[PATCH update 4/4] sched/fair: Do not balance task to a throttled cfs_rq

[Aaron Lu <ziqianlu@bytedance.com>]

When doing load balance and the target cfs_rq is in throttled hierarchy,
whether to allow balancing there is a question.

The good side to allow balancing is: if the target CPU is idle or less
loaded and the being balanced task is holding some kernel resources,
then it seems a good idea to balance the task there and let the task get
the CPU earlier and release kernel resources sooner. The bad part is, if
the task is not holding any kernel resources, then the balance seems not
that useful.

While theoretically it's debatable, a performance test[0] which involves
200 cgroups and each cgroup runs hackbench(20 sender, 20 receiver) in
pipe mode showed a performance degradation on AMD Genoa when allowing
load balance to throttled cfs_rq. Analysis[1] showed hackbench doesn't
like task migration across LLC boundary. For this reason, add a check in
can_migrate_task() to forbid balancing to a cfs_rq that is in throttled
hierarchy. This reduced task migration a lot and performance restored.

[0]: https://lore.kernel.org/lkml/20250822110701.GB289@bytedance/
[1]: https://lore.kernel.org/lkml/20250903101102.GB42@bytedance/
Signed-off-by: Aaron Lu <ziqianlu@bytedance.com>
---
update: fix build error reported by kernel test robot when
CONFIG_FAIR_GROUP_SCHED is not set.

 kernel/sched/fair.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3dbdfaa697477..18a30ae35441a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5737,6 +5737,11 @@ static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 	return cfs_bandwidth_used() && cfs_rq->throttle_count;
 }
 
+static inline int lb_throttled_hierarchy(struct task_struct *p, int dst_cpu)
+{
+	return throttled_hierarchy(task_group(p)->cfs_rq[dst_cpu]);
+}
+
 static inline bool task_is_throttled(struct task_struct *p)
 {
 	return cfs_bandwidth_used() && p->throttled;
@@ -6733,6 +6738,11 @@ static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
 	return 0;
 }
 
+static inline int lb_throttled_hierarchy(struct task_struct *p, int dst_cpu)
+{
+	return 0;
+}
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent) {}
 static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
@@ -9369,14 +9379,18 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	/*
 	 * We do not migrate tasks that are:
 	 * 1) delayed dequeued unless we migrate load, or
-	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
-	 * 3) running (obviously), or
-	 * 4) are cache-hot on their current CPU, or
-	 * 5) are blocked on mutexes (if SCHED_PROXY_EXEC is enabled)
+	 * 2) target cfs_rq is in throttled hierarchy, or
+	 * 3) cannot be migrated to this CPU due to cpus_ptr, or
+	 * 4) running (obviously), or
+	 * 5) are cache-hot on their current CPU, or
+	 * 6) are blocked on mutexes (if SCHED_PROXY_EXEC is enabled)
 	 */
 	if ((p->se.sched_delayed) && (env->migration_type != migrate_load))
 		return 0;
 
+	if (lb_throttled_hierarchy(p, env->dst_cpu))
+		return 0;
+
 	/*
 	 * We want to prioritize the migration of eligible tasks.
 	 * For ineligible tasks we soft-limit them and only allow
-- 
2.39.5