Re: [PATCH 3/6] sched: Restore preferred CPUs when hotplug removes the last one

[Fushuai Wang <fushuai.wang@linux.dev>]

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 04b62c16fa8c..98f4f9434965 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8712,6 +8712,14 @@ int sched_cpu_deactivate(unsigned int cpu)
>  
>  	set_cpu_active(cpu, false);
>  
> +#ifdef CONFIG_PREFERRED_CPU
> +	if (cpumask_empty(cpu_preferred_mask)) {
> +		pr_notice("sched: last preferred CPU%u offlined; make all active CPUs preferred\n",
> +			  cpu);
> +		reset_preferred_cpumask();

Hi, Yury

If the CPU being offlined is the only preferred CPU, after
reset_preferred_cpumask(), the steal governor may still need
some time to tighten the mask again, and eventually only one
preferred CPU may remain. Would this introduce additional
task migration overhead?

Regards,
Fushuai.
> +	}
> +#endif
> +
>  	/*
>  	 * From this point forward, this CPU will refuse to run any task that
>  	 * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively