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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: 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>
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1 changed files with 44 additions and 18 deletions
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@ -664,6 +664,48 @@ static int effective_prio(task_t *p)
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return prio;
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return prio;
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}
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}
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/*
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* We place interactive tasks back into the active array, if possible.
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*
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* To guarantee that this does not starve expired tasks we ignore the
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* interactivity of a task if the first expired task had to wait more
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* than a 'reasonable' amount of time. This deadline timeout is
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* load-dependent, as the frequency of array switched decreases with
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* increasing number of running tasks. We also ignore the interactivity
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* if a better static_prio task has expired, and switch periodically
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* regardless, to ensure that highly interactive tasks do not starve
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* the less fortunate for unreasonably long periods.
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*/
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static inline int expired_starving(runqueue_t *rq)
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{
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int limit;
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/*
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* Arrays were recently switched, all is well
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*/
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if (!rq->expired_timestamp)
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return 0;
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limit = STARVATION_LIMIT * rq->nr_running;
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/*
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* It's time to switch arrays
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*/
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if (jiffies - rq->expired_timestamp >= limit)
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return 1;
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/*
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* There's a better selection in the expired array
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*/
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if (rq->curr->static_prio > rq->best_expired_prio)
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return 1;
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/*
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* All is well
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*/
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return 0;
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}
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/*
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/*
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* __activate_task - move a task to the runqueue.
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* __activate_task - move a task to the runqueue.
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*/
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*/
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@ -671,7 +713,7 @@ static void __activate_task(task_t *p, runqueue_t *rq)
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{
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{
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prio_array_t *target = rq->active;
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prio_array_t *target = rq->active;
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if (batch_task(p))
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if (unlikely(batch_task(p) || expired_starving(rq)))
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target = rq->expired;
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target = rq->expired;
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enqueue_task(p, target);
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enqueue_task(p, target);
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rq->nr_running++;
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rq->nr_running++;
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@ -2489,22 +2531,6 @@ unsigned long long current_sched_time(const task_t *tsk)
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return ns;
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return ns;
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}
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}
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/*
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* We place interactive tasks back into the active array, if possible.
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*
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* To guarantee that this does not starve expired tasks we ignore the
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* interactivity of a task if the first expired task had to wait more
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* than a 'reasonable' amount of time. This deadline timeout is
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* load-dependent, as the frequency of array switched decreases with
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* increasing number of running tasks. We also ignore the interactivity
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* if a better static_prio task has expired:
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*/
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#define EXPIRED_STARVING(rq) \
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((STARVATION_LIMIT && ((rq)->expired_timestamp && \
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(jiffies - (rq)->expired_timestamp >= \
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STARVATION_LIMIT * ((rq)->nr_running) + 1))) || \
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((rq)->curr->static_prio > (rq)->best_expired_prio))
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/*
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/*
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* Account user cpu time to a process.
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* Account user cpu time to a process.
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* @p: the process that the cpu time gets accounted to
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* @p: the process that the cpu time gets accounted to
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@ -2640,7 +2666,7 @@ void scheduler_tick(void)
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if (!rq->expired_timestamp)
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if (!rq->expired_timestamp)
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rq->expired_timestamp = jiffies;
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rq->expired_timestamp = jiffies;
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if (!TASK_INTERACTIVE(p) || EXPIRED_STARVING(rq)) {
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if (!TASK_INTERACTIVE(p) || expired_starving(rq)) {
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enqueue_task(p, rq->expired);
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enqueue_task(p, rq->expired);
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if (p->static_prio < rq->best_expired_prio)
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if (p->static_prio < rq->best_expired_prio)
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rq->best_expired_prio = p->static_prio;
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rq->best_expired_prio = p->static_prio;
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