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

[Chen, Yu C]

On 6/9/2026 12:26 PM, K Prateek Nayak wrote:
> 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 ...

Right, this can leverage if (!nr_running) to skip the idle i, thus
avoid doing is_core_idle().

> >                  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;

We might not want to skip i if it has SMT busy siblings?
if ((!sched_smt_active() || is_core_idle(i)) && ...

thanks,
Chenyu