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SubjectRe: [RFC PATCH] sched/fair: Choose the CPU where short task is running during wake up
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On Fri, 2022-09-16 at 00:54 +0800, Chen Yu wrote:
>
> +/*
> + * If a task switches in and then voluntarily relinquishes the
> + * CPU quickly, it is regarded as a short running task.
> + * sysctl_sched_min_granularity is chosen as the threshold,
> + * as this value is the minimal slice if there are too many
> + * runnable tasks, see __sched_period().
> + */
> +static int is_short_task(struct task_struct *p)
> +{
> + return (p->se.sum_exec_runtime <=
> + (p->nvcsw * sysctl_sched_min_granularity));
> +}
> +
> /*
> * The purpose of wake_affine() is to quickly determine on which CPU we can run
> * soonest. For the purpose of speed we only consider the waking and previous
> @@ -6050,7 +6063,8 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
> if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
> return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu;
>
> - if (sync && cpu_rq(this_cpu)->nr_running == 1)
> + if ((sync && cpu_rq(this_cpu)->nr_running == 1) ||
> + is_short_task(cpu_curr(this_cpu)))
> return this_cpu;
>
> if (available_idle_cpu(prev_cpu))
> @@ -6434,6 +6448,21 @@ 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. If the waker on target CPU is a short
> + * task and the wakee is also a short task, pick
> + * target directly.
> + */
> + if (!has_idle_core && (5 * nr < 3 * sd->span_weight) &&
> + is_short_task(p) && is_short_task(cpu_curr(target)))

Should we check if target's rq's nr_running is 1, and if there's pending waking
task before picking it?

> + return target;
> }
> }
>

Thanks.

Tim

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