[tip: sched/core] sched/fair: Allow load balancing between CPUs of identical capacity

[tip-bot2 for Ricardo Neri]

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

Commit-ID:     7fd540b1bcaf59289e6e921463037d4eadc1d75b
Gitweb:        https://git.kernel.org/tip/7fd540b1bcaf59289e6e921463037d4eadc1d75b
Author:        Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
AuthorDate:    Mon, 20 Jul 2026 19:43:21 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 07 Aug 2026 18:27:11 +02:00

sched/fair: Allow load balancing between CPUs of identical capacity

sched_balance_find_src_rq() avoids selecting a runqueue with a single
running task as busiest if doing so results in migrating the task to a
CPU with less than ~5% of extra capacity. It also unintentionally
prevents migrations between CPUs of identical capacity.

When CONFIG_SCHED_CLUSTER is enabled, load should be balanced across
clusters of CPUs with the same capacity. Allowing migration between CPUs
of identical capacity is necessary to meet this goal.

Use get_actual_cpu_capacity() to reflect architectural capacity as well
as diminished capacity due to hardware or cpufreq pressure. Guard this
check with the sched_cluster_active static key so that systems without
cluster topology are unaffected.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Christian Loehle <christian.loehle@xxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: Christian Loehle <christian.loehle@xxxxxxx>
Tested-by: Andrea Righi <arighi@xxxxxxxxxx>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-5-bb500bf4afd4@xxxxxxxxxxxxxxx
---
 kernel/sched/fair.c |  9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f1b4db3..dcf860c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13102,13 +13102,20 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
 		    nr_running == 1) {
+			bool cluster_equal_cap = static_branch_unlikely(&sched_cluster_active) &&
+						 (get_actual_cpu_capacity(env->dst_cpu) ==
+						  get_actual_cpu_capacity(i));
 			bool smt_degraded_cap = sched_smt_active() && !is_core_idle(i);
 
 			/*
 			 * Busy SMT siblings reduce the capacity of CPU @i. Do
 			 * not skip it in this case.
+			 *
+			 * CONFIG_SCHED_CLUSTER requires balancing load across
+			 * clusters of identical capacity, accounting for
+			 * hardware and cpufreq pressure.
 			 */
-			if (!smt_degraded_cap &&
+			if (!smt_degraded_cap && !cluster_equal_cap &&
 			    !capacity_greater(capacity_of(env->dst_cpu), capacity))
 				continue;
 		}