Re: [PATCH] sched: Add SCHED_BGND (background) scheduling policy

[Mike Galbraith <efault@gmx.de>]

On Wed, 2006-07-05 at 09:35 +1000, Peter Williams wrote:

> @@ -3332,23 +3447,25 @@ need_resched_nonpreemptible:
>  	}
>  
>  	array = rq->active;
> -	if (unlikely(!array->nr_active)) {
> -		/*
> -		 * Switch the active and expired arrays.
> -		 */
> -		schedstat_inc(rq, sched_switch);
> -		rq->active = rq->expired;
> -		rq->expired = array;
> -		array = rq->active;
> -		rq->expired_timestamp = 0;
> -		rq->best_expired_prio = MAX_PRIO;
> -	}
> +	if (unlikely(!array->nr_active))
> +		array = switch_arrays(rq, MAX_PRIO);
>  
>  	idx = sched_find_first_bit(array->bitmap);
> +get_next:
>  	queue = array->queue + idx;
>  	next = list_entry(queue->next, struct task_struct, run_list);
> +	/* very strict backgrounding */
> +	if (unlikely(task_in_background(next) && rq->expired->nr_active)) {
> +		int tmp = sched_find_first_bit(rq->expired->bitmap);
> +
> +		if (likely(tmp < idx)) {
> +			array = switch_arrays(rq, idx);
> +			idx = tmp;
> +			goto get_next;

Won't this potentially expire the mutex holder which you specifically
protect in scheduler_tick() if it was preempted before being ticked?
The task in the expired array could also be a !safe_to_background() task
who already had a chance to run, and who's slice expired.

If it's worth protecting higher priority tasks from mutex holders ending
up in the expired array, then there's a case that should be examined.
There's little difference between a background task acquiring a mutex,
and a normal task with one tick left on it's slice.  Best for sleepers
is of course to just say no to expiring mutex holders period.

	-Mike

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