[PATCH] sched: remove starvation in check_preempt_equal_prio()

From: Hillf Danton
Date: Tue May 24 2011 - 09:34:12 EST


If there are pushable tasks and they are high enough in priority, in which
case task p is covered, the current could keep holding its CPU.

Even if current task has to release its CPU, requeuing task p could result in
starvation of tasks that are of same priority and have been waiting on RQ for
a couple of hours:/

Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
---

--- tip-git/kernel/sched_rt.c Sun May 22 20:12:01 2011
+++ sched_rt.c Tue May 24 21:01:51 2011
@@ -1028,24 +1028,23 @@ select_task_rq_rt(struct task_struct *p,
return cpu;
}

+static struct task_struct *pick_next_pushable_task(struct rq *);
+
static void check_preempt_equal_prio(struct rq *rq, struct task_struct *p)
{
- if (rq->curr->rt.nr_cpus_allowed == 1)
- return;
-
- if (p->rt.nr_cpus_allowed != 1
- && cpupri_find(&rq->rd->cpupri, p, NULL))
+ if (rq->curr->rt.nr_cpus_allowed > 1) {
+ struct task_struct *push = pick_next_pushable_task(rq);
+ /*
+ * Though curr is pushable, if there are other pushable tasks,
+ * we keep curr busy.
+ */
+ if (push && !(push->prio > p->prio))
+ return;
+ } else
return;

- if (!cpupri_find(&rq->rd->cpupri, rq->curr, NULL))
- return;
-
- /*
- * There appears to be other cpus that can accept
- * current and none to run 'p', so lets reschedule
- * to try and push current away:
- */
- requeue_task_rt(rq, p, 1);
+ /* yield curr */
+ requeue_task_rt(rq, rq->curr, 0);
resched_task(rq->curr);
}

@@ -1091,7 +1090,7 @@ static struct sched_rt_entity *pick_next
BUG_ON(idx >= MAX_RT_PRIO);

queue = array->queue + idx;
- next = list_entry(queue->next, struct sched_rt_entity, run_list);
+ next = list_first_entry(queue, struct sched_rt_entity, run_list);

return next;
}
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