linux-kernel - Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it

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Date:	Tue, 27 Mar 2012 17:21:34 +0200
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Zen Lin <zen@...nhuawei.org>
Subject: Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it

On Thu, Mar 22, 2012 at 6:18 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Thu, 22 Mar 2012, Gilad Ben-Yossef wrote:
>
>> > Is there any way for userspace to know that the tick is not off yet due to
>> > this? It would make sense for us to have busy loop in user space that
>> > waits until the OS has completed all processing if that avoids future
>> > latencies for the application.
>> >
>>
>> I previously suggested having the user register to receive a signal
>> when the tick
>> is turned off. Since the tick is always turned off the user task is
>> the current task
>> by design, *I think* you can simply mark the signal pending when you
>> turn the tick off.
>
> Ok that sounds good. You would define a new signal for this?
>

My gut instinct is to let the process register with a specific signal
(properly the RT range)
it wants to receive when the tick goes off and/or on.

> So we would startup the application. App will do all prep work (memory
> allocation, device setup etc etc) and then wait for the signal to be
> received. After that it would enter the low latency processing phase.
>
> Could we also get a signal if something disrupts the peace and switches
> the timer interrupt on again?
>

I think you'll have to since once you have the tick turned off there
is no guarantee that
it wont get turned on by a timer scheduling an task or an IPI.


-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru
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