[PATCH 23/63] sched: Resist moving tasks towards nodes with fewer hinting faults

[Mel Gorman]

Just as "sched: Favour moving tasks towards the preferred node" favours
moving tasks towards nodes with a higher number of recorded NUMA hinting
faults, this patch resists moving tasks towards nodes with lower faults.

[mgorman@xxxxxxx: changelog]
Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 kernel/sched/fair.c     | 33 +++++++++++++++++++++++++++++++++
 kernel/sched/features.h |  8 ++++++++
 2 files changed, 41 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8c2b779..21cad59 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4106,12 +4106,43 @@ static bool migrate_improves_locality(struct task_struct *p, struct lb_env *env)
 
 	return false;
 }
+
+
+static bool migrate_degrades_locality(struct task_struct *p, struct lb_env *env)
+{
+	int src_nid, dst_nid;
+
+	if (!sched_feat(NUMA) || !sched_feat(NUMA_RESIST_LOWER))
+		return false;
+
+	if (!p->numa_faults || !(env->sd->flags & SD_NUMA))
+		return false;
+
+	src_nid = cpu_to_node(env->src_cpu);
+	dst_nid = cpu_to_node(env->dst_cpu);
+
+	if (src_nid == dst_nid ||
+	    p->numa_migrate_seq >= sysctl_numa_balancing_settle_count)
+		return false;
+
+	if (p->numa_faults[dst_nid] < p->numa_faults[src_nid])
+		return true;
+
+	return false;
+}
+
 #else
 static inline bool migrate_improves_locality(struct task_struct *p,
 					     struct lb_env *env)
 {
 	return false;
 }
+
+static inline bool migrate_degrades_locality(struct task_struct *p,
+					     struct lb_env *env)
+{
+	return false;
+}
 #endif
 
 /*
@@ -4174,6 +4205,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	 * 3) too many balance attempts have failed.
 	 */
 	tsk_cache_hot = task_hot(p, rq_clock_task(env->src_rq), env->sd);
+	if (!tsk_cache_hot)
+		tsk_cache_hot = migrate_degrades_locality(p, env);
 
 	if (migrate_improves_locality(p, env)) {
 #ifdef CONFIG_SCHEDSTATS
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index d9278ce..5716929 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -74,4 +74,12 @@ SCHED_FEAT(NUMA,	false)
  * balancing.
  */
 SCHED_FEAT(NUMA_FAVOUR_HIGHER, true)
+
+/*
+ * NUMA_RESIST_LOWER will resist moving tasks towards nodes where a
+ * lower number of hinting faults have been recorded. As this has
+ * the potential to prevent a task ever migrating to a new node
+ * due to CPU overload it is disabled by default.
+ */
+SCHED_FEAT(NUMA_RESIST_LOWER, false)
 #endif
-- 
1.8.4

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