Re: [PATCH 4/5] sched/fair: Reject misfit pulls onto busy SMT siblings on asym-capacity

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 5/9/26 11:37 PM, Andrea Righi wrote:
> 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.
> 
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Christian Loehle <christian.loehle@arm.com>
> Cc: Koba Ko <kobak@nvidia.com>
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Reported-by: Felix Abecassis <fabecassis@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>   kernel/sched/fair.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6f0835c15ee11..2ddba8bd27e59 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9693,6 +9693,7 @@ struct lb_env {
>   
>   	int			dst_cpu;
>   	struct rq		*dst_rq;
> +	bool			dst_core_idle;
>   
>   	struct cpumask		*dst_grpmask;
>   	int			new_dst_cpu;
> @@ -10918,10 +10919,16 @@ 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;
>   
> @@ -11485,6 +11492,8 @@ 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;


This is kind of similar to what ASYM_PACKING would have done at MC domain with
equal CPU capacities. i.e pull the load if the core is idle.

In your table in the cover-letter, if you do "NO ASYM + SIS_UTIL + ASYM_PACKING (at MC)"
does it achieve close to "ASYM + SMT + SIS_UTIL"?