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: <20111115183026.GB3234@somewhere>
Date:	Tue, 15 Nov 2011 19:30:29 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	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 Thu, Nov 10, 2011 at 09:22:19AM -0800, Paul E. McKenney wrote:
> On Wed, Nov 09, 2011 at 05:48:11PM +0100, Frederic Weisbecker wrote:
> > On Thu, Nov 03, 2011 at 09:06:56AM -0700, Paul E. McKenney wrote:
> > > On Thu, Nov 03, 2011 at 08:31:02AM -0700, Josh Triplett wrote:
> > > > On Thu, Nov 03, 2011 at 06:32:31AM -0700, Paul E. McKenney wrote:
> > > > > On Thu, Nov 03, 2011 at 12:54:33PM +0100, Frederic Weisbecker wrote:
> > > > > > 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.
> > > > > 
> > > > > How about tick_nohz_rcu_idle_enter() vs. tick_nohz_idle_enter() on
> > > > > entry to the idle loop and tick_nohz_rcu_idle_exit() vs
> > > > > tick_nohz_idle_exit() on exit?
> > > > > 
> > > > > That said, I don't feel all that strongly on this naming topic.
> > > > 
> > > > Mostly I think that since this series tries to separate the concepts of
> > > > "idle nohz" and "rcu extended quiescent state", we should end up with
> > > > two entirely separate functions delimiting those two, without any
> > > > functions that poke both with correspondingly complex compound names.
> > > 
> > > Having four API members rather than the current six does seem quite
> > > attractive to me.  Frederic, any reason why this approach won't work?
> > 
> > The approach I took might sound silly but it's mostly an optimization:
> > 
> > I did the *_norcu() variant mostly to be able to keep rcu_idle_enter()
> > call under the same local_irq_disable() section.
> > 
> > This way we can't have an interrupt in between that can needlessly perform
> > RCU work (and trigger the softirq in the worst case), delaying the point
> > where we actually put the CPU to sleep.
> 
> But we have to tolerate this sort of thing on some architectures (x86
> and Power) in order to allow idle-task use of RCU read-side primitives,
> right?
> 
> So consolidating from six to four APIs doesn't expand the overall state
> space.

Well, we tolerate that, the two more APIs are there for optimization, not
to provide correctness.
But if you want me to remove the optimization and keep only the four APIs I can do it.
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