[RFC PATCH v2 2/4] mm: compaction: target mTHP order for proactive compaction

[Bo Zhang]

Proactive compaction triggers against COMPACTION_HPAGE_ORDER (order-9),
which is too heavyweight and misdirected for mTHP systems where the
pressure is on smaller orders like order-2.

Add compact_hpage_order() to return the highest always-enabled anonymous
mTHP order, and use it as the proactive compaction target in
should_proactive_compact_node(), __compact_finished() and kcompactd().

Fragmentation scores are not comparable across orders, a small order
scores much lower for the same state, so scale the proactive watermarks
down in proportion to the target order, otherwise order-2 would rarely
reach the order-9 threshold.

Signed-off-by: Bo Zhang <zhangbo56@xxxxxxxxxx>
---
 mm/compaction.c | 40 ++++++++++++++++++++++++++++++++--------
 1 file changed, 32 insertions(+), 8 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index ecd3e6cf54fc..3928a5483efc 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -24,6 +24,7 @@
 #include <linux/page_owner.h>
 #include <linux/psi.h>
 #include <linux/cpuset.h>
+#include <linux/huge_mm.h>
 #include "page_alloc.h"
 #include "internal.h"
 
@@ -81,6 +82,17 @@ static inline bool is_via_compact_memory(int order) { return false; }
 #define COMPACTION_HPAGE_ORDER	(PMD_SHIFT - PAGE_SHIFT)
 #endif
 
+static inline int compact_hpage_order(void)
+{
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
+	unsigned long orders = READ_ONCE(huge_anon_orders_always);
+
+	if (orders)
+		return __fls(orders);
+#endif
+	return COMPACTION_HPAGE_ORDER;
+}
+
 static struct page *mark_allocated_noprof(struct page *page, unsigned int order, gfp_t gfp_flags)
 {
 	post_alloc_hook(page, order, __GFP_MOVABLE, ALLOC_DEFAULT);
@@ -2257,24 +2269,36 @@ static unsigned int fragmentation_score_node(pg_data_t *pgdat, unsigned int orde
 	return score;
 }
 
-static unsigned int fragmentation_score_wmark(bool low)
+/*
+ * Fragmentation scores are not comparable across orders: a lower order
+ * scores much lower than a higher order for the same fragmentation state.
+ * Scale the watermark proportionally to the target order so a small order
+ * can still trigger proactive compaction; COMPACTION_HPAGE_ORDER keeps the
+ * original threshold.
+ */
+static unsigned int fragmentation_score_wmark(bool low, unsigned int order)
 {
-	unsigned int wmark_low, leeway;
+	unsigned int wmark_low, leeway, wmark;
 
 	wmark_low = 100U - sysctl_compaction_proactiveness;
 	leeway = min(10U, wmark_low / 2);
-	return low ? wmark_low : min(wmark_low + leeway, 100U);
+	wmark = low ? wmark_low : min(wmark_low + leeway, 100U);
+
+	if (order < COMPACTION_HPAGE_ORDER)
+		wmark = wmark * order / COMPACTION_HPAGE_ORDER;
+
+	return wmark;
 }
 
 static bool should_proactive_compact_node(pg_data_t *pgdat)
 {
 	int wmark_high;
-	unsigned int order = COMPACTION_HPAGE_ORDER;
+	unsigned int order = compact_hpage_order();
 
 	if (!sysctl_compaction_proactiveness || kswapd_is_running(pgdat))
 		return false;
 
-	wmark_high = fragmentation_score_wmark(false);
+	wmark_high = fragmentation_score_wmark(false, order);
 	return fragmentation_score_node(pgdat, order) > wmark_high;
 }
 
@@ -2306,7 +2330,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 
 	if (cc->proactive_compaction) {
 		int score, wmark_low;
-		unsigned int order = COMPACTION_HPAGE_ORDER;
+		unsigned int order = compact_hpage_order();
 		pg_data_t *pgdat;
 
 		pgdat = cc->zone->zone_pgdat;
@@ -2314,7 +2338,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 			return COMPACT_PARTIAL_SKIPPED;
 
 		score = fragmentation_score_zone(cc->zone, order);
-		wmark_low = fragmentation_score_wmark(true);
+		wmark_low = fragmentation_score_wmark(true, order);
 
 		if (score > wmark_low)
 			ret = COMPACT_CONTINUE;
@@ -3239,7 +3263,7 @@ static int kcompactd(void *p)
 		timeout = default_timeout;
 		if (should_proactive_compact_node(pgdat)) {
 			unsigned int prev_score, score;
-			unsigned int order = COMPACTION_HPAGE_ORDER;
+			unsigned int order = compact_hpage_order();
 
 			prev_score = fragmentation_score_node(pgdat, order);
 			compact_node(pgdat, true);
-- 
2.34.1