Re: [PATCH v2] sched/fair: combine detach into dequeue when migrating task

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 09/06/2022 05:53, Chengming Zhou wrote:
> When we are migrating task out of the CPU, we can combine detach
> into dequeue_entity() to save the independent detach_entity_cfs_rq()
> in migrate_task_rq_fair().
> 
> This optimization is like combining DO_ATTACH in the enqueue_entity()
> when migrating task to the CPU.
> 
> So we don't have to traverse the CFS tree twice to do these load
> detach and propagation.

By `propagation` you refer to the detach_entity_cfs_rq() ->
propagate_entity_cfs_rq() call?
This one wouldn't be called anymore with your change.

[...]

> @@ -4426,6 +4435,14 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq);
>  static void
>  dequeue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
>  {
> +	int action = UPDATE_TG;
> +
> +	/*
> +	 * If we are migrating task from the CPU, detach load_avg when dequeue.
> +	 */
> +	if (entity_is_task(se) && task_of(se)->on_rq == TASK_ON_RQ_MIGRATING)

- if (entity_is_task(se) && task_of(se)->on_rq == TASK_ON_RQ_MIGRATING)
+ if (entity_is_task(se) && task_on_rq_migrating(task_of(se)))

> +		action |= DO_DETACH;

With the `entity_is_task(se)` you make sure we only call
detach_entity_load_avg() and update_tg_load_avg() for the se
representing the task itself (and not taskgroups the task might run in).
So IMHO this looks good.

You save the propagate_entity_cfs_rq(&p->se) call from (2) by doing the
detach_entity_load_avg(), update_tg_load_avg() for a migrating task
inside (1) (the task being the first se in the loop )

detach_task()
  deactivate_task()
    dequeue_task_fair()
      for_each_sched_entity(se)
        dequeue_entity()
          update_load_avg() /* (1) */

  set_task_cpu()
    migrate_task_rq_fair()
      /* called detach_entity_cfs_rq() before the patch (2) */

[...]

> @@ -6940,15 +6957,10 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>  		se->vruntime -= min_vruntime;
>  	}
>  
> -	if (p->on_rq == TASK_ON_RQ_MIGRATING) {
> -		/*
> -		 * In case of TASK_ON_RQ_MIGRATING we in fact hold the 'old'
> -		 * rq->lock and can modify state directly.
> -		 */
> -		lockdep_assert_rq_held(task_rq(p));
> -		detach_entity_cfs_rq(&p->se);
> -
> -	} else {
> +	/*
> +	 * In case of TASK_ON_RQ_MIGRATING we already detach in dequeue_entity.
> +	 */
> +	if (p->on_rq != TASK_ON_RQ_MIGRATING) {

- if (p->on_rq != TASK_ON_RQ_MIGRATING) {
+ if (!task_on_rq_migrating(p)) {

[...]