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

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Message-ID: <20110830142705.GO9748@somewhere.redhat.com>
Date:	Tue, 30 Aug 2011 16:27:08 +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 04:13:57PM +0200, Peter Zijlstra wrote:
> On Tue, 2011-08-30 at 16:11 +0200, Frederic Weisbecker wrote:
> > On Tue, Aug 30, 2011 at 01:17:42PM +0200, Peter Zijlstra wrote:
> > > On Tue, 2011-08-30 at 01:35 +0200, Frederic Weisbecker wrote:
> > > > But rcu read side critical sections (preemption disabled, rcu_read_lock(),
> > > > softirq disabled) don't need the tick to enforce the critical section
> > > > itself. 
> > > 
> > > Note that with PREEMPT_RCU only the rcu_read_lock() is actually an rcu
> > > read side critical section, non of the others should be used as such.
> > > Relying on preempt_disable(), local_bh_disable() and similar is broken
> > > as per a long while ago.
> > 
> > Sure yeah.
> > 
> > My point was that the patchset doesn't care about all that anyway. Read
> > side critical section still work as usual. What changes is the way we
> > notice periods where we are *not* in rcu read side critical sections.
> > This was previously partly made through the tick. Now it's still the case
> > but we need to remotely wake up that tick first.
> 
> But you wake up the tick for any callback - systemwide - right? That's a
> massive fail.

Until we find a better solution yeah.
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