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

[Marcelo Tosatti]

On Mon, Apr 08, 2024 at 10:16:24AM -0700, Sean Christopherson wrote:
> On Fri, Apr 05, 2024, Paul E. McKenney wrote:
> > On Fri, Apr 05, 2024 at 07:42:35AM -0700, Sean Christopherson wrote:
> > > On Fri, Apr 05, 2024, Marcelo Tosatti wrote:
> > > > rcuc wakes up (which might exceed the allowed latency threshold
> > > > for certain realtime apps).
> > > 
> > > Isn't that a false negative? (RCU doesn't detect that a CPU is about to (re)enter
> > > a guest)  I was trying to ask about the case where RCU thinks a CPU is about to
> > > enter a guest, but the CPU never does (at least, not in the immediate future).
> > > 
> > > Or am I just not understanding how RCU's kthreads work?
> > 
> > It is quite possible that the current rcu_pending() code needs help,
> > given the possibility of vCPU preemption.  I have heard of people doing
> > nested KVM virtualization -- or is that no longer a thing?
> 
> Nested virtualization is still very much a thing, but I don't see how it is at
> all unique with respect to RCU grace periods and quiescent states.  More below.
> 
> > But the help might well involve RCU telling the hypervisor that a given
> > vCPU needs to run.  Not sure how that would go over, though it has been
> > prototyped a couple times in the context of RCU priority boosting.
> >
> > > > > > 3 - It checks if the guest exit happened over than 1 second ago. This 1
> > > > > >     second value was copied from rcu_nohz_full_cpu() which checks if the
> > > > > >     grace period started over than a second ago. If this value is bad,
> > > > > >     I have no issue changing it.
> > > > > 
> > > > > IMO, checking if a CPU "recently" ran a KVM vCPU is a suboptimal heuristic regardless
> > > > > of what magic time threshold is used.  
> > > > 
> > > > Why? It works for this particular purpose.
> > > 
> > > Because maintaining magic numbers is no fun, AFAICT the heurisitic doesn't guard
> > > against edge cases, and I'm pretty sure we can do better with about the same amount
> > > of effort/churn.
> > 
> > Beyond a certain point, we have no choice.  How long should RCU let
> > a CPU run with preemption disabled before complaining?  We choose 21
> > seconds in mainline and some distros choose 60 seconds.  Android chooses
> > 20 milliseconds for synchronize_rcu_expedited() grace periods.
> 
> Issuing a warning based on an arbitrary time limit is wildly different than using
> an arbitrary time window to make functional decisions.  My objection to the "assume
> the CPU will enter a quiescent state if it exited a KVM guest in the last second"
> is that there are plenty of scenarios where that assumption falls apart, i.e. where
> _that_ physical CPU will not re-enter the guest.
> 
> Off the top of my head:
> 
>  - If the vCPU is migrated to a different physical CPU (pCPU), the *old* pCPU
>    will get false positives, and the *new* pCPU will get false negatives (though
>    the false negatives aren't all that problematic since the pCPU will enter a
>    quiescent state on the next VM-Enter.
> 
>  - If the vCPU halts, in which case KVM will schedule out the vCPU/task, i.e.
>    won't re-enter the guest.  And so the pCPU will get false positives until the
>    vCPU gets a wake event or the 1 second window expires.
> 
>  - If the VM terminates, the pCPU will get false positives until the 1 second
>    window expires.
> 
> The false positives are solvable problems, by hooking vcpu_put() to reset
> kvm_last_guest_exit.  And to help with the false negatives when a vCPU task is
> scheduled in on a different pCPU, KVM would hook vcpu_load().

Sean,

It seems that fixing the problems you pointed out above is a way to go.