[2.6.16-rc6 patch] fix interactive task starvation

Mike Galbraith
2006-03-18T05:06:20Z

Greetings,

The patch below fixes 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.

This has not been verified by anyone. Comments?

-Mike

Signed-off-by: Mike Galbraith <efault@gmx.de>

--- linux-2.6.16-rc6/kernel/sched.c.org 2006-03-17 14:48:35.000000000 +0100
+++ linux-2.6.16-rc6/kernel/sched.c 2006-03-17 17:41:25.000000000 +0100
@@ -662,11 +662,30 @@
}

/*
+ * 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))
+
+/*
* __activate_task - move a task to the runqueue.
*/
static inline void __activate_task(task_t *p, runqueue_t *rq)
{
- enqueue_task(p, rq->active);
+ prio_array_t *array = rq->active;
+ if (unlikely(EXPIRED_STARVING(rq)))
+ array = rq->expired;
+ enqueue_task(p, array);
rq->nr_running++;
}

@@ -2461,22 +2480,6 @@
}

/*
- * 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
* @hardirq_offset: the offset to subtract from hardirq_count()