Date: Fri, 6 Feb 2015 08:19:38 -0800
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>, riel@redhat.com,
        kvm@vger.kernel.org, borntraeger@de.ibm.com,
        linux-kernel@vger.kernel.org, mtosatti@redhat.com, mingo@kernel.orgm,
        ak@linux.intel.com, oleg@redhat.com, masami.hiramatsu.pt@hitachi.com,
        lcapitulino@redhat.com
Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state
 when running KVM guest
Message-ID: <20150206161938.GB5418@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
References: <1423167832-17609-1-git-send-email-riel@redhat.com>
 <20150206134631.GA21905@lerouge>
 <54D4C6B4.9090507@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <54D4C6B4.9090507@redhat.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1534
Lines: 41

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.
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/