linux-kernel - Re: [sched-devel, patch-rfc] rework of "prioritize non-migratable tasks over migratable ones"

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Date:	Tue, 1 Jul 2008 12:46:20 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Cc:	"Gregory Haskins" <ghaskins@...ell.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [sched-devel, patch-rfc] rework of "prioritize non-migratable tasks over migratable ones"

2008/6/16 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Mon, 2008-06-16 at 19:59 +0200, Dmitry Adamushko wrote:
>
>> One way or another, we have different aritifacts (and mine have likely
>> more) but conceptually, both "violates" POSIX if a strict round-robin
>> scheduling is required.
>
> http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_08.html#tag_02_08_04_01
>
> Is quite strict on what FIFO should do, and I know of two points where
> we deviate and should work to match.
>

btw., rt group scheduling seems to well, slightly wreck this (per-rq)
FIFO model as well.

say, group_A has N SCHED_FIFO tasks of equal prio. So far so good,
they all run strictly one after another.

Now group_B gets task_S. On a group layer, group_B gets enqueued after group_A.

This changes when a current task (that belongs to group_A)
relinquishes a CPU: dequeue_stack -> __enqueue_rt_entity() will place
group_A in the tail of its list.

So the next task to run is task_S, although group_A migth have plenty
of tasks of the same prio that were enqueued ealrier.

We can't get a strict FIFO ordering with this pure tree-like hierarchy.

btw #2,

Gregory, our new modification also doesn't work nicely with group-scheduling.

We may place a task in the head of its queue, yes. But its group will
still remain where it was.

rt_se->nr_cpus_allowed just has no adequat sense for groups and
__enqueue_rt_entity() always places a group at the tail.

IOW, even if check_preempt_curr_rt() calls resched_task() based on
analysis of the newly arrived task 'p', 'p' won't be necessarily
picked up by pick_next_task_rt(). Although, there is a way to fix it.

-- 
Best regards,
Dmitry Adamushko
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