linux-kernel - Re: [PATCH RFC tip/core/rcu 19/28] nohz: Allow rcu extended quiescent state handling seperately from tick stop

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Message-ID: <20111103115426.GD8198@somewhere.redhat.com>
Date:	Thu, 3 Nov 2011 12:54:33 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	patches@...aro.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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"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 RFC tip/core/rcu 19/28] nohz: Allow rcu extended
 quiescent state handling seperately from tick stop

On Wed, Nov 02, 2011 at 09:00:03PM -0700, Josh Triplett wrote:
> On Wed, Nov 02, 2011 at 01:30:40PM -0700, Paul E. McKenney wrote:
> > From: Frederic Weisbecker <fweisbec@...il.com>
> > 
> > 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 two new APIs:
> > tick_nohz_idle_enter_norcu() and tick_nohz_idle_exit_norcu().
> > 
> > If no use of RCU is made in the idle loop between
> > tick_nohz_enter_idle() and tick_nohz_exit_idle() calls, the arch
> > must instead call the new *_norcu() version such that the arch doesn't
> > need to call rcu_idle_enter() and rcu_idle_exit().
> 
> The _norcu names confused me a bit.  At first, I thought they meant
> "idle but not RCU idle, so you can use RCU", but from re-reading the
> commit message, apparently they mean "idle and RCU idle, so don't use
> RCU".  What about something like _forbid_rcu instead?  Or,
> alternatively, why not just go ahead and separate the two types of idle
> entirely rather than introducing the _norcu variants first?

Or tick_nohz_idle_enter_rcu_stop() and tick_nohz_idle_exit_rcu_restart()?

Sounds clear but too long. May be we can shorten the tick_nohz thing in the
beginning.
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