Re: [PATCH] add requeue task

[Nick Piggin]

Con Kolivas wrote:
> add requeue task
>
>
>
> ------------------------------------------------------------------------
>
> We can requeue tasks for cheaper then doing a complete dequeue followed by
> an enqueue. Add the requeue_task function and perform it where possible.
>
> Change the granularity code to requeue tasks at their best priority
> instead of changing priority while they're running. This keeps tasks at
> their top interactive level during their whole timeslice.

I wonder... these things are all in sufficiently rarely used places,
that the icache miss might be more costly than the operations saved.

But....

> Signed-off-by: Con Kolivas <kernel@xxxxxxxxxxx>
>
> Index: linux-2.6.10-rc1-mm2/kernel/sched.c
> ===================================================================
> --- linux-2.6.10-rc1-mm2.orig/kernel/sched.c	2004-11-02 14:48:54.686316718 +1100
> +++ linux-2.6.10-rc1-mm2/kernel/sched.c	2004-11-02 14:52:51.805763544 +1100
> @@ -579,6 +579,16 @@ static void enqueue_task(struct task_str
>  }
>  
>  /*
> + * Put task to the end of the run list without the overhead of dequeue
> + * followed by enqueue.
> + */
> +static void requeue_task(struct task_struct *p, prio_array_t *array)
> +{
> +	list_del(&p->run_list);
> +	list_add_tail(&p->run_list, array->queue + p->prio);
> +}
> +
> +/*
>   * Used by the migration code - we pull tasks from the head of the
>   * remote queue so we want these tasks to show up at the head of the
>   * local queue:
> @@ -2425,8 +2435,7 @@ void scheduler_tick(void)
>  			set_tsk_need_resched(p);
>  
>  			/* put it at the end of the queue: */
> -			dequeue_task(p, rq->active);
> -			enqueue_task(p, rq->active);
> +			requeue_task(p, rq->active);
>  		}
>  		goto out_unlock;
>  	}
> @@ -2467,10 +2476,8 @@ void scheduler_tick(void)
>  			(p->time_slice >= TIMESLICE_GRANULARITY(p)) &&
>  			(p->array == rq->active)) {
>  
> -			dequeue_task(p, rq->active);
> +			requeue_task(p, rq->active);
>  			set_tsk_need_resched(p);
> -			p->prio = effective_prio(p);
> -			enqueue_task(p, rq->active);
>  		}
>  	}
>  out_unlock:

This isn't a 1:1 transformation. Looks like the effective_prio there
might be superfluous, but if so that should be a different patch.

> @@ -3569,8 +3576,14 @@ asmlinkage long sys_sched_yield(void)
>  	} else if (!rq->expired->nr_active)
>  		schedstat_inc(rq, yld_exp_empty);
>  
> -	dequeue_task(current, array);
> -	enqueue_task(current, target);
> +	if (array != target) {
> +		dequeue_task(current, array);
> +		enqueue_task(current, target);
> +	} else
> +		/*
> +		 * requeue_task is cheaper so perform that if possible.
> +		 */
> +		requeue_task(current, array);
>  
>  	/*
>  	 * Since we are going to call schedule() anyway, there's

Hmm if you have to go to this trouble I'd say its not worth it.
Ingo may want to weigh in though.
-
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