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

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

Load imbalance is observed when the workload frequently forks new threads.
Due to CPU affinity, the workload can run on CPU 0-7 in the first
group, and only on CPU 8-11 in the second group. CPU 12-15 are always idle.

{ 0 1 2 3 4 5 6 7 } {8 9 10 11 12 13 14 15}
  * * * * * * * *    * * *  *

When looking for dst group for newly forked threads, in many times
update_sg_wakeup_stats() reports the second group has more idle CPUs
than the first group. The scheduler thinks the second group is less
busy. Then it selects least busy CPUs among CPU 8-11. Therefore CPU 8-11
can be crowded with newly forked threads, at the same time CPU 0-7
can be idle.

A task may not use all the CPUs in a schedule group due to CPU affinity.
Only update schedule group statistics for allowed CPUs.

Signed-off-by: Adam Li <adamli@os.amperecomputing.com>
---
Resending this patch from the patchset:
https://lore.kernel.org/lkml/20250717062036.432243-2-adamli@os.amperecomputing.com/

Only changed commit message. The single patch may be easier for reviewing.
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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) {
 		struct rq *rq = cpu_rq(i);
 		unsigned int local;
 
-- 
2.34.1