[tip: sched/core] sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

[tip-bot2 for Ricardo Neri]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     180ff97c186971c2106e68470e4370ace0b8fbbc
Gitweb:        https://git.kernel.org/tip/180ff97c186971c2106e68470e4370ace0b8fbbc
Author:        Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
AuthorDate:    Mon, 20 Jul 2026 19:43:17 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 07 Aug 2026 18:27:09 +02:00

sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Tested-by: Andrea Righi <arighi@xxxxxxxxxx>
Link: https://patch.msgid.link/20260720-rneri-fix-cas-clusters-v6-1-bb500bf4afd4@xxxxxxxxxxxxxxx
---
 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 a24dd20..974e091 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13086,9 +13086,17 @@ 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