linux-kernel - Re: [RFC PATCH 09/15] rcu: Make rcu_enter,exit

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Message-ID: <20101221192739.GR1750@nowhere>
Date:	Tue, 21 Dec 2010 20:27:41 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 09/15] rcu: Make rcu_enter,exit_nohz() callable
 from irq

On Tue, Dec 21, 2010 at 11:26:35AM -0800, Paul E. McKenney wrote:
> On Mon, Dec 20, 2010 at 04:24:16PM +0100, Frederic Weisbecker wrote:
> > In order to be able to enter/exit into rcu extended quiescent
> > state from interrupt, we need to make rcu_enter_nohz() and
> > rcu_exit_nohz() callable from interrupts.
> > 
> > So, this proposes a new implementation of the rcu nohz fast path
> > related helpers, where rcu_enter_nohz() or rcu_exit_nohz() can
> > be called between rcu_enter_irq() and rcu_exit_irq() while keeping
> > the existing semantics.
> > 
> > We maintain three per cpu fields:
> > 
> > - nohz indicates we entered into extended quiescent state mode,
> > we may or not be in an interrupt even if that state is set though.
> > 
> > - irq_nest indicates we are in an irq. This number is incremented on
> > irq entry and decreased on irq exit. This includes NMIs
> > 
> > - qs_seq is increased everytime we see a true extended quiescent
> > state:
> > 	* When we call rcu_enter_nohz() and we are not in an irq.
> > 	* When we exit the outer most nesting irq and we are in
> > 	  nohz mode (rcu_enter_nohz() was called without a pairing
> > 	  rcu_exit_nohz() yet).
> > 
> > >From that three-part we can deduce the extended grace periods like
> > we did before on top of snapshots and comparisons.
> > 
> > If nohz == 1 and irq_nest == 0, we are in a quiescent state. qs_seq
> > is used to keep track of elapsed extended quiescent states, useful
> > to compare snapshots of rcu nohz state.
> > 
> > This is experimental and does not take care of barriers yet.
> 
> Indeed!!!
> 
> I will likely be reworking the dyntick interface soon anyway, so will
> try to make sure that your requirements are met.

Great, thanks!
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