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

Peter Williams
2006-07-05T13:59:22Z

Mike Galbraith wrote:
> 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?

I don't think so as its prio value should cause task_in_background() to
fail.

> 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 !safe_to_background() it's in our interest to let it run in
order to free up the resource that it's holding.

>
> 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.

It's more than just stopping them end up in the expired array. It's
stopping them being permanently in the expired array.

> There's little difference between a background task acquiring a mutex,
> and a normal task with one tick left on it's slice.

The difference is that the background task could stay there forever.

> Best for sleepers
> is of course to just say no to expiring mutex holders period.

In spite of my comments above, I agree that not expiring mutex holders
might (emphasis on the "might") be good for overall system performance
by reducing the time for which locks are held. Giving them a whole new
time slice on the active array might be too generous though. It could
become quite complex.

Peter
--
Peter Williams pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce