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

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Date:	Thu, 22 Mar 2012 11:18:42 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Gilad Ben-Yossef <gilad@...yossef.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, 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?

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?

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