From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest Date: Fri, 06 Feb 2015 14:50:44 +0100 Message-ID: <54D4C6B4.9090507@redhat.com> References: <1423167832-17609-1-git-send-email-riel@redhat.com> <20150206134631.GA21905@lerouge> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: 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, paulmck@linux.vnet.ibm.com, lcapitulino@redhat.com To: Frederic Weisbecker , riel@redhat.com Return-path: In-Reply-To: <20150206134631.GA21905@lerouge> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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.