[PATCH] numa,sched: reduce conflict between fbq_classify_rq and migration

[Rik van Riel]

It is possible for fbq_classify_rq to indicate that a CPU has tasks that
should be moved to another NUMA node, but for migrate_improves_locality
and migrate_degrades_locality to not identify those tasks.

This patch always gives preference to preferred node evaluations, and
only checks the number of faults when evaluating moves between two
non-preferred nodes on a larger NUMA system.

On a two node system, the number of faults is never evaluated. Either
a task is about to be pulled off its preferred node, or migrated onto
it.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
---
 kernel/sched/fair.c | 60 +++++++++++++++++++++++++++++------------------------
 1 file changed, 33 insertions(+), 27 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ffeaa4105e48..9c9f225b14fc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5467,10 +5467,15 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
 }
 
 #ifdef CONFIG_NUMA_BALANCING
-/* Returns true if the destination node has incurred more faults */
+/*
+ * Returns true if the destination node is the preferred node.
+ * Needs to match fbq_classify_rq: if there is a runnable task
+ * that is not on its preferred node, we should identify it.
+ */
 static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
 {
 	struct numa_group *numa_group = rcu_dereference(p->numa_group);
+	unsigned long src_faults, dst_faults;
 	int src_nid, dst_nid;
 
 	if (!sched_feat(NUMA_FAVOUR_HIGHER) || !p->numa_faults ||
@@ -5484,29 +5489,30 @@ static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
 	if (src_nid == dst_nid)
 		return false;
 
-	if (numa_group) {
-		/* Task is already in the group's interleave set. */
-		if (node_isset(src_nid, numa_group->active_nodes))
-			return false;
-
-		/* Task is moving into the group's interleave set. */
-		if (node_isset(dst_nid, numa_group->active_nodes))
-			return true;
-
-		return group_faults(p, dst_nid) > group_faults(p, src_nid);
-	}
-
 	/* Encourage migration to the preferred node. */
 	if (dst_nid == p->numa_preferred_nid)
 		return true;
 
-	return task_faults(p, dst_nid) > task_faults(p, src_nid);
+	/* Migrating away from the preferred node is bad. */
+	if (src_nid == p->numa_preferred_nid)
+		return false;
+
+	if (numa_group) {
+		src_faults = group_faults(p, src_nid);
+		dst_faults = group_faults(p, dst_nid);
+	} else {
+		src_faults = task_faults(p, src_nid);
+		dst_faults = task_faults(p, dst_nid);
+	}
+
+	return dst_faults > src_faults;
 }
 
 
 static bool migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
 {
 	struct numa_group *numa_group = rcu_dereference(p->numa_group);
+	unsigned long src_faults, dst_faults;
 	int src_nid, dst_nid;
 
 	if (!sched_feat(NUMA) || !sched_feat(NUMA_RESIST_LOWER))
@@ -5521,23 +5527,23 @@ static bool migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
 	if (src_nid == dst_nid)
 		return false;
 
-	if (numa_group) {
-		/* Task is moving within/into the group's interleave set. */
-		if (node_isset(dst_nid, numa_group->active_nodes))
-			return false;
+	/* Migrating away from the preferred node is bad. */
+	if (src_nid == p->numa_preferred_nid)
+		return true;
 
-		/* Task is moving out of the group's interleave set. */
-		if (node_isset(src_nid, numa_group->active_nodes))
-			return true;
+	/* Encourage migration to the preferred node. */
+	if (dst_nid == p->numa_preferred_nid)
+		return false;
 
-		return group_faults(p, dst_nid) < group_faults(p, src_nid);
+	if (numa_group) {
+		src_faults = group_faults(p, src_nid);
+		dst_faults = group_faults(p, dst_nid);
+	} else {
+		src_faults = task_faults(p, src_nid);
+		dst_faults = task_faults(p, dst_nid);
 	}
 
-	/* Migrating away from the preferred node is always bad. */
-	if (src_nid == p->numa_preferred_nid)
-		return true;
-
-	return task_faults(p, dst_nid) < task_faults(p, src_nid);
+	return dst_faults < src_faults;
 }
 
 #else
--
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