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

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Message-ID: <20101220235158.GE1715@nowhere>
Date:	Tue, 21 Dec 2010 00:52:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 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.
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