From 78d048a8f987b856290d5352dd250d6a324b7256 Mon Sep 17 00:00:00 2001
From: Mel Gorman <mgorman@techsingularity.net>
Date: Fri, 20 May 2022 11:35:17 +0100
Subject: [PATCH] sched/numa: Do not swap tasks between nodes when spare
 capacity is available

References: bnc#1193431
Patch-mainline: v6.0-rc1
Git-commit: 13ede33150877d44756171e33570076882b17b0b

If a destination node has spare capacity but there is an imbalance then
two tasks are selected for swapping. If the tasks have no numa group
or are within the same NUMA group, it's simply shuffling tasks around
without having any impact on the compute imbalance. Instead, it's just
punishing one task to help another.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lore.kernel.org/r/20220520103519.1863-3-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e815bf0d6980..750272c54ba8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1812,6 +1812,15 @@ static bool task_numa_compare(struct task_numa_env *env,
 	 */
 	cur_ng = rcu_dereference(cur->numa_group);
 	if (cur_ng == p_ng) {
+		/*
+		 * Do not swap within a group or between tasks that have
+		 * no group if there is spare capacity. Swapping does
+		 * not address the load imbalance and helps one task at
+		 * the cost of punishing another.
+		 */
+		if (env->dst_stats.node_type == node_has_spare)
+			goto unlock;
+
 		imp = taskimp + task_weight(cur, env->src_nid, dist) -
 		      task_weight(cur, env->dst_nid, dist);
 		/*