linux-kernel - Re: [ANNOUNCE] 3.8-rc6-nohz4

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Date:	Thu, 7 Feb 2013 18:18:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [ANNOUNCE] 3.8-rc6-nohz4

2013/2/7 Steven Rostedt <rostedt@...dmis.org>:
> On Thu, 2013-02-07 at 17:41 +0100, Frederic Weisbecker wrote:
>
>> I'm not convinced that "single task" must be a fundamental component
>> of this. It's an implementation detail. We should be able to keep the
>> tick off in the future when more than one task are on the runqueue and
>> hrtick is on. May be this will never show up as a performance gain but
>> we don't know yet.
>>
>> Ok let's talk about that single task constraint in the Kconfig help so
>> that the user knows the practical constraint as of today. But I
>> suggest we keep that as an internal detail that we can deal with in
>> the future.
>
> Hmm, but isn't time slices still implemented by ticks?

Not with hrtick.

> I would think
> implementing multiple tasks would be another huge change.

I don't think so. Really hrtick should take of everything.

>
> Maybe have:
>
> NO_HZ_IDLE
> NO_HZ_SINGLE_TASK
> NO_HZ_MULTI_TASK
> NO_HZ_COMPLETE

I still see single task, multitask or complete as implementation
constraints. Once we make hrtick support dynticks, it should be
dynamically handled: if hrtick is enabled then stop the tick even on
multitask, otherwise only stop it when we have one task.

Then when we remove jiffies, the complete coverage comes along.

>
> And as Ingo has suggested, maybe in the future we can remove SINGLE and
> MULTI and have just COMPLETE.

But really, turning these constraints into single built-in optable
choices doesn't make much sense to me.

> But anyway, the current method has a strict requirement of a single
> task, and that is user visible. I would want to keep the config name
> implying that requirement.

As long as it's specified in the Kconfig help, does it matter? It''s a
constraint amongst many others: you need to keep one CPU with a
periodic tick, you need to avoid posix cpu timers, etc...
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