From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752644AbcEPIOo (ORCPT ); Mon, 16 May 2016 04:14:44 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:35865 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751921AbcEPIOm (ORCPT ); Mon, 16 May 2016 04:14:42 -0400 Date: Mon, 16 May 2016 10:14:39 +0200 From: Michal Hocko To: Vlastimil Babka Cc: linux-mm@kvack.org, Andrew Morton , Joonsoo Kim , Rik van Riel , David Rientjes , Mel Gorman , Johannes Weiner , Tetsuo Handa , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [RFC 12/13] mm, compaction: more reliably increase direct compaction priority Message-ID: <20160516081439.GD23146@dhcp22.suse.cz> References: <1462865763-22084-1-git-send-email-vbabka@suse.cz> <1462865763-22084-13-git-send-email-vbabka@suse.cz> <20160513141539.GR20141@dhcp22.suse.cz> <57397760.4060407@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <57397760.4060407@suse.cz> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 16-05-16 09:31:44, Vlastimil Babka wrote: > On 05/13/2016 04:15 PM, Michal Hocko wrote: > > On Tue 10-05-16 09:36:02, Vlastimil Babka wrote: > > > > > > - should_compact_retry() is only called when should_reclaim_retry() returns > > > false. This means that compaction priority cannot get increased as long > > > as reclaim makes sufficient progress. Theoretically, reclaim should stop > > > retrying for high-order allocations as long as the high-order page doesn't > > > exist but due to races, this may result in spurious retries when the > > > high-order page momentarily does exist. > > > > This is intentional behavior and I would like to preserve it if it is > > possible. For higher order pages should_reclaim_retry retries as long > > as there are some eligible high order pages present which are just hidden > > by the watermark check. So this is mostly to get us over watermarks to > > start carrying about fragmentation. If we race there then nothing really > > terrible should happen and we should eventually converge to a terminal > > state. > > > > Does this make sense to you? > > Yeah it should work, my only worry was that this may get subtly wrong (as > experience shows us) and due to e.g. slightly different watermark checks > and/or a corner-case zone such as ZONE_DMA, should_reclaim_retry() would > keep returning true, even if reclaim couldn't/wouldn't help anything. Then > compaction would be needlessly kept at ineffective priority. watermark check for ZONE_DMA should always fail because it fails even when is completely free to the lowmem reserves. I had a subtle bug in the original code to check highzone_idx rather than classzone_idx but that should the fix has been posted recently: http://lkml.kernel.org/r/1463051677-29418-2-git-send-email-mhocko@kernel.org > Also my understanding of the initial compaction priorities is to lower the > latency if fragmentation is just light and there's enough memory. Once we > start struggling, I don't see much point in not switching to the full > compaction priority quickly. That is true but why to compact when there are high order pages and they are just hidden by the watermark check. -- Michal Hocko SUSE Labs