linux-kernel - Re: [patch 6/8] pull RT tasks

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Message-ID: <b647ffbd0710221534h7c3e7da8g3311c39c10cb341c@mail.gmail.com>
Date:	Tue, 23 Oct 2007 00:34:21 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Gregory Haskins" <ghaskins@...ell.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 6/8] pull RT tasks

Hi Steven,

agreed with your comments in the previous message. Indeed, I missed some points.

> On wakeup, we can wake up several RT tasks (as my test case does) and if
> we only push one task, then the other tasks may not migrate over to the
> other run queues. I logged this happening in my tests.

I guess, what may happen is that while we are running push_rt_tasks()
on CPU-k (say, as a result on try_to_wake_up(task_1))
and as this_rq->lock may get released (in double_lock_balance()) , we
may get in a 'race'
with try_to_wake_up(task_2) from (another) CPU-m.
It places a woken up task on the same run-queue (for which
push_rt_task() is already running on CPU-k) and, actually, run
push_rt_task() for the same rq again!

So it may happen that both task_1 and task_2 will be pushed from the same CPU...

Do you see an error in my description? (it's a late hour so I can miss
something again ... sure, otherwise I'm almost perfect :-/ ) Can it
correlate with what you have observed in your tests?

Otherwise, there is 1:1 relation : push_rt_task() is called for every
new (single) task activated by try_to_wake_up() and for a preempted
task... so it's not like a few tasks are placed on the run-queue and
then push_rt_tasks() is called once.

btw., if this scenario may take place... maybe it would make sense to
have something like RTLB_INPROGRESS/PENDING and to avoid competing
push_rt_tasks() calls for the same 'rq' from different CPUs?
(although, there can be some disadvantages here as well. e.g. we would
likely need to remove 'max 3 tasks at once' limit and get,
theoretically, unbounded time spent in push_rt_tasks() on a single
CPU).

>
> -- Steve
>

-- 
Best regards,
Dmitry Adamushko
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