[PATCH v5 5/6] sched/fair: Allow load balancing between CPUs of identical capacity

[Ricardo Neri]

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 arch_scale_cpu_capacity() to reflect architectural capacity, excluding
runtime reductions due to side activity or thermal pressure. Guard this
check with the sched_cluster_active static key so that systems without
cluster topology are unaffected.

Tested-by: Christian Loehle <christian.loehle@xxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes in v5:
 * Optimized logic to identify same-arch clusters only when needed.
 * Added Tested-by tag from Christian. Thanks!

Changes in v4:
 * Implemented the check for cluster with a local variable for improved
   readability.

Changes in v3:
 * Reverted the inverted capacity check; the inverted form incorrectly
   allows migrations to CPUs of slightly less capacity.
 * Guarded the check for architectural capacity with the
   sched_cluster_active static key.

Changes in v2:
 * Used arch_scale_cpu_capacity() instead of capacity_of() to ignore
   runtime variability.
 * Inverted the check for runtime capacity. (Christian)
 * Reworded patch description for clarity.
---
 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 e55eb019d2c9..f4eb55cad54d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12992,13 +12992,20 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
 		    nr_running == 1) {
+			bool same_arch_cluster = static_branch_unlikely(&sched_cluster_active) &&
+						 (arch_scale_cpu_capacity(env->dst_cpu) ==
+						  arch_scale_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. Use architectural capacity to ignore
+			 * runtime variability.
 			 */
-			if (!smt_degraded_cap &&
+			if (!smt_degraded_cap && !same_arch_cluster &&
 			    !capacity_greater(capacity_of(env->dst_cpu), capacity))
 				continue;
 		}

-- 
2.43.0