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: <1320922326.13800.5.camel@twins>
Date:	Thu, 10 Nov 2011 11:52:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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,
	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>,
	"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, 2011-11-09 at 17:48 +0100, Frederic Weisbecker wrote:
> > 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. 

I'm not sure I get what you're saying. A fully decoupled RCU/NO_HZ API
looks like:

  rcu_idle_enter();
  rcu_idle_exit();

  tick_nohz_idle_enter();
  tick_nohz_idle_exit();

And done you are, no funny interactions, 4 functions.

There is no _norcu variant simply because nohz will never touch rcu. If
you want the old coupled behaviour simply call both
tick_nohz_idle_enter() and rcu_idle_enter().


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