From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest Date: Fri, 6 Feb 2015 08:19:38 -0800 Message-ID: <20150206161938.GB5418@linux.vnet.ibm.com> References: <1423167832-17609-1-git-send-email-riel@redhat.com> <20150206134631.GA21905@lerouge> <54D4C6B4.9090507@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Frederic Weisbecker , 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 To: Paolo Bonzini Return-path: Content-Disposition: inline In-Reply-To: <54D4C6B4.9090507@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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. >