linux-kernel - Re: [RFC 4/4] mm, page_alloc: disallow migratetype fallback in fastpath

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Date:   Wed, 12 Oct 2016 16:51:48 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...com
Subject: Re: [RFC 4/4] mm, page_alloc: disallow migratetype fallback in
 fastpath

On 09/29/2016 11:05 PM, Vlastimil Babka wrote:
> The previous patch has adjusted async compaction so that it helps against
> longterm fragmentation when compacting for a non-MOVABLE high-order allocation.
> The goal of this patch is to force such allocations go through compaction
> once before being allowed to fallback to a pageblock of different migratetype
> (e.g. MOVABLE). In contexts where compaction is not allowed (and for order-0
> allocations), this delayed fallback possibility can still help by trying a
> different zone where fallback might not be needed and potentially waking up
> kswapd earlier.
> 
> Not-yet-signed-off-by: Vlastimil Babka <vbabka@...e.cz>

I forgot that compaction itself also needs to be told to not allow fallback,
otherwise it finishes with COMPACT_SUCCESS without actually doing anything...

>From 93acabcc744eab5a4aa965322e9083d0d9f990fc Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Wed, 12 Oct 2016 16:36:35 +0200
Subject: fixup! mm, page_alloc: disallow migratetype fallback in fastpath

We want to force compaction to run even when the requested page is potentially
available, but of a wrong migratetype. This won't work unless the compaction
itself is modified to not declare imediatelly success when it sees such page.
---
 mm/compaction.c | 22 +++++++++++++++++-----
 mm/internal.h   |  3 ++-
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index eb4ccd403543..eeb9200f7b7e 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1354,9 +1354,11 @@ static enum compact_result __compact_finished(struct zone *zone,
 #endif
 		/*
 		 * Job done if allocation would steal freepages from
-		 * other migratetype buddy lists.
+		 * other migratetype buddy lists. This is not allowed
+		 * for async direct compaction.
 		 */
-		if (find_suitable_fallback(area, order, migratetype,
+		if (!cc->prevent_fallback &&
+			find_suitable_fallback(area, order, migratetype,
 						true, &can_steal) != -1)
 			return COMPACT_SUCCESS;
 	}
@@ -1509,8 +1511,17 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
 	cc->migratetype = gfpflags_to_migratetype(cc->gfp_mask);
 	ret = compaction_suitable(zone, cc->order, cc->alloc_flags,
 							cc->classzone_idx);
-	/* Compaction is likely to fail */
-	if (ret == COMPACT_SUCCESS || ret == COMPACT_SKIPPED)
+	/*
+	 * Compaction should not be needed. If we don't allow stealing from
+	 * pageblocks of different migratetype, the watermark checks cannot
+	 * distinguish that, so assume we would need to steal, and leave the
+	 * thorough check to compact_finished().
+	 */
+	if (ret == COMPACT_SUCCESS && !cc->prevent_fallback)
+		return ret;
+
+	/* Compaction is likely to fail due to insufficient free pages */
+	if (ret == COMPACT_SKIPPED)
 		return ret;
 
 	/* huh, compaction_suitable is returning something unexpected */
@@ -1678,7 +1689,8 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
 		.direct_compaction = true,
 		.whole_zone = (prio == MIN_COMPACT_PRIORITY),
 		.ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
-		.ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
+		.ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY),
+		.prevent_fallback = (prio == COMPACT_PRIO_ASYNC)
 	};
 	INIT_LIST_HEAD(&cc.freepages);
 	INIT_LIST_HEAD(&cc.migratepages);
diff --git a/mm/internal.h b/mm/internal.h
index a46eab383e8d..bb01d9bd60a8 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -181,13 +181,14 @@ struct compact_control {
 	bool ignore_block_suitable;	/* Scan blocks considered unsuitable */
 	bool direct_compaction;		/* False from kcompactd or /proc/... */
 	bool whole_zone;		/* Whole zone should/has been scanned */
+	bool contended;			/* Signal lock or sched contention */
+	bool prevent_fallback;		/* Stealing migratetypes not allowed */
 	int order;			/* order a direct compactor needs */
 	int migratetype;		/* migratetype of direct compactor */
 	const gfp_t gfp_mask;		/* gfp mask of a direct compactor */
 	const unsigned int alloc_flags;	/* alloc flags of a direct compactor */
 	const int classzone_idx;	/* zone index of a direct compactor */
 	struct zone *zone;
-	bool contended;			/* Signal lock or sched contention */
 };
 
 unsigned long
-- 
2.10.0