linux-kernel - Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs

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Message-ID: <20110830143207.GP9748@somewhere.redhat.com>
Date:	Tue, 30 Aug 2011 16:32:09 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle
 enter/exit APIs

On Tue, Aug 30, 2011 at 01:21:55PM +0200, Peter Zijlstra wrote:
> On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > > That means it has to be in an extended grace period when we stop the
> > > tick.
> > 
> > You mean extended quiescent state?
> 
> Yeah that :-)
> 
> > As a summary here is what we do:
> > 
> > - if we are in the kernel, we can't run into extended quiescent state because
> > we may make use of rcu anytime there. But if we run nohz we don't have the tick
> > to notice quiescent states to the RCU machinery and help completing grace periods
> > so as soon as we receive an rcu IPI from another CPU (due to the grace period
> > beeing extended because our nohz CPU doesn't report quiescent states), we restart
> > the tick. We are optimistic enough to consider that we may avoid a lot of ticks
> > even if there are some risks to be disturbed in some random rates.
> > So even with the IPI we consider it as an upside.
> > 
> > - if we are in userspace we can run in extended quiescent state.
> 
> But you can only disable the tick/enter extended quiescent state while
> in kernel-space. Thus the second clause is precluded from ever being
> true.

No, we have a specific stacking in the irq:

	rcu_irq_enter()

	disable tick...
	if (user)
		rcu_enter_nohz();

	rcu_irq_exit() <-- extended quiescent state entry effective only there

And by the time we call rcu_irq_exit() and we resume to userspace, we are
not supposed to have rcu read side critical section (minus the case of
a signal with do_notify_resume() which I have yet to handle).
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