linux-kernel - Re: [RFC PATCH 07/15] nohz_task: Restart tick when RCU forces nohz task cpu quiescent state

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Date:	Tue, 21 Dec 2010 08:41:14 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 07/15] nohz_task: Restart tick when RCU forces nohz
 task cpu quiescent state

On Tue, 2010-12-21 at 00:52 +0100, Frederic Weisbecker wrote:
> On Mon, Dec 20, 2010 at 05:02:09PM +0100, Peter Zijlstra wrote:
> > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > > If a cpu is in nohz mode due to a nohz task running, then
> > > it is not able to notify quiescent states requested by other
> > > CPUs.
> > > 
> > > Then restart the tick to remotely force the quiescent states on the
> > > nohz task cpus.
> > 
> > -ENOPARSE.. if its in NOHZ state, it couldn't possibly need to
> > participate in the quiescent state machine because the cpu is in a
> > quiescent state and has 0 RCU activity.
> 
> 
> But it can be in nohz state in the kernel in which case it can have
> any RCU activity.

That still doesn't make sense.. if you're in nohz state there shouldn't
be any rcu activity, otherwise its not nohz is it?


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