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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 B407AC5ACCC for ; Thu, 18 Oct 2018 08:23:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 626B6208E4 for ; Thu, 18 Oct 2018 08:23:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 626B6208E4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727526AbeJRQXS (ORCPT ); Thu, 18 Oct 2018 12:23:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:45760 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727363AbeJRQXS (ORCPT ); Thu, 18 Oct 2018 12:23:18 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id D7764B041; Thu, 18 Oct 2018 08:23:23 +0000 (UTC) Subject: Re: [RFC v4 PATCH 2/5] mm/__free_one_page: skip merge for order-0 page unless compaction failed To: Aaron Lu Cc: Mel Gorman , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Huang Ying , Dave Hansen , Kemi Wang , Tim Chen , Andi Kleen , Michal Hocko , Matthew Wilcox , Daniel Jordan , Tariq Toukan , Jesper Dangaard Brouer References: <20181017063330.15384-1-aaron.lu@intel.com> <20181017063330.15384-3-aaron.lu@intel.com> <20181017104427.GJ5819@techsingularity.net> <20181017131059.GA9167@intel.com> <20181017135807.GL5819@techsingularity.net> <6d4d1a59-bb70-d4c9-bd18-8c398a09f25f@suse.cz> <20181018064839.GA6468@intel.com> From: Vlastimil Babka Openpgp: preference=signencrypt Autocrypt: addr=vbabka@suse.cz; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <20181018064839.GA6468@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/18/18 8:48 AM, Aaron Lu wrote: > On Wed, Oct 17, 2018 at 07:03:30PM +0200, Vlastimil Babka wrote: >> On 10/17/18 3:58 PM, Mel Gorman wrote: >>> Again, as compaction is not guaranteed to find the pageblocks, it would >>> be important to consider whether a) that matters or b) find an >>> alternative way of keeping unmerged buddies on separate lists so they >>> can be quickly discovered when a high-order allocation fails. >> >> Agree, unmerged buddies could be on separate freelist from regular >> order-0 freelist. That list could be also preferred to allocations >> before the regular one. Then one could e.g. try "direct merging" via >> this list when compaction fails, or prefer direct merging to compaction >> for non-costly-order allocations, do direct merging when allocation >> context doesn't even allow compaction (atomic etc). > > One concern regarding "direct merging" these unmerged pages via this > separate freelist(let's call it unmerged_free_list) is: adjacent > unmerged pages on the unmerged_free_list could be far away from each > other regarding their physical positions, so during the process of > merging them, the needed high order page may not be able to be formed > in a short time. Actually, the time could be unbound in a bad condition > when: > 1 unmerged pages adjacent on the unmerged_free_list happen to be far > away from each other regarding their physical positions; and I'm not sure I understand. Why should it matter for merging if pages are adjacent on the unmerged_free_list? The buddy for merging is found the usual way, no? > 2 there are a lot of unmerged pages on unmerged_free_list. That will affect allocation latency, yeah. Still might be faster than direct compaction. And possible to do in GFP_ATOMIC context, unlike direct compaction. > That's the reason I hooked the merging of unmerged pages in compaction > when isolate_migratepages_block() is scanning every page of a pageblock > in PFN order. > > OTOH, if there is a kernel thread trying to reduce fragmentation by > doing merges for these unmerged pages, I think it's perfect fine to let > it iterate all unmerged pages of that list and do_merge() for all of > them. > > So what about this: if kcompactd is running, let it handle these > unmerged pages on the list and after that, do its usual job of > compaction. If direct compaction is running, do not handle unmerged > pages on that list but rely on isolate_migratepages_block() to do the > merging as is done in this patchset. > > This of course has the effect of tying compaction with 'lazy merging'. > If it is not desirable, what about creating a new kernel thread to do > the merging of unmerged pages on the list while keeping the behaviour of > isolate_migratepages_block() in this patchset to improve compaction > success rate. Note that anything based on daemons will seem like reducing latency for allocations, but if we delay merging and then later do it from a daemon, the overall zone lock times will be essentially the same, right? The reduced zone lock benefits only happen when the unmerged pages get reallocated. >> Also I would definitely consider always merging pages freed to >> non-MOVABLE pageblocks. We really don't want to increase the >> fragmentation in those. However that means it probably won't help the >> netperf case? > > Yes, that would be unfortunate for all in-kernel users of page > allocator... In that case there should definitely be a direct merging possibility IMHO, even if only as a last resort before stealing from another pageblock.