[PATCH v2 3/4] 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.

We are interested in the architectural capacity of the involved CPUs,
excluding any reductions due to side activity or thermal pressure. Use
arch_scale_cpu_capacity().

While here, invert the check for runtime capacity for clarity.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes since v1:
 * 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 | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 166a5b109e0e..4105717e64fe 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11816,9 +11816,14 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 * eventually lead to active_balancing high->low capacity.
 		 * Higher per-CPU capacity is considered better than balancing
 		 * average load.
+		 *
+		 * Cluster scheduling requires balancing load across clusters
+		 * of identical capacity. Use architectural capacity to ignore
+		 * runtime variability.
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
+		    arch_scale_cpu_capacity(env->dst_cpu) != arch_scale_cpu_capacity(i) &&
+		    capacity_greater(capacity, capacity_of(env->dst_cpu)) &&
 		    nr_running == 1)
 			continue;
 

-- 
2.43.0