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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
