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[PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype

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  [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype Vlastimil Babka <vbabka@suse.cz> - 2017-02-10 19:20 +0100
    Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to  pageblocks of same migratetype Mel Gorman <mgorman@techsingularity.net> - 2017-02-13 12:00 +0100

#1578677 — [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype

The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
pageblocks. This is a heuristic intended to reduce latency, based on the
assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.

However, with the exception of THP's, most high-order allocations are not
movable. Should the async compaction succeed, this increases the chance that
the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
long-term fragmentation worse.

This patch attempts to help the situation by changing async direct compaction
so that the migrate scanner only scans the pageblocks of the requested
migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
contain movable pages, chances are that the allocation can succeed within one
of such pageblocks, removing the need for a fallback. If that fails, the
subsequent sync attempt will ignore this restriction.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/compaction.c | 11 +++++++++--
 mm/page_alloc.c | 20 +++++++++++++-------
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index b7094700712b..84ef44c3b1c9 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -994,10 +994,17 @@ isolate_migratepages_range(struct compact_control *cc, unsigned long start_pfn,
 static bool suitable_migration_source(struct compact_control *cc,
 							struct page *page)
 {
-	if (cc->mode != MIGRATE_ASYNC)
+	int block_mt;
+
+	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
 		return true;
 
-	return is_migrate_movable(get_pageblock_migratetype(page));
+	block_mt = get_pageblock_migratetype(page);
+
+	if (cc->migratetype == MIGRATE_MOVABLE)
+		return is_migrate_movable(block_mt);
+	else
+		return block_mt == cc->migratetype;
 }
 
 /* Returns true if the page is within a block suitable for migration to */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a7d33818610f..6d9ba640a12d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3523,6 +3523,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
 {
 	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
+	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
 	struct page *page = NULL;
 	unsigned int alloc_flags;
 	unsigned long did_some_progress;
@@ -3572,12 +3573,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 
 	/*
 	 * For costly allocations, try direct compaction first, as it's likely
-	 * that we have enough base pages and don't need to reclaim. Don't try
-	 * that for allocations that are allowed to ignore watermarks, as the
-	 * ALLOC_NO_WATERMARKS attempt didn't yet happen.
+	 * that we have enough base pages and don't need to reclaim. For non-
+	 * movable high-order allocations, do that as well, as compaction will
+	 * try prevent permanent fragmentation by migrating from blocks of the
+	 * same migratetype.
+	 * Don't try this for allocations that are allowed to ignore
+	 * watermarks, as the ALLOC_NO_WATERMARKS attempt didn't yet happen.
 	 */
-	if (can_direct_reclaim && order > PAGE_ALLOC_COSTLY_ORDER &&
-		!gfp_pfmemalloc_allowed(gfp_mask)) {
+	if (can_direct_reclaim &&
+			(costly_order ||
+			   (order > 0 && ac->migratetype != MIGRATE_MOVABLE))
+			&& !gfp_pfmemalloc_allowed(gfp_mask)) {
 		page = __alloc_pages_direct_compact(gfp_mask, order,
 						alloc_flags, ac,
 						INIT_COMPACT_PRIORITY,
@@ -3589,7 +3595,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		 * Checks for costly allocations with __GFP_NORETRY, which
 		 * includes THP page fault allocations
 		 */
-		if (gfp_mask & __GFP_NORETRY) {
+		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
 			/*
 			 * If compaction is deferred for high-order allocations,
 			 * it is because sync compaction recently failed. If
@@ -3684,7 +3690,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	 * Do not retry costly high order allocations unless they are
 	 * __GFP_REPEAT
 	 */
-	if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT))
+	if (costly_order && !(gfp_mask & __GFP_REPEAT))
 		goto nopage;
 
 	/* Make sure we know about allocations which stall for too long */
-- 
2.11.0

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#1579612 — Re: [PATCH v2 07/10] mm, compaction: restrict async compaction to pageblocks of same migratetype

On Fri, Feb 10, 2017 at 06:23:40PM +0100, Vlastimil Babka wrote:
> The migrate scanner in async compaction is currently limited to MIGRATE_MOVABLE
> pageblocks. This is a heuristic intended to reduce latency, based on the
> assumption that non-MOVABLE pageblocks are unlikely to contain movable pages.
> 
> However, with the exception of THP's, most high-order allocations are not
> movable. Should the async compaction succeed, this increases the chance that
> the non-MOVABLE allocations will fallback to a MOVABLE pageblock, making the
> long-term fragmentation worse.
> 
> This patch attempts to help the situation by changing async direct compaction
> so that the migrate scanner only scans the pageblocks of the requested
> migratetype. If it's a non-MOVABLE type and there are such pageblocks that do
> contain movable pages, chances are that the allocation can succeed within one
> of such pageblocks, removing the need for a fallback. If that fails, the
> subsequent sync attempt will ignore this restriction.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Ok, I really like this idea. The thinking of async originally was to
reduce latency but that was also at the time when THP allocations were
stalling for long periods of time. Now that the default has changed,
this idea makes a lot of sense. A few months ago I would have thought
that this will increase the changes that a high-order allocation for the
stack may have a higher chance of failing but with VMAP_STACK, this is
much less of a concern. It would be very nice to know for this patch if
the number of times the extfrag tracepoint is triggered is reduced or
increased by this patch. Do you have that data?

-- 
Mel Gorman
SUSE Labs

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