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

Mike Galbraith
2006-07-05T11:37:27Z

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