Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Feb 06, 2015 at 02:50:44PM +0100, Paolo Bonzini wrote:
> 
> 
> On 06/02/2015 14:46, Frederic Weisbecker wrote:
> > > When running a KVM guest on a system with NOHZ_FULL enabled
> > 
> > I just need to clarify the motivation first, does the above situation
> > really happen? Ok some distros enable NOHZ_FULL to let the user stop
> > the tick in userspace. So most of the time, CONFIG_NOHZ_FULL=y but
> > nohz full is runtime disabled (we need to pass a nohz_full= boot
> > parameter to enable it). And when it is runtime disabled, there should
> > be no rcu nocb CPU.
> > 
> > (Although not setting CPUs in nocb mode when nohz full is runtime disabled
> > is perhaps a recent change.)
> > 
> > So for the problem to arise, one need to enable nohz_full and run KVM
> > guest. And I never heard about such workloads.
> 
> Yeah, it's a new thing but Marcelo, Luiz and Rik have been having a lot
> of fun with them (with PREEMPT_RT too).  They're getting pretty good
> results given the right tuning.
> 
> I'll let Paul queue the patches for 3.21 then!

Frederic, given the background from Paolo, Rik, and Christian, are you
OK with these patches?

							Thanx, Paul

> Paolo
> 
> > That said it's potentially
> > interesting to turn off the tick on the host when the guest runs.
>