Re: [PATCH] sched_rt_rq_enqueue() resched idle

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2008-08-26 at 15:09 -0400, John Blackwood wrote:
> Hi Peter,
> 
> When sysctl_sched_rt_runtime is set to something other than -1 and the
> CONFIG_RT_GROUP_SCHED kernel parameter is NOT enabled, we get into a state
> where we see one or more CPUs idling forvever even though there are
> real-time
> tasks in their rt runqueue that are able to run (no longer throttled).
> 
> The sequence is:
> 
> - A real-time task is running when the timer sets the rt runqueue
>     to throttled, and the rt task is resched_task()ed and switched
>     out, and idle is switched in since there are no non-rt tasks to
>     run on that cpu.
> 
> - Eventually the do_sched_rt_period_timer() runs and un-throttles
>     the rt runqueue, but we just exit the timer interrupt and go back
>     to executing the idle task in the idle loop forever.
> 
> If we change the sched_rt_rq_enqueue() routine to use some of the code
> from the CONFIG_RT_GROUP_SCHED enabled version of this same routine and
> resched_task() the currently executing task (idle in our case) if it is
> a lower priority task than the higher rt task in the now un-throttled
> runqueue, the problem is no longer observed.

Very good spotting, Thanks!

However I think the patch isn't quite good, as highest_prio is only
available on SMP || RT_GROUP_SCHED.

Furthermore, on !RT_GROUP_SCHED any RT task will be higher than current,
so we can do the below, do you agree?

---
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 94daace..f672aee 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -199,6 +199,8 @@ static inline struct rt_rq *group_rt_rq(struct sched_rt_entity *rt_se)
 
 static inline void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
 {
+	if (rt_rq->rt_nr_running)
+		resched_task(rq_of_rt_rq(rt_rq)->curr);
 }
 
 static inline void sched_rt_rq_dequeue(struct rt_rq *rt_rq)