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 EB004C433E6 for ; Mon, 11 Jan 2021 10:26:41 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ABFF022795 for ; Mon, 11 Jan 2021 10:26:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729134AbhAKK0l (ORCPT ); Mon, 11 Jan 2021 05:26:41 -0500 Received: from foss.arm.com ([217.140.110.172]:52732 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729103AbhAKK0k (ORCPT ); Mon, 11 Jan 2021 05:26:40 -0500 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 A86DF101E; Mon, 11 Jan 2021 02:25:54 -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 119DA3F70D; Mon, 11 Jan 2021 02:25:52 -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> <5138b97e-41f7-11c3-9a28-7fb2e2f5c387@arm.com> <6764cfe0-00ad-20b5-7fc8-2c7d4170751f@redhat.com> From: Anshuman Khandual Message-ID: <72a04d85-439d-d224-230e-08450720e709@arm.com> Date: Mon, 11 Jan 2021 15:56:12 +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: <6764cfe0-00ad-20b5-7fc8-2c7d4170751f@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/11/21 3:43 PM, David Hildenbrand wrote: > On 11.01.21 05:17, Anshuman Khandual wrote: >> >> >> 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. >> > > Note that there is > > https://lkml.kernel.org/r/15cf9a2359197fee0168f820c5c904650d07939e.1610146597.git.sudaraja@codeaurora.org > > (Sudarshan missed to cc linux-mm) > Right, some how missed that. Anyways, ended up spending some time testing the change for various configs.