Re: [PATCH 1/2] sched/fair: Record the short sleeping time of a task

[Aaron Lu]

On Tue, Sep 26, 2023 at 01:11:02PM +0800, Chen Yu wrote:
> During task wakeup, the wakee firstly checks if its previous
> running CPU is idle. If yes, choose that CPU as its first
> choice. However, in most cases, the wakee's previous CPU
> could be chosen by someone else, which breaks the cache
> locality.
> 
> Proposes a mechanism to reserve the task's previous
> CPU for a short while. In this reservation period, other
> tasks are not allowed to pick that CPU until a timeout.
> The reservation period is defined as the average short
> sleep time of the task. To be more specific, it is the
> time delta between this task being dequeued and enqueued.
> Only the sleep time shorter than sysctl_sched_migration_cost
> will be recorded. If the sleep time is longer than
> sysctl_sched_migration_cost, give the reservation period
> a penalty by shrinking it to half. In this way, the 'burst'
> sleeping time of the task is honored, meanwhile, if that
> task becomes a long-sleeper, the reservation time of that
> task is shrunk to reduce the impact on task wakeup.
> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> ---
>  include/linux/sched.h |  3 +++
>  kernel/sched/fair.c   | 21 +++++++++++++++++++++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index dc37ae787e33..4a0ac0276384 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -561,6 +561,9 @@ struct sched_entity {
>  	u64				vruntime;
>  	s64				vlag;
>  	u64				slice;
> +	u64				prev_dequeue_time;
> +	/* the reservation period of this task during wakeup */
> +	u64				sis_rsv_avg;

Nit: these info are only relavant for task, not group so might be better
to move them to task_struct to save a little memory?

>  
>  	u64				nr_migrations;
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d0877878bcdb..297b9470829c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6456,6 +6456,24 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	struct sched_entity *se = &p->se;
>  	int idle_h_nr_running = task_has_idle_policy(p);
>  	int task_new = !(flags & ENQUEUE_WAKEUP);
> +	u64 last_dequeue = p->se.prev_dequeue_time;
> +	u64 now = sched_clock_cpu(task_cpu(p));

I think cpu_of(rq) is more clear than task_cpu(p). Using task_cpu(p)
seems to suggest task_cpu(p) can be different from cpu_of(rq).

> +
> +	/*
> +	 * If the task is a short-sleepting task, there is no need
> +	 * to migrate it to other CPUs. Estimate the average short sleeping
> +	 * time of the wakee. This sleep time is used as a hint to reserve
> +	 * the dequeued task's previous CPU for a short while. During this
> +	 * reservation period, select_idle_cpu() prevents other wakees from
> +	 * choosing this CPU. This could bring a better cache locality.
> +	 */
> +	if ((flags & ENQUEUE_WAKEUP) && last_dequeue && cpu_online(task_cpu(p)) &&

Hmm...the cpu_online() check looks weird. If the cpu is offlined, no task
will be enqueued there, right?

Thanks,
Aaron

> +	    now > last_dequeue) {
> +		if (now - last_dequeue < sysctl_sched_migration_cost)
> +			update_avg(&p->se.sis_rsv_avg, now - last_dequeue);
> +		else
> +			p->se.sis_rsv_avg >>= 1;
> +	}
>  
>  	/*
>  	 * The code below (indirectly) updates schedutil which looks at
> @@ -6550,6 +6568,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  	int task_sleep = flags & DEQUEUE_SLEEP;
>  	int idle_h_nr_running = task_has_idle_policy(p);
>  	bool was_sched_idle = sched_idle_rq(rq);
> +	u64 now;
>  
>  	util_est_dequeue(&rq->cfs, p);
>  
> @@ -6611,6 +6630,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  dequeue_throttle:
>  	util_est_update(&rq->cfs, p, task_sleep);
>  	hrtick_update(rq);
> +	now = sched_clock_cpu(cpu_of(rq));
> +	p->se.prev_dequeue_time = task_sleep ? now : 0;
>  }
>  
>  #ifdef CONFIG_SMP
> -- 
> 2.25.1
>