From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933333AbcDKNqt (ORCPT ); Mon, 11 Apr 2016 09:46:49 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:34965 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932372AbcDKNqr (ORCPT ); Mon, 11 Apr 2016 09:46:47 -0400 Date: Mon, 11 Apr 2016 15:46:44 +0200 From: Michal Hocko To: Vlastimil Babka Cc: Andrew Morton , Linus Torvalds , Johannes Weiner , Mel Gorman , David Rientjes , Tetsuo Handa , Joonsoo Kim , Hillf Danton , linux-mm@kvack.org, LKML Subject: Re: [PATCH 06/11] mm, compaction: distinguish between full and partial COMPACT_COMPLETE Message-ID: <20160411134633.GJ23157@dhcp22.suse.cz> References: <1459855533-4600-1-git-send-email-mhocko@kernel.org> <1459855533-4600-7-git-send-email-mhocko@kernel.org> <570B9432.9090600@suse.cz> <20160411124653.GG23157@dhcp22.suse.cz> <570B9E50.9040000@suse.cz> <20160411132745.GH23157@dhcp22.suse.cz> <570BA9BD.2030404@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <570BA9BD.2030404@suse.cz> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 11-04-16 15:42:21, Vlastimil Babka wrote: > On 04/11/2016 03:27 PM, Michal Hocko wrote: > >On Mon 11-04-16 14:53:36, Vlastimil Babka wrote: > >>On 04/11/2016 02:46 PM, Michal Hocko wrote: > >> > >>The racy part is negligible but I didn't realize the sync/async migrate > >>scanner part until now. So yeah, free_pfn could have got to middle of zone > >>when it was in the async mode. But that also means that the async mode > >>recently used up all free pages in the second half of the zone. WRT free > >>pages isolation, async mode is not trying less than sync, so it shouldn't be > >>a considerable missed opportunity if we don't rescan the it, though. > > > >I am not really sure I understand. The primary intention of this patch > >is to distinguish where we have scanned basically whole zones from cases > >where a new scan started off previous mark and so it was just unlucky to > >see only tiny bit of the zone where we would clearly give up too early. > >FWIU this shouldn't be the case if we start scanning from the beginning > >of the zone even if we raced on the other end of the zone because the > >missed part would be negligible. Is that understanding correct? > > Yes, it should be less unlucky than seeing a tiny bit of the zone. Just > wanted to point out that you might still not see the whole zone in one > compaction attempt. E.g. async compaction is first, advances the free > scanner and caches its position when it bails out due to being contended. > Then direct reclaim frees some pages behind the cached position. Sync > compaction attempts starts migration scanner from start_pfn, but picks up > the cached free scanner pfn. The result is missing some free pages and the > scanners meeting somewhat earlier than they otherwise would. Probably not > critical even for OOM decisions, as that's also racy anyway. OK, I see now. I agree this shouldn't be critical and thanks for the clarification. -- Michal Hocko SUSE Labs