[PATCH] mm/migrate: wake kcompactd when huge folio migration fails in numa balancing

[Joshua Hahn]

NUMA balancing migrates a misplaced folio if migrate_balanced_pgdat()
finds enough headroom on the destination node. For large folios, it then
allocates a folio from the destination with GFP_TRANSHUGE_LIGHT, which
wakes up neither kswapd nor kcompactd. This means a hot large folio
trying to promote to a fragmented destination may never succeed in
promotion even after repeated retries.

Wake kcompactd on the target node when the destination allocation fails
for a large folio. Don't wake kswapd, since this path can only be
reached if there is enough free memory to service the allocation, but it
is too fragmented to serve the large folio allocation.

This patch has been A/B tested on tiered Meta systems (DRAM + CXL)
running identical cacheing workloads. Without this patch, we accumulate
around 4.5k failed THP migrations over the span of a day, while the host
with this patch accumulates only about 80.

Those migration differences leads to about 25% reduced CXL bandwidth
usage after applying this patch.

Signed-off-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
---
 mm/migrate.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 7bdcdb57652f8..67364fe0851c0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2671,6 +2671,7 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
 	int nid = (int) data;
 	int order = folio_order(src);
 	gfp_t gfp = __GFP_THISNODE;
+	struct folio *dst;
 
 	if (order > 0)
 		gfp |= GFP_TRANSHUGE_LIGHT;
@@ -2679,7 +2680,17 @@ static struct folio *alloc_misplaced_dst_folio(struct folio *src,
 			__GFP_NOWARN;
 		gfp &= ~__GFP_RECLAIM;
 	}
-	return __folio_alloc_node(gfp, order, nid);
+
+	dst = __folio_alloc_node(gfp, order, nid);
+
+	/*
+	 * A node with enough free memory might still be too fragmented to serve
+	 * a large folio. Wake kcompactd to help the next huge folio migration.
+	 */
+	if (!dst && order > 0)
+		wakeup_kcompactd(NODE_DATA(nid), order, gfp_zone(gfp));
+
+	return dst;
 }
 
 /*
-- 
2.53.0-Meta