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

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Date:	Wed, 23 Apr 2008 10:05:03 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Gregory Haskins" <ghaskins@...ell.com>, mingo@...e.hu,
	suresh.b.siddha@...el.com, chinang.ma@...el.com,
	arjan@...ux.intel.com, willy@...ux.intel.com,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: push rt tasks only if newly activated tasks have been added

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
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