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=-1.0 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 E8D9EC43387 for ; Fri, 18 Jan 2019 09:09:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BF4612086D for ; Fri, 18 Jan 2019 09:09:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726741AbfARJJ4 (ORCPT ); Fri, 18 Jan 2019 04:09:56 -0500 Received: from mx2.suse.de ([195.135.220.15]:39882 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1725938AbfARJJz (ORCPT ); Fri, 18 Jan 2019 04:09:55 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay1.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 8D4EDAC85; Fri, 18 Jan 2019 09:09:54 +0000 (UTC) Subject: Re: [PATCH 20/25] mm, compaction: Reduce unnecessary skipping of migration target scanner To: Mel Gorman Cc: Linux-MM , David Rientjes , Andrea Arcangeli , ying.huang@intel.com, kirill@shutemov.name, Andrew Morton , Linux List Kernel Mailing References: <20190104125011.16071-1-mgorman@techsingularity.net> <20190104125011.16071-21-mgorman@techsingularity.net> <8e310c2a-5f2e-ee99-24c5-10a71972699a@suse.cz> <20190117193909.GO27437@techsingularity.net> From: Vlastimil Babka Message-ID: <7a4dcc5e-9c31-c3bb-e38a-5ef955ca409e@suse.cz> Date: Fri, 18 Jan 2019 10:09:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190117193909.GO27437@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15 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 1/17/19 8:39 PM, Mel Gorman wrote: > On Thu, Jan 17, 2019 at 06:58:30PM +0100, Vlastimil Babka wrote: >> On 1/4/19 1:50 PM, Mel Gorman wrote: >> > The fast isolation of pages can move the scanner faster than is necessary >> > depending on the contents of the free list. This patch will only allow >> > the fast isolation to initialise the scanner and advance it slowly. The >> > primary means of moving the scanner forward is via the linear scanner >> > to reduce the likelihood the migration source/target scanners meet >> > prematurely triggering a rescan. >> >> Maybe I've seen enough code today and need to stop, but AFAICS the description >> here doesn't match the actual code changes? What I see are some cleanups, and a >> change in free scanner that will set pageblock skip bit after a pageblock has >> been scanned, even if there were pages isolated, while previously it would set >> the skip bit only if nothing was isolated. >> > > The first three hunks could have been split out but it wouldn't help > overall. Maybe a changelog rewrite will help; > > mm, compaction: Reduce premature advancement of the migration target scanner > > The fast isolation of free pages allows the cached PFN of the free > scanner to advance faster than necessary depending on the contents > of the free list. The key is that fast_isolate_freepages() can update > zone->compact_cached_free_pfn via isolate_freepages_block(). When the > fast search fails, the linear scan can start from a point that has skipped > valid migration targets, particularly pageblocks with just low-order > free pages. This can cause the migration source/target scanners to meet > prematurely causing a reset. > > This patch starts by avoiding an update of the pageblock skip information > and cached PFN from isolate_freepages_block() and puts the responsibility > of updating that information in the callers. The fast scanner will update > the cached PFN if and only if it finds a block that is higher than the > existing cached PFN and sets the skip if the pageblock is full or nearly > full. The linear scanner will update skipped information and the cached > PFN only when a block is completely scanned. The total impact is that > the free scanner advances more slowly as it is primarily driven by the > linear scanner instead of the fast search. > > Does that help? Yeah, now I get it, thanks! Acked-by: Vlastimil Babka