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. > > 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. -- Gleb. -- 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
