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

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Message-ID: <20110913164933.GF2416@linux.vnet.ibm.com>
Date:	Tue, 13 Sep 2011 09:49:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 Tue, Sep 13, 2011 at 02:06:45AM +0200, Frederic Weisbecker wrote:
> 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.

Thank you for checking this!

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

OK, I guess my next step is to put a WARN_ON_ONCE() in the idle loop
to check to see if there might be some other idle loop buried in
PowerPC somewhere...

							Thanx, Paul
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