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

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Date:	Sun, 9 Dec 2007 18:16:52 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Steven Rostedt" <srostedt@...hat.com>
Cc:	"Gregory Haskins" <ghaskins@...ell.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Balbir Singh" <balbir@...ibm.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	vatsa@...ux.vnet.ibm.com,
	"LKML Kernel" <linux-kernel@...r.kernel.org>
Subject: RT Load balance changes in sched-devel

[ 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).

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.

-- 
Best regards,
Dmitry Adamushko
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