[PATCH v4 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.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
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 77554d7410ff..74b9669d149b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12939,6 +12939,9 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 	int i;
 
 	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
+		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);
 		unsigned long capacity, load, util;
 		unsigned int nr_running;
@@ -12983,9 +12986,13 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 *
 		 * 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 (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-		    !smt_degraded_cap &&
+		    !smt_degraded_cap && !same_arch_cluster &&
 		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
 		    nr_running == 1)
 			continue;

-- 
2.43.0