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

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Date:	Fri, 06 Feb 2015 14:50:44 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>, riel@...hat.com
CC:	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 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!

Paolo

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