Re: [2.6.16-rc6 patch] fix interactive task starvation

Andrew Morton
2006-03-18T08:08:47Z

Mike Galbraith <efault@gmx.de> wrote:
>
> 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?

What does that question mean, btw?

-mm is looking like linux-2.6.38 at present so of course things got tangled
up - sched-activate-sched-batch-expired.patch modifies __activate_task().

I ended up with the below.

Which do we think is more likely to be true - batch_task(p) or
expired_starving(rq)? batch_task() looks cheaper to evaluate so I put that
first. But I guess it's less likely to be true. hmm.

diff -puN kernel/sched.c~sched-fix-interactive-task-starvation kernel/sched.c
--- devel/kernel/sched.c~sched-fix-interactive-task-starvation 2006-03-17 23:55:12.000000000 -0800
+++ devel-akpm/kernel/sched.c 2006-03-17 23:59:03.000000000 -0800
@@ -733,14 +733,56 @@ static inline void dec_nr_running(task_t
}

/*
+ * 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.
*/
static void __activate_task(task_t *p, runqueue_t *rq)
{
prio_array_t *target = rq->active;

- if (batch_task(p))
- target = rq->expired;
+ if (unlikely(batch_task(p) || expired_starving(rq)))
+ target = rq->expired;
enqueue_task(p, target);
inc_nr_running(p, rq);
}
@@ -2614,22 +2656,6 @@ unsigned long long current_sched_time(co
}

/*
- * 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()
@@ -2764,7 +2790,7 @@ 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;
_