From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by kanga.kvack.org (Postfix) with ESMTP id 4AED28E0002 for ; Wed, 16 Jan 2019 10:00:26 -0500 (EST) Received: by mail-pg1-f198.google.com with SMTP id q62so4024254pgq.9 for ; Wed, 16 Jan 2019 07:00:26 -0800 (PST) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id k38si6182618pgi.235.2019.01.16.07.00.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 16 Jan 2019 07:00:25 -0800 (PST) Subject: Re: [PATCH 11/25] mm, compaction: Use free lists to quickly locate a migration source References: <20190104125011.16071-1-mgorman@techsingularity.net> <20190104125011.16071-12-mgorman@techsingularity.net> <20190116143308.GE27437@techsingularity.net> From: Vlastimil Babka Message-ID: Date: Wed, 16 Jan 2019 16:00:22 +0100 MIME-Version: 1.0 In-Reply-To: <20190116143308.GE27437@techsingularity.net> Content-Type: text/plain; charset=iso-8859-15 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Mel Gorman Cc: Linux-MM , David Rientjes , Andrea Arcangeli , ying.huang@intel.com, kirill@shutemov.name, Andrew Morton , Linux List Kernel Mailing On 1/16/19 3:33 PM, Mel Gorman wrote: >>> + break; >>> + } >>> + >>> + /* >>> + * If low PFNs are being found and discarded then >>> + * limit the scan as fast searching is finding >>> + * poor candidates. >>> + */ >> >> I wonder about the "low PFNs are being found and discarded" part. Maybe >> I'm missing it, but I don't see them being discarded above, this seems >> to be the first check against cc->migrate_pfn. With the min() part in >> update_fast_start_pfn(), does it mean we can actually go back and rescan >> (or skip thanks to skip bits, anyway) again pageblocks that we already >> scanned? >> > > Extremely poor phrasing. My mind was thinking in terms of discarding > unsuitable candidates as they were below the migration scanner and it > did not translate properly. > > Based on your feedback, how does the following untested diff look? IMHO better. Meanwhile I noticed that the next patch removes the set_pageblock_skip() so maybe it's needless churn to introduce the fast_find_block, but I'll check more closely. The new comment about pfns below cc->migrate_pfn is better but I still wonder if it would be better to really skip over those candidates (they are still called unsuitable) and not go backwards with cc->migrate_pfn. But if you think the pageblock skip bits and halving of limit minimizes pointless rescan sufficiently, then fine.