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[PATCH] sched: fix interactive task starvation

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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: Mike Galbraith <efault@gmx.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Acked-by: Con Kolivas <kernel@kolivas.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Mike Galbraith 2006-04-10 22:52:44 -07:00 committed by Linus Torvalds
parent 019ff2d57b
commit 5ce74abe78
1 changed files with 44 additions and 18 deletions
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62
kernel/sched.c
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@ -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;