Re: sched/rt: Pick up the throttled rt tasks in case no other non-rt tasks

[Mike Galbraith <bitbucket@online.de>]

On Wed, 2013-10-16 at 23:43 +0800, Chuansheng Liu wrote: 
> Meet the case that on CPU2, there are just two tasks to be run,
> one is one rt task A, another is idle task, but at this time,
> and the rt_rq is throttled.
> 
> CPU1		                 CPU2
> 				 staying in idle task;
> waking up rt task A on
> target CPU 2;
> 
>                                  exiting from idle task;
>                                  DO schedule()
>                                    pick up RT next task
> 					pick_next_task_rt()
> 						_pick_next_task_rt()
> 						rt_rq is throttled
> 					return NULL rt task to be picked
> 
> 				 CPU2 then went into idle task AGAIN;
>                                  after 2s, CPU2 is waken up again;
>                                  rt task A is get running;
> 
> Here rt task A is delayed for some time, even when CPU2 is in idle state.
> 
> so in case there are just rt tasks running, we can pick up one of them
> even rt_rq is throttled.

And shortly thereafter, a SCHED_NORMAL task wakes.. only to find that
its alleged bodyguard slit its throat while it slept.  Bad bodyguard.

-Mike