[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>

diff --git a/kernel/sched.c b/kernel/sched.c
index dd153d6f8a04b7c2ba8c4954b8628ef70ad307ce..2e8a146dd0660a8e8a914ee0936e66832934b33d 100644 (file)
--- 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;