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

["Chen, Yu C" <yu.c.chen@intel.com>]

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