Re: [RFC PATCH v2 2/2] sched/fair: Choose the CPU where short task is running during wake up

[Peter Zijlstra]

On Sun, Oct 23, 2022 at 11:33:39PM +0800, Chen Yu wrote:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8820d0d14519..3a8ee6232c59 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6249,6 +6249,11 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
>  	if (available_idle_cpu(prev_cpu))
>  		return prev_cpu;
>  
> +	/* The only running task is a short duration one. */
> +	if (cpu_rq(this_cpu)->nr_running == 1 &&
> +	    is_short_task(cpu_curr(this_cpu)))
> +		return this_cpu;
> +
>  	return nr_cpumask_bits;
>  }

This is very close to using is_short_task() as dynamic WF_SYNC hint, no?

> @@ -6623,6 +6628,23 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>  			/* overloaded LLC is unlikely to have idle cpu/core */
>  			if (nr == 1)
>  				return -1;
> +
> +			/*
> +			 * If nr is smaller than 60% of llc_weight, it
> +			 * indicates that the util_avg% is higher than 50%.
> +			 * This is calculated by SIS_UTIL in
> +			 * update_idle_cpu_scan(). The 50% util_avg indicates
> +			 * a half-busy LLC domain. System busier than this
> +			 * level could lower its bar to choose a compromised
> +			 * "idle" CPU, so as to avoid the overhead of cross
> +			 * CPU wakeup. If the task on target CPU is a short
> +			 * duration one, and it is the only running task, pick
> +			 * target directly.
> +			 */
> +			if (!has_idle_core && (5 * nr < 3 * sd->span_weight) &&
> +			    cpu_rq(target)->nr_running == 1 &&
> +			    is_short_task(cpu_curr(target)))
> +				return target;
>  		}
>  	}

And here you're basically saying that if the domain is 'busy' and the
task is short, don't spend time searching for a better location.

Should we perhaps only consider shortness; after all, spending more time
searching for an idle cpu than the task would've taken to run is daft.
Business of the domain seems unrelated to that.


Also, I'm not sure on your criteria for short; but I don't have enough
thoughts on that yet.