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

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Message-ID: <20150206161938.GB5418@linux.vnet.ibm.com>
Date:	Fri, 6 Feb 2015 08:19:38 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>, riel@...hat.com,
	kvm@...r.kernel.org, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, mtosatti@...hat.com,
	mingo@...nel.orgm, ak@...ux.intel.com, oleg@...hat.com,
	masami.hiramatsu.pt@...achi.com, lcapitulino@...hat.com
Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state
 when running KVM guest

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.
> 

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