Date: Tue, 21 Feb 2012 01:02:30 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Avi Kivity <avi@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Gilad Ben-Yossef <gilad@benyossef.com>, linux-kernel@vger.kernel.org,
        Chris Metcalf <cmetcalf@tilera.com>, linux-mm@kvack.org,
        Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>,
        Sasha Levin <levinsasha928@gmail.com>, Rik van Riel <riel@redhat.com>,
        Andi Kleen <andi@firstfloor.org>, Mel Gorman <mel@csn.ul.ie>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Michal Nazarewicz <mina86@mina86.com>,
        Kosaki Motohiro <kosaki.motohiro@gmail.com>,
        Milton Miller <miltonm@bga.com>
Subject: Re: [v7 0/8] Reduce cross CPU IPI interference
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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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