linux-kernel - Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs

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Message-ID: <1314703315.2799.5.camel@twins>
Date:	Tue, 30 Aug 2011 13:21:55 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to
 idle enter/exit APIs

On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > That means it has to be in an extended grace period when we stop the
> > tick.
> 
> You mean extended quiescent state?

Yeah that :-)

> As a summary here is what we do:
> 
> - if we are in the kernel, we can't run into extended quiescent state because
> we may make use of rcu anytime there. But if we run nohz we don't have the tick
> to notice quiescent states to the RCU machinery and help completing grace periods
> so as soon as we receive an rcu IPI from another CPU (due to the grace period
> beeing extended because our nohz CPU doesn't report quiescent states), we restart
> the tick. We are optimistic enough to consider that we may avoid a lot of ticks
> even if there are some risks to be disturbed in some random rates.
> So even with the IPI we consider it as an upside.
> 
> - if we are in userspace we can run in extended quiescent state.

But you can only disable the tick/enter extended quiescent state while
in kernel-space. Thus the second clause is precluded from ever being
true.


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