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Commit 5ce74abe authored by Mike Galbraith's avatar Mike Galbraith Committed by Linus Torvalds
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[PATCH] sched: fix interactive task starvation 



Fix a starvation problem that occurs when a stream of highly interactive tasks
delay an array switch for extended periods despite EXPIRED_STARVING(rq) being
true.  AFAIKT, the only choice is to enqueue awakening tasks on the expired
array in this case.

Without this patch, it can be nearly impossible to remotely login to a busy
server, and interactive shell commands can starve for minutes.

Also, convert the EXPIRED_STARVING macro into an inline function which humans
can understand.

Signed-off-by: default avatarMike Galbraith <efault@gmx.de>
Acked-by: default avatarIngo Molnar <mingo@elte.hu>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Acked-by: default avatarCon Kolivas <kernel@kolivas.org>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 019ff2d5

kernel/sched.c

+44 −18
Original line number Diff line number Diff line
@@ -664,6 +664,48 @@ static int effective_prio(task_t *p) 
	return prio;

}



/*

 * We place interactive tasks back into the active array, if possible.

 *

 * To guarantee that this does not starve expired tasks we ignore the

 * interactivity of a task if the first expired task had to wait more

 * than a 'reasonable' amount of time. This deadline timeout is

 * load-dependent, as the frequency of array switched decreases with

 * increasing number of running tasks. We also ignore the interactivity

 * if a better static_prio task has expired, and switch periodically

 * regardless, to ensure that highly interactive tasks do not starve

 * the less fortunate for unreasonably long periods.

 */

static inline int expired_starving(runqueue_t *rq)

{

	int limit;



	/*

	 * Arrays were recently switched, all is well

	 */

	if (!rq->expired_timestamp)

		return 0;



	limit = STARVATION_LIMIT * rq->nr_running;



	/*

	 * It's time to switch arrays

	 */

	if (jiffies - rq->expired_timestamp >= limit)

		return 1;



	/*

	 * There's a better selection in the expired array

	 */

	if (rq->curr->static_prio > rq->best_expired_prio)

		return 1;



	/*

	 * All is well

	 */

	return 0;

}



/*

 * __activate_task - move a task to the runqueue.

 */
@@ -671,7 +713,7 @@ static void __activate_task(task_t *p, runqueue_t *rq) 
{

	prio_array_t *target = rq->active;



	if (batch_task(p))

	if (unlikely(batch_task(p) || expired_starving(rq)))

		target = rq->expired;

	enqueue_task(p, target);

	rq->nr_running++;
@@ -2489,22 +2531,6 @@ unsigned long long current_sched_time(const task_t *tsk) 
	return ns;

}



/*

 * We place interactive tasks back into the active array, if possible.

 *

 * To guarantee that this does not starve expired tasks we ignore the

 * interactivity of a task if the first expired task had to wait more

 * than a 'reasonable' amount of time. This deadline timeout is

 * load-dependent, as the frequency of array switched decreases with

 * increasing number of running tasks. We also ignore the interactivity

 * if a better static_prio task has expired:

 */

#define EXPIRED_STARVING(rq) \

	((STARVATION_LIMIT && ((rq)->expired_timestamp && \

		(jiffies - (rq)->expired_timestamp >= \

			STARVATION_LIMIT * ((rq)->nr_running) + 1))) || \

			((rq)->curr->static_prio > (rq)->best_expired_prio))



/*

 * Account user cpu time to a process.

 * @p: the process that the cpu time gets accounted to
@@ -2640,7 +2666,7 @@ void scheduler_tick(void) 


		if (!rq->expired_timestamp)

			rq->expired_timestamp = jiffies;

		if (!TASK_INTERACTIVE(p) || EXPIRED_STARVING(rq)) {

		if (!TASK_INTERACTIVE(p) || expired_starving(rq)) {

			enqueue_task(p, rq->expired);

			if (p->static_prio < rq->best_expired_prio)

				rq->best_expired_prio = p->static_prio;