From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: RCU treating guest mode just like it does user-mode execution Date: Wed, 17 Aug 2011 15:05:20 -0700 Message-ID: <20110817220520.GN2419@linux.vnet.ibm.com> References: <20110817204327.GA10633@linux.vnet.ibm.com> <20110817215015.GA6423@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, avi@redhat.com, mtosatti@redhat.com, kvm@vger.kernel.org To: Gleb Natapov Return-path: Received: from e8.ny.us.ibm.com ([32.97.182.138]:56910 "EHLO e8.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751643Ab1HQWFh (ORCPT ); Wed, 17 Aug 2011 18:05:37 -0400 Content-Disposition: inline In-Reply-To: <20110817215015.GA6423@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Aug 18, 2011 at 12:50:15AM +0300, Gleb Natapov wrote: > On Wed, Aug 17, 2011 at 01:43:27PM -0700, Paul E. McKenney wrote: > > Hello, Gleb, > > > > I was looking at KVM's call to rcu_virt_note_context_switch() > > in kvm_guest_enter(), and noting the comment talking about treating > > guest mode like user-mode execution is. One difference between RCU's > > treatment of KVM guest execution and user-mode execution is that RCU > > notes a context switch only at the beginning of KVM guest execution, > > but notes user-mode execution at every scheduling-clock interrupt. > > > > Does it make sense to also note KVM guest execution on each > > scheduling-clock interrupt? One reason it might not make sense is > > if interrupts from KVM guest execution appear to rcu_check_callbacks() > > as interrupts from user-mode execution. (Do they? Given that people > > are reporting RCU CPU stall warnings in virtualized environments, I > > am beginning to suspect that the answer is "no".) > > > The answer is "no" because any interrupt kicks cpu out of a guest mode, so > it appears to be in the kernel for RCU. Do people still reporting RCU > stalls even with the my patch? > > > If KVM guest execution does not appear as user-mode execution to > > rcu_check_callback(), I would consider doing the following: > > > > 1. Rename rcu_virt_note_context_switch() to something like > > rcu_guest_execution_start(). > > > > 2. Place a call to a new rcu_guest_execution_end() in > > kvm_guest_exit(). > > > > 3. Make rcu_guest_execution_start() and rcu_guest_execution_end() > > set and clear a new per-CPU variable. > There is such variable already: current->flags & PF_VCPU. Good to know, thank you! > > 4. Make rcu_check_callbacks() check this per-CPU variable in > > much the same way that it currently checks its "user" > > argument, aside from needing to check that the CPU is > > not in an interrupt handler or some such. > > > > Of course, some thought is required to make sure that the checks for > > executing in an interrupt handler actually cover all of the needed > > situations, but so it goes! > > > > Thoughts? > > I wonder why it will be better than current situation. After cpu leaves > a guest mode there are only three options. It will either go to > userspace, execute schedule or go back to guest mode. At all those cases > RCU should note quiescent state. Might be that the current state is optimal. That would be a good thing. But if a CPU stays in guest mode for (say) 30 seconds, it will have called schedule() every jiffy in the meantime? In other words, if a CPU stays in guest mode for a long time, how does RCU know that this CPU is in an extended quiescent state? Thanx, Paul