On Mon, May 06, 2024, Marcelo Tosatti wrote: > On Fri, May 03, 2024 at 05:44:22PM -0300, Leonardo Bras wrote: > > > And that race exists in general, i.e. any IRQ that arrives just as the > > > idle task > > > is being scheduled in will unnecessarily wakeup rcuc. > > > > That's a race could be solved with the timeout (snapshot) solution, if we > > don't zero last_guest_exit on kvm_sched_out(), right? > > Yes.
And if KVM doesn't zero last_guest_exit on kvm_sched_out(), then we're right back in the situation where RCU can get false positives (see below). > > > > > > /* Is the RCU core waiting for a quiescent state from this > > > > > > CPU? */ > > > > > > > > > > > > The problem is: > > > > > > > > > > > > 1) You should only set that flag, in the VM-entry path, after the > > > > > > point > > > > > > where no use of RCU is made: close to guest_state_enter_irqoff call. > > > > > > > > > > Why? As established above, KVM essentially has 1 second to enter the > > > > > guest after > > > > > setting in_guest_run_loop (or whatever we call it). In the vast > > > > > majority of cases, > > > > > the time before KVM enters the guest can probably be measured in > > > > > microseconds. > > > > > > > > OK. > > > > > > > > > Snapshotting the exit time has the exact same problem of depending on > > > > > KVM to > > > > > re-enter the guest soon-ish, so I don't understand why this would be > > > > > considered > > > > > a problem with a flag to note the CPU is in KVM's run loop, but not > > > > > with a > > > > > snapshot to say the CPU recently exited a KVM guest. > > > > > > > > See the race above. > > > > > > Ya, but if kvm_last_guest_exit is zeroed in kvm_sched_out(), then the > > > snapshot > > > approach ends up with the same race. And not zeroing kvm_last_guest_exit > > > is > > > arguably much more problematic as encountering a false positive doesn't > > > require > > > hitting a small window. > > > > For the false positive (only on nohz_full) the maximum delay for the > > rcu_core() to be run would be 1s, and that would be in case we don't > > schedule out for some userspace task or idle thread, in which case we have > > a quiescent state without the need of rcu_core(). > > > > Now, for not being an userspace nor idle thread, it would need to be one or > > more kernel threads, which I suppose aren't usually many, nor usually take > > that long for completing, if we consider to be running on an isolated > > (nohz_full) cpu. > > > > So, for the kvm_sched_out() case, I don't actually think we are > > statistically introducing that much of a delay in the RCU mechanism. > > > > (I may be missing some point, though) My point is that if kvm_last_guest_exit is left as-is on kvm_sched_out() and vcpu_put(), then from a kernel/RCU safety perspective there is no meaningful difference between KVM setting kvm_last_guest_exit and userspace being allowed to mark a task as being exempt from being preempted by rcuc. Userspace can simply do KVM_RUN once to gain exemption from rcuc until the 1 second timeout expires. And if KVM does zero kvm_last_guest_exit on kvm_sched_out()/vcpu_put(), then the approach has the exact same window as my in_guest_run_loop idea, i.e. rcuc can be unnecessarily awakened in the time between KVM puts the vCPU and the CPU exits to userspace.
