Re: [PATCH RESEND] sched/fair: Only update stats for allowed CPUs when looking for dst group

[Adam Li <adamli@os.amperecomputing.com>]

Hi Chenyu,

Thanks for your comments.
On 10/12/2025 1:42 AM, Chen, Yu C wrote:
> On 10/11/2025 2:43 PM, Adam Li wrote:
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index bc0b7ce8a65d..d5ec15050ebc 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10671,7 +10671,7 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
>>       if (sd->flags & SD_ASYM_CPUCAPACITY)
>>           sgs->group_misfit_task_load = 1;
>>   -    for_each_cpu(i, sched_group_span(group)) {
>> +    for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
> 
> Looks good to me. One minor question, would pre-calculating the mask be better?

I do agree pre-calculating the cpumask can save cpu cycles, without
doing mask AND at each loop.

> Copied from select_idle_cpu():
> 
> cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
> cpumask_and(cpus, sched_group_span(sd), p->cpus_ptr);
> for_each_cpu(i, cpus) {
> 
But I am not sure if it is safe to use the percpu 'select_rq_mask'
in update_sg_wakeup_stats(). Or we have to allocate a 'struct cpumask'.

I tested bellow patch. It can work and fix the bug.
If it is safe to use 'select_rq_mask' , I can submit V2 patch.

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10664,6 +10664,7 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
                                          struct task_struct *p)
 {
        int i, nr_running;
+       struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_rq_mask);

        memset(sgs, 0, sizeof(*sgs));

@@ -10671,7 +10672,8 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
        if (sd->flags & SD_ASYM_CPUCAPACITY)
                sgs->group_misfit_task_load = 1;

-       for_each_cpu(i, sched_group_span(group)) {
+       cpumask_and(cpus, sched_group_span(group), p->cpus_ptr);
+       for_each_cpu(i, cpus) {
                struct rq *rq = cpu_rq(i);
                unsigned int local;


Thanks,
-adam