linux-kernel - Re: [PATCH 1/4] nohz: Split extended quiescent state handling from nohz switch

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Message-ID: <20110913000637.GA17335@somewhere.redhat.com>
Date:	Tue, 13 Sep 2011 02:06:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	Mike Frysinger <vapier@...too.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Russell King <linux@....linux.org.uk>,
	Paul Mackerras <paulus@...ba.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 1/4] nohz: Split extended quiescent state handling from
 nohz switch

On Wed, Sep 07, 2011 at 07:22:33AM -0700, Paul E. McKenney wrote:
> On Sun, Sep 04, 2011 at 04:36:43PM -0700, Paul E. McKenney wrote:
> > On Sat, Aug 20, 2011 at 07:30:49PM +0200, Frederic Weisbecker wrote:
> > > It is assumed that rcu won't be used once we switch to tickless
> > > mode and until we restart the tick. However this is not always
> > > true, as in x86-64 where we dereference the idle notifiers after
> > > the tick is stopped.
> > > 
> > > To prepare for fixing this, split the tickless mode switching and
> > > RCU extended quiescent state logics.
> > > Make tick_nohz_stop/restart_sched_tick() RCU agnostic but provide
> > > a new pair of APIs tick_nohz_enter/exit_idle() that keep the
> > > old behaviour by handling both the nohz mode and RCU extended
> > > quiescent states, then convert every archs to use these.
> > > 
> > > Archs that want to switch to RCU extended QS to some custom points
> > > can do it later by changing the parameter in tick_nohz_enter,exit_idle()
> > > to false and call rcu_enter,exit() separately.
> > 
> > This approach looks quite good to me!  A few comments below.
> 
> But I get RCU stall warnings when running it on powerpc on top of
> the patch set at:
> 
> 	https://lkml.org/lkml/2011/9/7/64
> 
> At first glance, it appears that CPUs are entering dyntick-idle
> mode without RCU being informed.  Any thoughts on diagnostics?
> 
> 							Thanx, Paul

I've just tested with your rcu/next branch from github and applied
my patches on top of it but did not experience a stall in x86.

That may be related to powerpc more precisely, or may be any arch
but on some particular condition.
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