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

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Message-ID: <d6b78d12-298d-4316-91f4-bf7d3d7d5776@os.amperecomputing.com>
Date: Tue, 14 Oct 2025 18:51:59 +0800
From: Adam Li <adamli@...amperecomputing.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, cl@...ux.com,
 linux-kernel@...r.kernel.org, patches@...erecomputing.com,
 shkaushik@...erecomputing.com, mingo@...hat.com, peterz@...radead.org,
 juri.lelli@...hat.com, vincent.guittot@...aro.org
Subject: Re: [PATCH RESEND] sched/fair: Only update stats for allowed CPUs
 when looking for dst group

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