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

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Message-ID: <20120328115304.GC17189@somewhere.redhat.com>
Date:	Wed, 28 Mar 2012 13:53:07 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.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, Mar 27, 2012 at 11:13:39AM -0500, Christoph Lameter wrote:
> 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.

tracing is probably the right place to log these things. But that's
about debugging. This won't be a notification on top of which your app
could recover.
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