Re: [PATCH v4 1/6] sched/fair: Do not skip CPUs of similar capacity with busy SMT siblings

[K Prateek Nayak]

Hello Ricardo,

On 6/8/2026 6:27 PM, Ricardo Neri wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f4ed841f766f..229f32cebf1f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12924,6 +12924,7 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
>         int i;
> 
>         for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> +               bool smt_degraded_cap = sched_smt_active() && !is_core_idle(i);

Based on a quick look at the generated asm, we seem to be calculating
"smt_degraded_cap" regardless of whether we are on a
"SD_ASYM_CPUCAPACITY" domain or not which is an unnecessary overhead of
doing a is_core_idle() on symmetric domains.

We can have an indicator up top like:

    bool asym_balance = !!(env->sd->flags & SD_ASYM_CPUCAPACITY);

and later use that when computing smt_degraded_cap:

    bool smt_degraded_cap = asym_balance && (sched_smt_active() && !is_core_idle(i));

I would even suggest pushing this until later and do ...

>                 unsigned long capacity, load, util;
>                 unsigned int nr_running;
>                 enum fbq_type rt;
> @@ -12964,8 +12965,12 @@ static struct rq *sched_balance_find_src_rq(struct lb_env *env,
>                  * eventually lead to active_balancing high->low capacity.
>                  * Higher per-CPU capacity is considered better than balancing
>                  * average load.
> +                *
> +                * Busy SMT siblings reduce the capacity of CPU i. Do not skip
> +                * it in this case.
>                  */
>                 if (env->sd->flags & SD_ASYM_CPUCAPACITY &&

...

    if (asym_balance) {
        /* Busy SMT reduces capacity */
        if (sched_smt_active() && !is_core_idle(i))
            continue;

        /* Other bits */
        if (...)
            continue;
    }

Thoughts?

Aside from that, the changes makes sense to me. Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>


> +                   !smt_degraded_cap &&
>                     !capacity_greater(capacity_of(env->dst_cpu), capacity) &&
>                     nr_running == 1)
>                         continue;
> 
> --
> 2.43.0
> 

-- 
Thanks and Regards,
Prateek