[PATCH v4 1/6] sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

[Ricardo Neri]

When picking a busiest CPU with only one running task, the function
sched_balance_find_src_rq() skips candidate CPUs if the destination CPU has
less than ~5% extra capacity. This condition only holds if all the SMT
siblings of a CPU are idle.

SMT siblings share the computing resources of a physical core and this
results in reduced capacity if more than one sibling is busy.

Skipping a CPU as described would prevent the load balancer from pulling
tasks from a scheduling group previously and correctly identified as
group_smt_balance (i.e., one with more than one task running).

Do not skip a candidate CPU of similar capacity if it has busy SMT
siblings.

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes in v4:
 * Introduced this patch.

Changes in v3:
 * N/A

Changes in v2:
 * N/A
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f4ed841f766f..229f32cebf1f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12924,6 +12924,7 @@ 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 smt_degraded_cap = sched_smt_active() && !is_core_idle(i);
 		unsigned long capacity, load, util;
 		unsigned int nr_running;
 		enum fbq_type rt;
@@ -12964,8 +12965,12 @@ 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.
+		 *
+		 * Busy SMT siblings reduce the capacity of CPU i. Do not skip
+		 * it in this case.
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
+		    !smt_degraded_cap &&
 		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
 		    nr_running == 1)
 			continue;

-- 
2.43.0