linux-kernel - Re: [v7 0/8] Reduce cross CPU IPI interference

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Message-ID: <20120221000225.GJ5752@somewhere.redhat.com>
Date:	Tue, 21 Feb 2012 01:02:30 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	linux-kernel@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	linux-mm@...ck.org, Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Michal Nazarewicz <mina86@...a86.com>,
	Kosaki Motohiro <kosaki.motohiro@...il.com>,
	Milton Miller <miltonm@....com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference

On Tue, Feb 14, 2012 at 03:18:03PM +0200, Avi Kivity wrote:
> On 02/10/2012 03:39 AM, Frederic Weisbecker wrote:
> > > 
> > > As long as the code doesn't enter RCU read-side critical sections in
> > > the time between rcu_idle_enter() and rcu_idle_exit(), this should
> > > work fine.
> >
> > This should work fine yeah but further the correctness, I wonder if this
> > is going to be a win.
> >
> > We use rcu_idle_enter() in idle to avoid to keep the tick for RCU. But
> > what about falling into guest mode? I guess the tick is kept there
> > so is it going to be a win in throughput or something to use rcu_idle_enter()?
> 
> We could disable the tick while in guest mode as well.  Interrupts in
> guest mode are even more expensive than interrupts in user mode.

Right, that's definitely something I need to explore with the adaptive tickless
thing.
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