[RFC PATCH 1/4] mm: compaction: make proactive compaction mTHP-aware

[Bo Zhang <zhangbo0325@gmail.com>]

Currently, proactive compaction only evaluates fragmentation relative
to COMPACTION_HPAGE_ORDER (typically order-9 for 2MB THP). This makes
it unsuitable for systems that primarily need smaller high-order pages,
such as order-2 (16KB) for mTHP.

Generalize the fragmentation score functions to accept an order parameter:
  - fragmentation_score_zone(zone, order)
  - fragmentation_score_zone_weighted(zone, order)
  - fragmentation_score_node(pgdat, order)

Calculate the min order in huge_anon_orders_always to configure the target
order for proactive compaction.

This enables proactive compaction to maintain free page availability
at any order, which is particularly useful for mTHP-enabled systems
where order-n (n < 9) allocation pressure is high.

Signed-off-by: Bo Zhang <zhangbo56@xiaomi.com>
---
 mm/compaction.c | 36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index a049415512c6..a4f87232ad80 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 +82,15 @@ 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)
+{
+	unsigned long orders = READ_ONCE(huge_anon_orders_always);
+
+	if (orders)
+		return __ffs(orders);
+	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);
@@ -2208,16 +2218,16 @@ static bool kswapd_is_running(pg_data_t *pgdat)
 
 /*
  * A zone's fragmentation score is the external fragmentation wrt to the
- * COMPACTION_HPAGE_ORDER. It returns a value in the range [0, 100].
+ * compact_hpage_order(). It returns a value in the range [0, 100].
  */
-static unsigned int fragmentation_score_zone(struct zone *zone)
+static unsigned int fragmentation_score_zone(struct zone *zone, unsigned int order)
 {
-	return extfrag_for_order(zone, COMPACTION_HPAGE_ORDER);
+	return extfrag_for_order(zone, order);
 }
 
 /*
  * A weighted zone's fragmentation score is the external fragmentation
- * wrt to the COMPACTION_HPAGE_ORDER scaled by the zone's size. It
+ * wrt to the compact_hpage_order() scaled by the zone's size. It
  * returns a value in the range [0, 100].
  *
  * The scaling factor ensures that proactive compaction focuses on larger
@@ -2225,11 +2235,11 @@ static unsigned int fragmentation_score_zone(struct zone *zone)
  * ZONE_DMA32. For smaller zones, the score value remains close to zero,
  * and thus never exceeds the high threshold for proactive compaction.
  */
-static unsigned int fragmentation_score_zone_weighted(struct zone *zone)
+static unsigned int fragmentation_score_zone_weighted(struct zone *zone, unsigned int order)
 {
 	unsigned long score;
 
-	score = zone->present_pages * fragmentation_score_zone(zone);
+	score = zone->present_pages * fragmentation_score_zone(zone, order);
 	return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
 }
 
@@ -2240,7 +2250,7 @@ static unsigned int fragmentation_score_zone_weighted(struct zone *zone)
  * the node's score falls below the low threshold, or one of the back-off
  * conditions is met.
  */
-static unsigned int fragmentation_score_node(pg_data_t *pgdat)
+static unsigned int fragmentation_score_node(pg_data_t *pgdat, unsigned int order)
 {
 	unsigned int score = 0;
 	int zoneid;
@@ -2251,7 +2261,7 @@ static unsigned int fragmentation_score_node(pg_data_t *pgdat)
 		zone = &pgdat->node_zones[zoneid];
 		if (!populated_zone(zone))
 			continue;
-		score += fragmentation_score_zone_weighted(zone);
+		score += fragmentation_score_zone_weighted(zone, order);
 	}
 
 	return score;
@@ -2269,12 +2279,13 @@ static unsigned int fragmentation_score_wmark(bool low)
 static bool should_proactive_compact_node(pg_data_t *pgdat)
 {
 	int wmark_high;
+	unsigned int order = compact_hpage_order();
 
 	if (!sysctl_compaction_proactiveness || kswapd_is_running(pgdat))
 		return false;
 
 	wmark_high = fragmentation_score_wmark(false);
-	return fragmentation_score_node(pgdat) > wmark_high;
+	return fragmentation_score_node(pgdat, order) > wmark_high;
 }
 
 static enum compact_result __compact_finished(struct compact_control *cc)
@@ -2311,7 +2322,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 		if (kswapd_is_running(pgdat))
 			return COMPACT_PARTIAL_SKIPPED;
 
-		score = fragmentation_score_zone(cc->zone);
+		score = fragmentation_score_zone(cc->zone, compact_hpage_order());
 		wmark_low = fragmentation_score_wmark(true);
 
 		if (score > wmark_low)
@@ -3238,10 +3249,11 @@ static int kcompactd(void *p)
 		timeout = default_timeout;
 		if (should_proactive_compact_node(pgdat)) {
 			unsigned int prev_score, score;
+			unsigned int order = compact_hpage_order();
 
-			prev_score = fragmentation_score_node(pgdat);
+			prev_score = fragmentation_score_node(pgdat, order);
 			compact_node(pgdat, true);
-			score = fragmentation_score_node(pgdat);
+			score = fragmentation_score_node(pgdat, order);
 			/*
 			 * Defer proactive compaction if the fragmentation
 			 * score did not go down i.e. no progress made.
-- 
2.34.1