[tip: sched/core] sched/numa: Do not swap tasks between nodes when spare capacity is available

[tip-bot2 for Mel Gorman]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     13ede33150877d44756171e33570076882b17b0b
Gitweb:        https://git.kernel.org/tip/13ede33150877d44756171e33570076882b17b0b
Author:        Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
AuthorDate:    Fri, 20 May 2022 11:35:17 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 13 Jun 2022 10:29:59 +02:00

sched/numa: Do not swap tasks between nodes when spare capacity is available

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@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://lore.kernel.org/r/20220520103519.1863-3-mgorman@xxxxxxxxxxxxxxxxxxx
---
 kernel/sched/fair.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 51836ef..23da36c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1790,6 +1790,15 @@ 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);
 		/*