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=-7.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,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 64480C433E0 for ; Wed, 27 Jan 2021 10:18:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1913C20723 for ; Wed, 27 Jan 2021 10:18:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235735AbhA0KRz (ORCPT ); Wed, 27 Jan 2021 05:17:55 -0500 Received: from mail.kernel.org ([198.145.29.99]:44416 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S314289AbhAZXDw (ORCPT ); Tue, 26 Jan 2021 18:03:52 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 0F9EA2065E; Tue, 26 Jan 2021 23:03:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1611702191; bh=19FsxoKkPI3hC+GmGLglLDAnCCE1nO9FzM/AvjFKYZ8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=A8/tiYkCfUWHAYm7MedPDYLwvu6EjnZUJrUcd+8GIEKIUIqgPp/bzwAO8cMDDSazE YhUWZg/yQ2IAmE3I1bS3AnZxocq8CyFID3aazvRqRDRWxhz9b5KZhlZEO9Z5y1BHcr /ehxxw3FAh4VGVfvyIoLZ5eQpYfsOB9jL6000nDmk+21Ai/cpdzhHD7eVY/nIsBAXN h19/6wvm2UF9oVRoJom44JSh6OEgXrtR1Mf0Sv1Li5tSCl0avY+caqnngDyhm3JhaY NlDbKVPLwEXvcENdLgkeSw0ZA1SChsZkZIwm+/AiiZ9UHtFekyVj1iwwS1Q/AL7/pu ppy5xK/NO4nUQ== Date: Tue, 26 Jan 2021 23:03:06 +0000 From: Will Deacon To: Vlastimil Babka Cc: Vincent Guittot , Bharata B Rao , Christoph Lameter , linux-kernel , linux-mm@kvack.org, David Rientjes , Joonsoo Kim , Andrew Morton , guro@fb.com, Shakeel Butt , Johannes Weiner , aneesh.kumar@linux.ibm.com, Jann Horn , Michal Hocko , Catalin Marinas Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order Message-ID: <20210126230305.GD30941@willie-the-truck> References: <20201118082759.1413056-1-bharata@linux.ibm.com> <20210121053003.GB2587010@in.ibm.com> <786571e7-b9a2-4cdb-06d5-aa4a4b439b7e@suse.cz> <20210123051607.GC2587010@in.ibm.com> <66652406-25e4-a9e7-45a1-8ad14d2e8a36@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <66652406-25e4-a9e7-45a1-8ad14d2e8a36@suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 25, 2021 at 12:20:14PM +0100, Vlastimil Babka wrote: > On 1/23/21 1:32 PM, Vincent Guittot wrote: > >> PowerPC PowerNV Host: (160 cpus) > >> num_online_cpus 1 num_present_cpus 160 num_possible_cpus 160 nr_cpu_ids 160 > >> > >> PowerPC pseries KVM guest: (-smp 16,maxcpus=160) > >> num_online_cpus 1 num_present_cpus 16 num_possible_cpus 160 nr_cpu_ids 160 > >> > >> That's what I see on powerpc, hence I thought num_present_cpus() could > >> be the correct one to use in slub page order calculation. > > > > num_present_cpus() is set to 1 on arm64 until secondaries cpus boot > > > > arm64 224cpus acpi host: > > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 224 nr_cpu_ids 224 > > arm64 8cpus DT host: > > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8 > > arm64 8cpus qemu-system-aarch64 (-smp 8,maxcpus=256) > > num_online_cpus 1 num_present_cpus 1 num_possible_cpus 8 nr_cpu_ids 8 > > I would have expected num_present_cpus to be 224, 8, 8, respectively. > > > Then present and online increase to num_possible_cpus once all cpus are booted > > > >> > >> > > >> > What about heuristic: > >> > - num_online_cpus() > 1 - we trust that and use it > >> > - otherwise nr_cpu_ids > >> > Would that work? Too arbitrary? > >> > >> Looking at the following snippet from include/linux/cpumask.h, it > >> appears that num_present_cpus() should be reasonable compromise > >> between online and possible/nr_cpus_ids to use here. > >> > >> /* > >> * The following particular system cpumasks and operations manage > >> * possible, present, active and online cpus. > >> * > >> * cpu_possible_mask- has bit 'cpu' set iff cpu is populatable > >> * cpu_present_mask - has bit 'cpu' set iff cpu is populated > >> * cpu_online_mask - has bit 'cpu' set iff cpu available to scheduler > >> * cpu_active_mask - has bit 'cpu' set iff cpu available to migration > >> * > >> * If !CONFIG_HOTPLUG_CPU, present == possible, and active == online. > >> * > >> * The cpu_possible_mask is fixed at boot time, as the set of CPU id's > >> * that it is possible might ever be plugged in at anytime during the > >> * life of that system boot. The cpu_present_mask is dynamic(*), > >> * representing which CPUs are currently plugged in. And > >> * cpu_online_mask is the dynamic subset of cpu_present_mask, > >> * indicating those CPUs available for scheduling. > >> * > >> * If HOTPLUG is enabled, then cpu_possible_mask is forced to have > >> * all NR_CPUS bits set, otherwise it is just the set of CPUs that > >> * ACPI reports present at boot. > >> * > >> * If HOTPLUG is enabled, then cpu_present_mask varies dynamically, > >> * depending on what ACPI reports as currently plugged in, otherwise > >> * cpu_present_mask is just a copy of cpu_possible_mask. > >> * > >> * (*) Well, cpu_present_mask is dynamic in the hotplug case. If not > >> * hotplug, it's a copy of cpu_possible_mask, hence fixed at boot. > >> */ > >> > >> So for host systems, present is (usually) equal to possible and for > > > > But "cpu_present_mask varies dynamically, depending on what ACPI > > reports as currently plugged in" > > > > So it should varies when secondaries cpus are booted > > Hm, but booting the secondaries is just a software (kernel) action? They are > already physically there, so it seems to me as if the cpu_present_mask is not > populated correctly on arm64, and it's just a mirror of cpu_online_mask? I think the present_mask retains CPUs if they are hotplugged off, whereas the online mask does not. We can't really do any better on arm64, as there's no way of telling that a CPU is present until we've seen it. Will