linux-kernel - Re: [PATCH 4/7] nohz: Allow rcu extended quiescent state handling seperately from tick stop

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Message-ID: <20110926160205.GB2399@linux.vnet.ibm.com>
Date:	Mon, 26 Sep 2011 09:02:06 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Frysinger <vapier@...too.org>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	David Miller <davem@...emloft.net>,
	Chris Metcalf <cmetcalf@...era.com>,
	Hans-Christian Egtvedt <hans-christian.egtvedt@...el.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Ingo Molnar <mingo@...hat.com>,
	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 4/7] nohz: Allow rcu extended quiescent state handling
 seperately from tick stop

On Mon, Sep 26, 2011 at 12:44:44PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-09-26 at 12:19 +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, add a parameter to tick_nohz_enter_idle()
> > named "rcu_ext_qs" that tells whether we want to enter RCU extended
> > quiescent state at the same time we stop the tick.
> > 
> > If no use of RCU is made in the idle loop between
> > tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the parameter
> > must be set to true and the arch doesn't need to call rcu_enter_nohz()
> > and rcu_exit_nohz() explicitly.
> > 
> > Otherwise the parameter must be set to false and the arch is
> > responsible of calling:
> > 
> > - rcu_enter_nohz() after its last use of RCU before the CPU is put
> > to sleep.
> > - rcu_exit_nohz() before the first use of RCU after the CPU is woken
> > up. 
> 
> I can't say this really makes sense:
> 
>   tick_nohz_idle_enter(false);
> 
> reads like, don't enter nohz state, not: enter nohz state but don't
> enter rcu-nohz state.
> 
> I realize you want to keep the per-arch frobbing low, but since you're
> already touching all of them, I think its more important to keep the
> functions readable.
> 
> Why not simply fully split nohz and rcu and modify all idle routines
> with both calls?

This might well be the correct thing to do, but one thing that gives
me pause is that some architectures have a large number of idle routines.
If such an architecture can use tick_nohz_idle_enter(true), then that
architecture needs only one change rather than one change to each of
potentially many idle loops.

Would your readability concerns be addressed by something like the
following?

#define RCU_NO_HZ_LATER 0
#define RCU_NO_HZ_NOW   1

Then we would have one of the following:

	tick_nohz_idle_enter(RCU_NO_HZ_LATER);
	tick_nohz_idle_enter(RCU_NO_HZ_NOW);

							Thanx, Paul
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