[PATCH v5 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.

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
Changes in v5:
 * Optimized logic to identify CPUs with busy SMT siblings only when
   needed. (Prateek, Chen Yu)
 * Added Reviewed-by tag from Prateek. Thanks!
 * Christian also provided his Reviewed-by tag, but the patch changed
   significantly since then. I did not think it was correct to keep it
   without him reviewing the updated patch first.

Changes in v4:
 * Introduced this patch.

Changes in v3:
 * N/A

Changes in v2:
 * N/A
---
 kernel/sched/fair.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d78467ec6ee1..892abd7fcc18 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12976,9 +12976,17 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
 		 * average load.
 		 */
 		if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-		    !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
-		    nr_running == 1)
-			continue;
+		    nr_running == 1) {
+			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.
+			 */
+			if (!smt_degraded_cap &&
+			    !capacity_greater(capacity_of(env->dst_cpu), capacity))
+				continue;
+		}
 
 		/*
 		 * Make sure we only pull tasks from a CPU of lower priority

-- 
2.43.0