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

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Date:	Tue, 15 Jul 2008 14:48:26 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
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"

On Tue, 2008-07-01 at 12:46 +0200, Dmitry Adamushko wrote:
> 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.


Yeah, I realized this quite a while back, but 1) posix doesn't say
anything about group scheduling like this, and 2) all the rt group
scheduling stuff is still experimental :-)

I'm open to suggestion though.

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