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

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Message-ID: <alpine.DEB.2.00.1203271111110.1170@router.home>
Date:	Tue, 27 Mar 2012 11:13:39 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	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>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	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 Tue, 27 Mar 2012, Frederic Weisbecker 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.
>
> What is the usecase you have in mind? Is it for realtime purpose?

Please do not use "realtime" since I am not sure what you mean by that.
Its for a low latency applications that cannot use "realtime" because that
implies high latencies.

> The "tick stopped" is a volatile and relative state.

The use case is an application that cannot tolerate the latencies
introduced by timer tick processing. It will only start running when the
system is in a sufficiently quiet state.

> Relative because if a timer list is enqueued to fire 1 second later,
> the tick will be stopped until that happens. How do we consider this (common)
> case?
>
> Also as Chris noted it is volatile because the tick can be restarted anytime
> for random reasons: the CPU receives an IPI which makes it restart the
> periodic tick.

Ok some sort of notification would be good for that case. If a timer tick
happens and that was unavoidable then it would be good to log the reason
why this occured so that the system can be configured in such a way that
these interruptions are minimized.

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