From 5ce74abe788a26698876e66b9c9ce7e7acc25413 Mon Sep 17 00:00:00 2001 From: Mike Galbraith Date: Mon, 10 Apr 2006 22:52:44 -0700 Subject: [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 Acked-by: Ingo Molnar Cc: Nick Piggin Acked-by: Con Kolivas Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/sched.c | 62 +++++++++++++++++++++++++++++++++++++++++----------------- 1 file changed, 44 insertions(+), 18 deletions(-) (limited to 'kernel') diff --git a/kernel/sched.c b/kernel/sched.c index dd153d6f8a04..2e8a146dd066 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -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; -- cgit v1.2.2