Re: [PATCH 1/2] sched: push rt tasks only if newly activated tasks have been added

Dmitry Adamushko
2008-04-23T08:05:28Z

Hi Steven,

> [ ... ]
> > square#0:
> >
> > cpu1: T0 is running
> >
> > T1 is of the same prio as T0 (shouldn't really matter but to get the
> > same result it would require altering the flow of events slightly)
> >
> > T1's affinity allows it to be run only on cpu1.
> > T0 can run on both.
> >
> > try_to_wake_up() is called for T1.
> > |
> > --> select_task_rq_rt() => gives cpu1
> > |
> > --> task_wake_up_rt()
> > |
> > ---> push_rt_tasks() -> rq->rt.pushed = 1
> >
> > now, neither T1 (due to its affinity), nor T0 (it's running) can be
> > pushed away to cpu0.
>
> Ah, this may be what you are talking about. T0 was running, but because
> T1 has its affinity set to cpu1 it wont cause a push. When T0 schedules
> away to give T1 its cpu time, T0 wont push away because of the pushed
> flag.
>
> Hmm, interesting. Of course my response is "Don't use SCHED_RR! It's
> evil!" ;-)

It's not just SCHED_RR ;-) They both can be of SCHED_FIFO.

T1 _preempts_ T0 and again

--> task_wake_up_rt()
|
---> push_rt_tasks() -> rq->rt.pushed = 1

and T0 won't be pushed away to cpu0 by post_schedule_rt().

As Gregory has pointed out, at the very least it's a test in
task_wake_up_rt() which is wrong.

push_rt_tasks() should not be called when 'p' (a newly woken up task)
is the next one to run.

IOW, it should be (p->prio < rq->curr->prio) instead of (p->prio >=
rq->rt.highest_prio).

--
Best regards,
Dmitry Adamushko