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Date: Thu, 2 Feb 2012 09:01:34 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Avi Kivity <avi@redhat.com>
Cc: Christoph Lameter <cl@linux.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Gilad Ben-Yossef <gilad@benyossef.com>, linux-kernel@vger.kernel.org,
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        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 Thu, Feb 02, 2012 at 06:14:36PM +0200, Avi Kivity wrote:
> On 02/02/2012 05:34 PM, Paul E. McKenney wrote:
> > On Thu, Feb 02, 2012 at 11:34:25AM +0200, Avi Kivity wrote:
> > > On 02/01/2012 10:13 PM, Paul E. McKenney wrote:
> > > > > 
> > > > > Could we also apply the same approach to processors busy doing
> > > > > computational work? In that case the OS is also not needed. Interrupting
> > > > > these activities is impacting on performance and latency.
> > > >
> > > > Yep, that is in fact what Frederic's dyntick-idle userspace work does.
> > > 
> > > Running in a guest is a special case of running in userspace, so we'd
> > > need to extend this work to kvm as well.
> >
> > As long as rcu_idle_enter() is called at the appropriate time, RCU will
> > happily ignore the CPU.  ;-)
> >
> 
> It's not called (since the cpu is not idle).  Instead we call
> rcu_virt_note_context_switch().

Frederic's work checks to see if there is only one runnable user task
on a given CPU.  If there is only one, then the scheduling-clock interrupt
is turned off for that CPU, and RCU is told to ignore it while it is
executing in user space.  Not sure whether this covers KVM guests.

In any case, this is not yet in mainline.

							Thanx, Paul