sched/fair: Reject misfit pulls onto busy SMT siblings on asym-capacity

BugLink: https://bugs.launchpad.net/bugs/2150671

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.

Reported-by: Felix Abecassis <fabecassis@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260509180955.1840064-5-arighi@nvidia.com
(cherry picked from commit bf6aa72 linux-next)
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Seth Forshee <sforshee@nvidia.com>
Acked-by: Nirmoy Das <nirmoyd@nvidia.com>
Acked-by: Matthew R. Ochs <mochs@nvidia.com>
Signed-off-by: Seth Forshee <sforshee@nvidia.com>

Author: arighi

kernel/sched/fair.c

@@ -9496,6 +9496,7 @@ struct lb_env {
 
 	int			dst_cpu;
 	struct rq		*dst_rq;
+	bool			dst_core_idle;
 
 	struct cpumask		*dst_grpmask;
 	int			new_dst_cpu;
@@ -10737,10 +10738,16 @@ 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) ||
+	    (!env->dst_core_idle ||
+	     !capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) ||
 	     sds->local_stat.group_type != group_has_spare))
 		return false;
 
@@ -11307,6 +11314,8 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 	unsigned long sum_util = 0;
 	bool sg_overloaded = 0, sg_overutilized = 0;
 
+	env->dst_core_idle = !sched_smt_active() || is_core_idle(env->dst_cpu);
+
 	do {
 		struct sg_lb_stats *sgs = &tmp_sgs;
 		int local_group;