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: <20150206180933.GF21905@lerouge>
Date:	Fri, 6 Feb 2015 19:09:35 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	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, paulmck@...ux.vnet.ibm.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.

Ok but, I'm still not sure about the details of what you're trying to do.
Whether it's only about RCU or it also involves ticks. What kind of tuning
you're doing and what kind of performance gain?

Thanks.

> 
> I'll let Paul queue the patches for 3.21 then!
> 
> Paolo
> 
> > That said it's potentially
> > interesting to turn off the tick on the host when the guest runs.
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