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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DBBABEB64DC for ; Thu, 15 Jun 2023 03:58:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236869AbjFOD6u (ORCPT ); Wed, 14 Jun 2023 23:58:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55602 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229711AbjFOD6p (ORCPT ); Wed, 14 Jun 2023 23:58:45 -0400 Received: from out30-131.freemail.mail.aliyun.com (out30-131.freemail.mail.aliyun.com [115.124.30.131]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 515802122 for ; Wed, 14 Jun 2023 20:58:44 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R201e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018046059;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=7;SR=0;TI=SMTPD_---0Vl8reEi_1686801520; Received: from 30.97.48.67(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0Vl8reEi_1686801520) by smtp.aliyun-inc.com; Thu, 15 Jun 2023 11:58:41 +0800 Message-ID: <5f340d98-4ee0-35a9-58ed-943834d68042@linux.alibaba.com> Date: Thu, 15 Jun 2023 11:59:00 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.11.2 Subject: Re: [PATCH v2] mm: compaction: skip memory hole rapidly when isolating migratable pages To: "Huang, Ying" , Mel Gorman , david@redhat.com Cc: akpm@linux-foundation.org, vbabka@suse.cz, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <770f9f61472b24b6bc89adbd71a77d9cf62bb54f.1686646361.git.baolin.wang@linux.alibaba.com> <20230614095501.m4porztaibchrgwx@techsingularity.net> <87ilbpo1d9.fsf@yhuang6-desk2.ccr.corp.intel.com> From: Baolin Wang In-Reply-To: <87ilbpo1d9.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/15/2023 11:22 AM, Huang, Ying wrote: > Hi, Mel, > > Mel Gorman writes: > >> On Tue, Jun 13, 2023 at 04:55:04PM +0800, Baolin Wang wrote: >>> On some machines, the normal zone can have a large memory hole like >>> below memory layout, and we can see the range from 0x100000000 to >>> 0x1800000000 is a hole. So when isolating some migratable pages, the >>> scanner can meet the hole and it will take more time to skip the large >>> hole. From my measurement, I can see the isolation scanner will take >>> 80us ~ 100us to skip the large hole [0x100000000 - 0x1800000000]. >>> >>> So adding a new helper to fast search next online memory section >>> to skip the large hole can help to find next suitable pageblock >>> efficiently. With this patch, I can see the large hole scanning only >>> takes < 1us. >>> >>> [ 0.000000] Zone ranges: >>> [ 0.000000] DMA [mem 0x0000000040000000-0x00000000ffffffff] >>> [ 0.000000] DMA32 empty >>> [ 0.000000] Normal [mem 0x0000000100000000-0x0000001fa7ffffff] >>> [ 0.000000] Movable zone start for each node >>> [ 0.000000] Early memory node ranges >>> [ 0.000000] node 0: [mem 0x0000000040000000-0x0000000fffffffff] >>> [ 0.000000] node 0: [mem 0x0000001800000000-0x0000001fa3c7ffff] >>> [ 0.000000] node 0: [mem 0x0000001fa3c80000-0x0000001fa3ffffff] >>> [ 0.000000] node 0: [mem 0x0000001fa4000000-0x0000001fa402ffff] >>> [ 0.000000] node 0: [mem 0x0000001fa4030000-0x0000001fa40effff] >>> [ 0.000000] node 0: [mem 0x0000001fa40f0000-0x0000001fa73cffff] >>> [ 0.000000] node 0: [mem 0x0000001fa73d0000-0x0000001fa745ffff] >>> [ 0.000000] node 0: [mem 0x0000001fa7460000-0x0000001fa746ffff] >>> [ 0.000000] node 0: [mem 0x0000001fa7470000-0x0000001fa758ffff] >>> [ 0.000000] node 0: [mem 0x0000001fa7590000-0x0000001fa7ffffff] >>> >>> Signed-off-by: Baolin Wang >> >> This may only be necessary for non-contiguous zones so a check for >> zone_contiguous could be made but I suspect the saving, if any, would be >> marginal. >> >> However, it's subtle that block_end_pfn can end up in an arbirary location >> past the end of the zone or past cc->free_pfn. As the "continue" will update >> cc->migrate_pfn, that might lead to errors in the future. It would be a >> lot safer to pass in cc->free_pfn and do two things with the value. First, >> there is no point scanning for a valid online section past cc->free_pfn so >> terminating after cc->free_pfn may save some cycles. Second, cc->migrate_pfn >> does not end up with an arbitrary value which is a more defensive approach >> to any future programming errors. > > I have thought about this before. Originally, I had thought that we > were safe because cc->free_pfn should be in a online section and > block_end_pfn should reach cc->free_pfn before the end of zone. But > after checking more code and thinking about it again, I found that the > underlying sections may go offline under us during compaction. So that, > cc->free_pfn may be in a offline section or after the end of zone. So, > you are right, we need to consider the range of block_end_pfn. > > But, if we thought in this way (memory online/offline at any time), it > appears that we need to check whether the underlying section was > offlined. For example, is it safe to use "pfn_to_page()" in > "isolate_migratepages_block()"? Is it possible for the underlying > section to be offlined under us? It is possible. There is a previous discussion[1] about the race between pfn_to_online_page() and memory offline. [1] https://lore.kernel.org/lkml/87zgc6buoq.fsf@nvidia.com/T/#m642d91bcc726437e1848b295bc57ce249c7ca399