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

[Chen, Yu C]

On 10/14/2025 6:51 PM, Adam Li wrote:
> 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'.

Allocating dynamically would be costly. Using percpu select_rq_mask is
safe in this scenario: the waker's CPU has already disabled local irq
via raw_spinlock_irqsave(&p->pi_lock), so I suppose no one can modify
it simultaneously. Moreover, if the fast wakeup path select_idle_sibling()
 can use it, the slow path sched_balance_find_dst_cpu() should also be able
to do so IMO.

> 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)) {

nice-to-have:
maybe add a comment here that cpus is not empty, because
we have cpumask_intersects() check in sched_balance_find_dst_group(),
(just in case sgs->group_type incorrectly remain 0 which is 
group_has_spare, if
the cpus is empty)

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

and from my understanding, for this percpu version,

Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

thanks,
Chenyu