Re: [PATCH] sched_rt_rq_enqueue() resched idle

[John Blackwood]

> 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!

You're welcome.

> 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?

Yes, I see what you are saying.
The patch version below looks good.
I re-tested with it and it works fine.

Thanks!

> 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)
>
>
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