org References: <20210917223504.C140445A@davehans-spike.ostc.intel.com> <20210917223505.F817CB6B@davehans-spike.ostc.intel.com> <87k0jeog7r.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <87k0jeog7r.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: multipart/mixed; boundary="------------B43186B552DFF601C99F8E08" Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is a multi-part message in MIME format. --------------B43186B552DFF601C99F8E08 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit On 9/17/21 5:55 PM, Huang, Ying wrote: >> @@ -3147,6 +3177,16 @@ static void __set_migration_target_nodes >> int node; >> >> /* >> + * The "migration path" array is heavily optimized >> + * for reads. This is the write side which incurs a >> + * very heavy synchronize_rcu(). Avoid this overhead >> + * when nothing of consequence has changed since the >> + * last write. >> + */ >> + if (!node_demotion_topo_changed()) >> + return; >> + >> + /* >> * Avoid any oddities like cycles that could occur >> * from changes in the topology. This will leave >> * a momentary gap when migration is disabled. > Now synchronize_rcu() is called in disable_all_migrate_targets(), which > is called for MEM_GOING_OFFLINE. Can we remove the synchronize_rcu() > from disable_all_migrate_targets() and call it in > __set_migration_target_nodes() before we update the node_demotion[]? I see what you are saying. This patch just targeted __set_migration_target_nodes() which is called in for MEM_ONLINE/OFFLINE. But, it missed MEM_GOING_OFFLINE's call to disable_all_migrate_targets(). I think I found something better than what I had in this patch, or the tweak you suggested: The 'memory_notify->status_change_nid' field is passed to all memory hotplug notifiers and tells us whether the node is going online/offline. Instead of trying to track the changes, I think we can simply rely on it to tell us when a node is going online/offline. This removes the need for the demotion code to track *any* state. I've attached a totally untested patch to do this. --------------B43186B552DFF601C99F8E08 Content-Type: text/x-patch; charset=UTF-8; name="faster-node-order.patch" Content-Transfer-Encoding: quoted-printable Content-Disposition: attachment; filename="faster-node-order.patch" diff -puN mm/migrate.c~faster-node-order mm/migrate.c --- a/mm/migrate.c~faster-node-order 2021-09-17 14:44:58.697476940 -0700 +++ b/mm/migrate.c 2021-09-20 14:31:43.570477095 -0700 @@ -3239,8 +3239,18 @@ static int migration_offline_cpu(unsigne * set_migration_target_nodes(). */ static int __meminit migrate_on_reclaim_callback(struct notifier_block *= self, - unsigned long action, void *arg) + unsigned long action, void *_arg) { + struct memory_notify *arg =3D _arg; + + /* + * Only update the node migration order when a node is + * changing status, like online->offline. This avoids + * the overhead of synchronize_rcu() in most cases. + */ + if (arg->status_change_nid < 0) + return notifier_from_errno(0); + switch (action) { case MEM_GOING_OFFLINE: /* _ --------------B43186B552DFF601C99F8E08--