[RFC PATCH v2 4/4] mm: compaction: skip unmovable blocks for non-costly proactive mTHP compaction

[Bo Zhang]

Proactive compaction scans all pageblocks including unmovable/reclaimable
ones. For non-costly mTHP orders (<= PAGE_ALLOC_COSTLY_ORDER), those blocks
are dominated by non-migratable kernel allocations (slab, page tables) and
scanning them wastes cycles: a small order is easily satisfied from movable
blocks alone, so there is no need to scavenge the few migratable fallback
pages inside unmovable blocks.

Costly orders keep scanning all blocks: they are hard to form and benefit
from any migratable page available, including allocator fallbacks inside
unmovable blocks.

Restrict non-costly proactive compaction to movable/CMA blocks.

Signed-off-by: Bo Zhang <zhangbo56@xxxxxxxxxx>
---
 mm/compaction.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/compaction.c b/mm/compaction.c
index 0ff454ee5604..deddf931d0f2 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1404,10 +1404,19 @@ static bool suitable_migration_source(struct compact_control *cc,
 							struct page *page)
 {
 	int block_mt;
+	unsigned int order = compact_hpage_order();
+	bool costly = order > PAGE_ALLOC_COSTLY_ORDER;
 
 	if (pageblock_skip_persistent(page))
 		return false;
 
+	if (is_via_compact_memory(cc->order) && !costly) {
+		block_mt = get_pageblock_migratetype(page);
+		if (block_mt == MIGRATE_MOVABLE || is_migrate_cma(block_mt))
+			return true;
+		return false;
+	}
+
 	/*
 	 * Background compaction produces blocks for the zone at
 	 * large, with no particular allocation context. Allow all
-- 
2.34.1