Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755749AbcDKQHx (ORCPT ); Mon, 11 Apr 2016 12:07:53 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:33759 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754620AbcDKQHw (ORCPT ); Mon, 11 Apr 2016 12:07:52 -0400 Date: Mon, 11 Apr 2016 18:07:48 +0200 From: Michal Hocko To: Andrew Morton Cc: Linus Torvalds , Johannes Weiner , Mel Gorman , David Rientjes , Tetsuo Handa , Joonsoo Kim , Hillf Danton , linux-mm@kvack.org, LKML , Vlastimil Babka Subject: [RFC PATCH] mm: use compaction feedback for thp backoff conditions Message-ID: <20160411160748.GO23157@dhcp22.suse.cz> References: <1459855533-4600-1-git-send-email-mhocko@kernel.org> <1459855533-4600-10-git-send-email-mhocko@kernel.org> <20160411154036.GN23157@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160411154036.GN23157@dhcp22.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 Content-Length: 3569 Lines: 93 On Mon 11-04-16 17:40:36, Michal Hocko wrote: > Hi Andrew, > Vlastimil has pointed out[1] that using compaction_withdrawn() for THP > allocations has some non-trivial consequences. While I still think that > the check is OK it is true we shouldn't sneak in a potential behavior > change into something that basically provides an API. So can you fold > the following partial revert into the original patch please? > > [1] http://lkml.kernel.org/r/570BB719.2030007@suse.cz This would be an RFC on top. --- >From 6cfed80ad41f3f1506930b9a3254fe135bf90d4c Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Mon, 11 Apr 2016 17:51:28 +0200 Subject: [PATCH] mm: use compaction feedback for thp backoff conditions THP requests skip the direct reclaim if the compaction is either deferred or contended to reduce stalls which wouldn't help the allocation success anyway. These checks are ignoring other potential feedback modes which we have available now. It clearly doesn't make much sense to go and reclaim few pages if the previous compaction has failed. We can also simplify the check by using compaction_withdrawn which checks for both COMPACT_CONTENDED and COMPACT_DEFERRED. This check is however covering more reasons why the compaction was withdrawn. None of them should be a problem for the THP case though. It is safe to back of if we see COMPACT_SKIPPED because that means that compaction_suitable failed and a single round of the reclaim is unlikely to make any difference here. We would have to be close to the low watermark to reclaim enough and even then there is no guarantee that the compaction would make any progress while the direct reclaim would have caused the stall. COMPACT_PARTIAL_SKIPPED is slightly different because that means that we have only seen a part of the zone so a retry would make some sense. But it would be a compaction retry not a reclaim retry to perform. We are not doing that and that might indeed lead to situations where THP fails but this should happen only rarely and it would be really hard to measure. Signed-off-by: Michal Hocko --- mm/page_alloc.c | 27 ++++++++------------------- 1 file changed, 8 insertions(+), 19 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 5eae9e0555ed..6d1da0ceaf1e 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3420,25 +3420,14 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, if (order && compaction_made_progress(compact_result)) compaction_retries++; - /* Checks for THP-specific high-order allocations */ - if (is_thp_gfp_mask(gfp_mask)) { - /* - * If compaction is deferred for high-order allocations, it is - * because sync compaction recently failed. If this is the case - * and the caller requested a THP allocation, we do not want - * to heavily disrupt the system, so we fail the allocation - * instead of entering direct reclaim. - */ - if (compact_result == COMPACT_DEFERRED) - goto nopage; - - /* - * Compaction is contended so rather back off than cause - * excessive stalls. - */ - if(compact_result == COMPACT_CONTENDED) - goto nopage; - } + /* + * Checks for THP-specific high-order allocations and back off + * if the the compaction backed off or failed + */ + if (is_thp_gfp_mask(gfp_mask) && + (compaction_withdrawn(compact_result) || + compaction_failed(compact_result))) + goto nopage; /* Try direct reclaim and then allocating */ page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac, -- 2.8.0.rc3 -- Michal Hocko SUSE Labs