Re: [PATCH v5 2/5] sched/fair: Limited scan for idle cores when overloaded

[Tim Chen <tim.c.chen@linux.intel.com>]

On Fri, 2022-09-09 at 13:53 +0800, Abel Wu wrote:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5af9bf246274..7abe188a1533 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6437,26 +6437,42 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  		time = cpu_clock(this);
>  	}
>  
> -	if (sched_feat(SIS_UTIL) && !has_idle_core) {
> +	if (sched_feat(SIS_UTIL)) {
>  		sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
>  		if (sd_share) {
>  			/* because !--nr is the condition to stop scan */
>  			nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
> -			/* overloaded LLC is unlikely to have idle cpu/core */
> -			if (nr == 1)
> +
> +			/*
> +			 * Overloaded LLC is unlikely to have idle cpus.
> +			 * But if has_idle_core hint is true, a limited
> +			 * speculative scan might help without incurring
> +			 * much overhead.
> +			 */
> +			if (has_idle_core)
> +				nr = nr > 1 ? INT_MAX : 3;

The choice of nr is a very abrupt function of utilization when has_idle_core==true,
it is either feast or famine.  Why is such choice better than a smoother
reduction of nr vs utilization?  I agree that we want to scan more aggressively than
!has_idle_core, but it is not obvious why the above work better, versus something
like nr = nr*2+1.

Tim

> +			else if (nr == 1)
>  				return -1;
>  		}
>  	}
>  
>  	for_each_cpu_wrap(cpu, cpus, target + 1) {
> +		/*
> +		 * This might get the has_idle_cores hint cleared for a
> +		 * partial scan for idle cores but the hint is probably
> +		 * wrong anyway. What more important is that not clearing
> +		 * the hint may result in excessive partial scan for idle
> +		 * cores introducing innegligible overhead.
> +		 */
> +		if (!--nr)
> +			break;
> +
>  		if (has_idle_core) {
>  			i = select_idle_core(p, cpu, cpus, &idle_cpu);
>  			if ((unsigned int)i < nr_cpumask_bits)
>  				return i;
>  
>  		} else {
> -			if (!--nr)
> -				return -1;
>  			idle_cpu = __select_idle_cpu(cpu, p);
>  			if ((unsigned int)idle_cpu < nr_cpumask_bits)
>  				break;