[PATCH 2/2] sched/fair: Reject misfit pulls onto busy SMT siblings on asym-capacity

[Andrea Righi]

When SD_ASYM_CPUCAPACITY load balancing considers pulling a misfit task,
capacity_of(dst_cpu) can overstate available compute if the SMT sibling
is busy: the core does not deliver its full nominal capacity.

If SMT is active and dst_cpu is not on a fully idle core, skip this
destination so we do not migrate a misfit expecting a capacity upgrade
we cannot actually provide.

No functional changes on systems with only asymmetric CPUs or only SMT.

Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Christian Loehle <christian.loehle@xxxxxxx>
Cc: Koba Ko <kobak@xxxxxxxxxx>
Reported-by: Felix Abecassis <fabecassis@xxxxxxxxxx>
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 kernel/sched/fair.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7f09191014d18..7bebceb5ed9df 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10607,10 +10607,16 @@ static bool update_sd_pick_busiest(struct lb_env *env,
 	 * We can use max_capacity here as reduction in capacity on some
 	 * CPUs in the group should either be possible to resolve
 	 * internally or be covered by avg_load imbalance (eventually).
+	 *
+	 * When SMT is active, only pull a misfit to dst_cpu if it is on a
+	 * fully idle core; otherwise the effective capacity of the core is
+	 * reduced and we may not actually provide more capacity than the
+	 * source.
 	 */
 	if ((env->sd->flags & SD_ASYM_CPUCAPACITY) &&
 	    (sgs->group_type == group_misfit_task) &&
-	    (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
+	    ((sched_smt_active() && !is_core_idle(env->dst_cpu)) ||
+	     !capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
 	     sds->local_stat.group_type != group_has_spare))
 		return false;
 
-- 
2.53.0