Re: [PATCH 2/5] sched/fair: Attach sched_domain_shared to sd_asym_cpucapacity

[K Prateek Nayak]

Hello Vincent,

On 5/6/2026 3:15 PM, Vincent Guittot wrote:
>> @@ -7925,7 +7925,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>>         struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
>>         int i, cpu, idle_cpu = -1, nr = INT_MAX;
>>
>> -       if (sched_feat(SIS_UTIL)) {
>> +       if (sched_feat(SIS_UTIL) && sd->shared) {
> 
> If shared is attached to sd_asym_cpucapacity instead of sd_llc we
> should never reach this point. Or I'm missing a case ?

So the hotpulg might race with a wakeup like:

  claim_asym_sched_domain_shared()
    init_sched_domain_shared(d, sd_asym);
    return true;
  update_top_cache_domain()
    rcu_assign_pointer(sd_llc, sd);
    ...                                              select_idle_sibling()
                                                       sd = rcu_dereference_all(sd_asym_cpucapacity)
                                                       /* sd_asym_cpucapacity still hasn't been updated */
                                                       if (sd /* NULL */) { ... }
                                                       sd = rcu_dereference_all(sd_llc); /* Valid */
                                                       select_idle_cpu(sd)
    rcu_assign_pointer(sd_asym_cpucapacity, sd);         sd->shared /* NULL */


This prevents that rare race where a remote CPU will see sd_llc
before sd_asym is published and take the !ASYM wakeup route only
to find sd->shared is NULL since sd_asym has claimed it.

> 
>>                 /*
>>                  * Increment because !--nr is the condition to stop scan.
>>                  *

-- 
Thanks and Regards,
Prateek