linux-kernel - Re: [PATCH 2/3] sched: terminate newidle balancing once at least one task has moved over

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Message-Id: <48746084.BA47.005A.0@novell.com>
Date:	Wed, 09 Jul 2008 04:53:56 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Nick Piggin" <nickpiggin@...oo.com.au>
Cc:	<mingo@...e.hu>, <rostedt@...dmis.org>, <peterz@...radead.org>,
	<npiggin@...e.de>, <linux-kernel@...r.kernel.org>,
	<linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched: terminate newidle balancing once at
	least one task has moved over

>>> On Wed, Jul 9, 2008 at  4:09 AM, in message
<200807091809.52293.nickpiggin@...oo.com.au>, Nick Piggin
<nickpiggin@...oo.com.au> wrote: 
> On Tuesday 08 July 2008 22:37, Gregory Haskins wrote:
>> >>> On Tue, Jul 8, 2008 at  1:00 AM, in message
>>
>> <200807081500.18245.nickpiggin@...oo.com.au>, Nick Piggin
>>
>> <nickpiggin@...oo.com.au> wrote:
>> > On Saturday 28 June 2008 06:29, Gregory Haskins wrote:
>> >> Inspired by Peter Zijlstra.
>> >>
>> >> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> >
>> > What happened to the feedback I sent about this?
>> >
>> > It is still nack from me.
>>
>> Ah yes.  Slipped through the cracks...sorry about that.
>>
>> What if we did "if (idle == CPU_NEWLY_IDLE && need_resched())" instead?
> 
> Isn't that exactly the same thing

Not quite.  The former version would break on *any* succesful enqueue (as a result of a
local move_task as well as a remote wake-up/migration).  The latter version will only
break on the the remote variety.  You were concerned about stopping a move_task
operation early because it would reduce efficiency, and I do not entirely disagree.
However, this really only concerns the local type (which have now been removed).

Remote preemptions should (IMO) always break immediately because it would have been likely
to invalidate the f_b_g() calculation anyway, and low-latency requirements dictate
its the right thing to do.

> because any task will preempt the idle thread?

During NEWIDLE this is a preempt-disabled section because we are still in the middle
of a schedule(). Therefore there will be no involuntary preemption and that is why we
are concerned with making sure we check for voluntary preemption.  The move_tasks()
will run to completion without this patch.  With this patch it will break the operation if
someone tries to preempt us.

I'll keep an open mind but I am pretty sure this is something we should be doing. As far
as I can tell, there should be no downside with this second version.  As a compromise
we could put an #ifdef CONFIG_PREEMPT around this new logic, but I don't think it
is strictly necessary.

Regards,
-Greg
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