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 09:38:19 +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 Wed, Mar 21, 2012 at 4:54 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
>
>> If RCU is waiting for the current CPU to complete a grace
>> period, don't turn off the tick. Unlike dynctik-idle, we
>> are not necessarily going to enter into rcu extended quiescent
>> state, so we may need to keep the tick to note current CPU's
>> quiescent states.
>
> 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.

The user would register a signal handler to set a flag when it is
called and then busy
loop waiting for a flag to clear.

Checking this approach is on my todo list, but alas I had no progress
with it yet :-)

Gilad



-- 
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@...yossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
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"If you take a class in large-scale robotics, can you end up in a
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 -- Jean-Baptiste Queru
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