[PATCH v2 1/5] mm: demotion: Set demotion list differently

[Jagdish Gediya]

Sharing used_targets between multiple nodes in a single
pass limits some of the opportunities for demotion target
sharing.

Don't share the used targets between multiple nodes in a
single pass, instead accumulate all the used targets in
source nodes shared by all pass, and reset 'used_targets'
to source nodes while finding demotion targets for any new
node.

This results into some more opportunities to share demotion
targets between multiple source nodes, e.g. with below NUMA
topology, where node 0 & 1 are cpu + dram nodes, node 2 & 3
are equally slower memory only nodes, and node 4 is slowest
memory only node,

available: 5 nodes (0-4)
node 0 cpus: 0 1
node 0 size: n MB
node 0 free: n MB
node 1 cpus: 2 3
node 1 size: n MB
node 1 free: n MB
node 2 cpus:
node 2 size: n MB
node 2 free: n MB
node 3 cpus:
node 3 size: n MB
node 3 free: n MB
node 4 cpus:
node 4 size: n MB
node 4 free: n MB
node distances:
node   0   1   2   3   4
  0:  10  20  40  40  80
  1:  20  10  40  40  80
  2:  40  40  10  40  80
  3:  40  40  40  10  80
  4:  80  80  80  80  10

The existing implementation gives below demotion targets,

node    demotion_target
 0              3, 2
 1              4
 2              X
 3              X
 4              X

With this patch applied, below are the demotion targets,

node    demotion_target
 0              3, 2
 1              3, 2
 2              4
 3              4
 4              X

e.g. with below NUMA topology, where node 0, 1 & 2 are
cpu + dram nodes and node 3 is slow memory node,

available: 4 nodes (0-3)
node 0 cpus: 0 1
node 0 size: n MB
node 0 free: n MB
node 1 cpus: 2 3
node 1 size: n MB
node 1 free: n MB
node 2 cpus: 4 5
node 2 size: n MB
node 2 free: n MB
node 3 cpus:
node 3 size: n MB
node 3 free: n MB
node distances:
node   0   1   2   3
  0:  10  20  20  40
  1:  20  10  20  40
  2:  20  20  10  40
  3:  40  40  40  10

The existing implementation gives below demotion targets,

node    demotion_target
 0              3
 1              X
 2              X
 3              X

With this patch applied, below are the demotion targets,

node    demotion_target
 0              3
 1              3
 2              3
 3              X

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Signed-off-by: Jagdish Gediya <jvgediya@xxxxxxxxxxxxx>
---
 mm/migrate.c | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index de175e2fdba5..516f4e1348c1 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -2383,7 +2383,7 @@ static void __set_migration_target_nodes(void)
 {
 	nodemask_t next_pass	= NODE_MASK_NONE;
 	nodemask_t this_pass	= NODE_MASK_NONE;
-	nodemask_t used_targets = NODE_MASK_NONE;
+	nodemask_t source_nodes = NODE_MASK_NONE;
 	int node, best_distance;
 
 	/*
@@ -2401,20 +2401,23 @@ static void __set_migration_target_nodes(void)
 again:
 	this_pass = next_pass;
 	next_pass = NODE_MASK_NONE;
+
 	/*
-	 * To avoid cycles in the migration "graph", ensure
-	 * that migration sources are not future targets by
-	 * setting them in 'used_targets'.  Do this only
-	 * once per pass so that multiple source nodes can
-	 * share a target node.
-	 *
-	 * 'used_targets' will become unavailable in future
-	 * passes.  This limits some opportunities for
-	 * multiple source nodes to share a destination.
+	 * Accumulate source nodes to avoid the cycle in migration
+	 * list.
 	 */
-	nodes_or(used_targets, used_targets, this_pass);
+	nodes_or(source_nodes, source_nodes, this_pass);
 
 	for_each_node_mask(node, this_pass) {
+		/*
+		 * To avoid cycles in the migration "graph", ensure
+		 * that migration sources are not future targets by
+		 * setting them in 'used_targets'. Reset used_targets
+		 * to source nodes for each node in this pass so that
+		 * multiple source nodes can share a target node.
+		 */
+		nodemask_t used_targets = source_nodes;
+
 		best_distance = -1;
 
 		/*
-- 
2.35.1