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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 4B5FAC433E0 for ; Mon, 11 Jan 2021 04:17:41 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id C14A7224B1 for ; Mon, 11 Jan 2021 04:17:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C14A7224B1 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 32BD66B0123; Sun, 10 Jan 2021 23:17:40 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 2B4278D0021; Sun, 10 Jan 2021 23:17:40 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 17D668D0020; Sun, 10 Jan 2021 23:17:40 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0139.hostedemail.com [216.40.44.139]) by kanga.kvack.org (Postfix) with ESMTP id EF6636B0123 for ; Sun, 10 Jan 2021 23:17:39 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id B32111F1B for ; Mon, 11 Jan 2021 04:17:39 +0000 (UTC) X-FDA: 77692185438.30.play14_4e06ac127509 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin30.hostedemail.com (Postfix) with ESMTP id 94766180B3AA7 for ; Mon, 11 Jan 2021 04:17:39 +0000 (UTC) X-HE-Tag: play14_4e06ac127509 X-Filterd-Recvd-Size: 4686 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf04.hostedemail.com (Postfix) with ESMTP for ; Mon, 11 Jan 2021 04:17:38 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 18F2B101E; Sun, 10 Jan 2021 20:17:38 -0800 (PST) Received: from [10.163.88.153] (unknown [10.163.88.153]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6BD2F3F719; Sun, 10 Jan 2021 20:17:36 -0800 (PST) Subject: Re: [PATCH 1/1] arm64: make section size configurable for memory hotplug To: David Hildenbrand , Sudarshan Rajagopalan , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <66f79b0c06602c22df4da8ff4a5c2b97c9275250.1609895500.git.sudaraja@codeaurora.org> <055b0aca-af60-12ad-cd68-e15440ade64b@arm.com> <3ae8c16d-50c4-c6cc-62b8-922cfc308c95@arm.com> <7939710a-5d03-de2b-73b2-bca472de431a@redhat.com> From: Anshuman Khandual Message-ID: <5138b97e-41f7-11c3-9a28-7fb2e2f5c387@arm.com> Date: Mon, 11 Jan 2021 09:47:56 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <7939710a-5d03-de2b-73b2-bca472de431a@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 1/8/21 9:00 PM, David Hildenbrand wrote: >> To summarize, the section size bits for each base page size config >> should always >> >> a. Avoid (MAX_ORDER - 1 + PAGE_SHIFT) > SECTION_SIZE_BITS > > Pageblocks must also always fall completely into a section. > >> >> b. Provide minimum possible section size for a given base page config to >> have increased agility during memory hotplug operations and reduced >> vmemmap wastage for sections with holes. > > OTOH, making the section size too small (e.g., 16MB) creates way to many > memory block devices in /sys/devices/system/memory/, and might consume > too many page->flags bits in the !vmemmap case. > > For bigger setups, we might, similar to x86-64 (e.g., >= 64 GiB), > determine the memory_block_size_bytes() at runtime (spanning multiple > sections then), once it becomes relevant. > >> >> c. Allow 4K base page configs to have PMD based vmemmap mappings > > Agreed. > >> >> Because CONFIG_FORCE_MAX_ZONEORDER is always defined on arm64 platform, >> the following would always avoid the condition (a) >> >> SECTION_SIZE_BITS (CONFIG_FORCE_MAX_ZONEORDER - 1 + PAGE_SHIFT) >> >> - 22 (11 - 1 + 12) for 4K pages >> - 24 (11 - 1 + 14) for 16K pages without THP >> - 25 (12 - 1 + 14) for 16K pages with THP >> - 26 (11 - 1 + 16) for 64K pages without THP >> - 29 (14 - 1 + 16) for 64K pages with THP >> >> Apart from overriding 4K base page size config to have 27 as section size >> bits, should not all other values be okay here ? But then wondering what >> benefit 128MB (27 bits) section size for 16K config would have ? OR the >> objective here is to match 16K page size config with default x86-64. > > We don't want to have sections that are too small. We don't want to have > sections that are too big :) > > Not sure if we really want to allow setting e.g., a section size of 4 > MB. That's just going to hurt. IMHO, something in the range of 64..256 > MB is quite a good choice, where possible. > >> >>> >>> (If we worry about the number of section bits in page->flags, we could >>> glue it to vmemmap support where that does not matter) >> >> Could you please elaborate ? Could smaller section size bits numbers like >> 22 or 24 create problems in page->flags without changing other parameters >> like NR_CPUS or NODES_SHIFT ? A quick test with 64K base page without THP > > Yes, in the !vmemmap case, we have to store the section_nr in there. > IIRC, it's less of an issue with section sizes like 128 MB. > >> i.e 26 bits in section size, fails to boot. > > 26 bits would mean 64 MB, no? Not sure if that's possible even without > THP (MAX_ORDER - 1, pageblock_order ...) on 64k pages. I'd assume 512 MB > is the lowest we can go. I'd assume this would crash :) > >> >> As you have suggested, probably constant defaults (128MB for 4K/16K, 512MB >> for 64K) might be better than depending on the CONFIG_FORCE_MAX_ZONEORDER, >> at least for now. > > That's also what I would prefer, keeping it simple. Okay sure, will send a RFC to begin with.