Re: [RFC PATCH v1 0/2] Avoid rcu_core() if CPU just left guest vcpu

[Sean Christopherson <seanjc@google.com>]

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.