linux-kernel - Re: RT Load balance changes in sched-devel

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Date:	Sun, 09 Dec 2007 13:32:27 -0500
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>,
	"Steven Rostedt" <srostedt@...hat.com>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Ingo Molnar" <mingo@...e.hu>, "Balbir Singh" <balbir@...ibm.com>,
	<vatsa@...ux.vnet.ibm.com>,
	"LKML Kernel" <linux-kernel@...r.kernel.org>
Subject: Re: RT Load balance changes in sched-devel

Hi Dmitry,

>>> On Sun, Dec 9, 2007 at 12:16 PM, in message
<b647ffbd0712090916l4eb9a944r4726680a5fdcae46@...l.gmail.com>, "Dmitry
Adamushko" <dmitry.adamushko@...il.com> wrote: 
> [ cc'ed lkml ]
> 
> I guess, one possible load-balancing point is out of consideration --
> sched_setscheduler()
> (also rt_mutex_setprio()).
> 
> (1)  NORMAL --> RT, when p->se.on_rq == 1 && ! task_running(rq, p)
> 
> (2) RT --> NORMAL, when task_running(rq, p) == 1
> 
> e.g. for (2) we may even get a completely idle rq (schedule() -->
> schedule_balance_rt() will not help due to schedule_balance_rt()
> having a rt_task(prev) check in place... and 'prev' is of NORMAL type
> when it's scheduled out).

Indeed.  I think you are correct on both counts.  This is an oversight, so good eyes!

> 
> 
> btw., both cases would be addressed by placing load-balance points
> into sched_class_rt->{enqueue,dequeue}_task_rt()... push_rt_tasks()
> and pull_rt_tasks() respectively. As a side effect (I think,
> technically, it would be possible), 3 out of 4 *_balance_rt() calls
> (the exception: schedule_tail_balance_rt()) in schedule() would become
> unnecessary.
> 
> _BUT_
> 
> the enqueue/dequeue() interface would become less straightforward,
> logically-wise.
> Something like:
> 
> rq = activate_task(rq, ...) ; /* may unlock rq and lock/return another one 
> */
> 
> would complicate the existing use cases.
> 

I think I would prefer to just fix the setscheduler/setprio cases for the class transition than change the behavior of these enqueue/dequeue calls.  But I will keep an open mind as I look into this issue.

Thanks for the review!
-Greg



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