Re: [RFC PATCH RESEND 01/10] sched/fair: Do not set_rd_overloaded() if rd->online != env->cpus

[K Prateek Nayak]

Hello Xin,

On 9/10/2026 9:59 AM, Xin Zhao wrote:
> In update_sg_lb_stats(), it only traverses sched_group that belongs to
> env->cpus, but env->cpus may not necessarily equal rd->online. This can
> lead to the incorrect clearing of the overloaded flag of rd. For example,
> if cpuA belongs to the online CPU mask of the rd but does not belong to
> env->cpus, and cpuA consistently maintains nr_running >= 2, while other
> CPUs in rd->online keep rq->nr_running <= 1, the overloaded flag of rd
> will not be set until next update of update_sd_lb_stats() for that rd.
> During this period, sched_balance_newidle() will prematurely return due to
> the incorrect assumption that the rd is in a non-overloaded state.

Only two cases manipulate env.cpus:

1. LBF_DST_PINNED: The CPU doing the load balancing clears itself from
   env.cpus since pinned tasks cannot be moved to it and goes to
   "more_balance" but "more_balance" does not recompute stats and never
   reaches update_sd_lb_stats().

2. LBF_ALL_PINNED: CPU with no movable task is cleared from env.cpus.
   How will rd->overload being set for a CPU that cannot be helped make
   newidle balance any more efficient?

  Since LBF_ALL_PINNED is known with busiest's rq_lock held, maybe you
  can set a rq->flag and later consume it in add_nr_running() to
  do set_rd_overloaded() selectively.


Now a combination of (1) -> (2) -> redo can actually leave the original
"dest_cpu" out of the env.cpus which might be problematic.

Vincent, do you know why we clear the original dest_cpu (the CPU doing
load balancing) from "env.cpus" in LBF_DST_PINNED?

We update the destination to env.new_dst_cpu, "busiest" is till the same
and instead of moving load from A -> B, we are moving it from A -> C.

Later, if we do a "redo", B can still be a valid target for a different
busiest CPU with movable tasks right?

> 
> In update_sd_lb_stats(), add a check to verify whether rd->online is equal
> to env->cpus before calling set_rd_overloaded() to avoid such incorrect
> settings.
> 
> Signed-off-by: Xin Zhao <jackzxcui1989@xxxxxxx>
> ---
>  kernel/sched/fair.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index dcf860c59a14..13e873b1ef58 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -12679,8 +12679,12 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  		env->fbq_type = fbq_classify_group(&sds->busiest_stat);
>  
>  	if (!env->sd->parent) {
> -		/* update overload indicator if we are at root domain */
> -		set_rd_overloaded(env->dst_rq->rd, sg_overloaded);
> +		/*
> +		 * Update overload indicator if we are at root domain.
> +		 * Note that env->cpus may change during sched_balance_rq().
> +		 */
> +		if (cpumask_equal(env->dst_rq->rd->online, env->cpus))
> +			set_rd_overloaded(env->dst_rq->rd, sg_overloaded);
>  
>  		/* Update over-utilization (tipping point, U >= 0) indicator */
>  		set_rd_overutilized(env->dst_rq->rd, sg_overutilized);

-- 
Thanks and Regards,
Prateek