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

Paul E. McKenney
2024-04-08T21:02:37Z

On Mon, Apr 08, 2024 at 01:06:00PM -0700, Sean Christopherson wrote:
> On Mon, Apr 08, 2024, Paul E. McKenney wrote:
> > On Mon, Apr 08, 2024 at 10:16:24AM -0700, Sean Christopherson wrote:
> > > On Fri, Apr 05, 2024, Paul E. McKenney wrote:
> > > 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().
> >
> > Here you are arguing against the heuristic in the original patch, correct?
>
> Yep, correct.

Whew!!! ;-)

> > As opposed to the current RCU heuristic that ignores certain quiescent
> > states for nohz_full CPUs until the grace period reaches an age of
> > one second?
> >
> > If so, no argument here. In fact, please consider my ack cancelled.
>
> ...
>
> > > That's a largely orthogonal discussion. As above, boosting the scheduling priority
> > > of a vCPU because that vCPU is in critical section of some form is not at all
> > > unique to nested virtualization (or RCU).
> > >
> > > For basic functional correctness, the L0 hypervisor already has the "hint" it
> > > needs. L0 knows that the L1 CPU wants to run by virtue of the L1 CPU being
> > > runnable, i.e. not halted, not in WFS, etc.
> >
> > And if the system is sufficiently lightly loaded, all will be well, as is
> > the case with my rcutorture usage. However, if the system is saturated,
> > that basic functional correctness might not be enough. I haven't heard
> > many complaints, other than research work, so I have been assuming that
> > we do not yet need hinting. But you guys tell me. ;-)
>
> We should never use hinting for basic, *default* functionality. If the host is
> so overloaded that it can induce RCU stalls with the default threshold of 21
> seconds, then something in the host's domain is broken/misconfigured. E.g. it
> doesn't necessary have to be a host kernel/userspace bug, it could be an issue
> with VM scheduling at the control plane. But it's still a host issue, and under
> no circumstance should the host need a hint in order for the guest to not complain
> after 20+ seconds.
>
> And _if_ we were to push the default lower, e.g. all the way down to Android's
> aggressive 20 milliseconds, a boosting hint would still be the wrong way to go
> about it, because no sane hypervisor would ever back such a hint with strong
> guarantees for all scenarios.
>
> It's very much possible to achieve a 20ms deadline when running as a VM, but it
> would require strong guarantees about the VM's configuration and environment,
> e.g. that memory isn't overcommited, that each vCPU has a fully dedicated pCPU,
> etc.

Agreed, and again, you guys need to tell me what is necessary here.

> > > > > + rcu_nohz_full_cpu())
> > > >
> > > > And rcu_nohz_full_cpu() has a one-second timeout, and has for quite
> > > > some time.
> > >
> > > That's not a good reason to use a suboptimal heuristic for determining whether
> > > or not a CPU is likely to enter a KVM guest, it simply mitigates the worst case
> > > scenario of a false positive.
> >
> > Again, are you referring to the current RCU code, or the original patch
> > that started this email thread?
>
> Original patch.
>
> > > > > /* Is the RCU core waiting for a quiescent state from this CPU? */
> > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > > > index bfb2b52a1416..5a7efc669a0f 100644
> > > > > --- a/virt/kvm/kvm_main.c
> > > > > +++ b/virt/kvm/kvm_main.c
> > > > > @@ -209,6 +209,9 @@ void vcpu_load(struct kvm_vcpu *vcpu)
> > > > > {
> > > > > int cpu = get_cpu();
> > > > >
> > > > > + if (vcpu->wants_to_run)
> > > > > + context_tracking_guest_start_run_loop();
> > > >
> > > > At this point, if this is a nohz_full CPU, it will no longer report
> > > > quiescent states until the grace period is at least one second old.
> > >
> > > I don't think I follow the "will no longer report quiescent states" issue. Are
> > > you saying that this would prevent guest_context_enter_irqoff() from reporting
> > > that the CPU is entering a quiescent state? If so, that's an issue that would
> > > need to be resolved regardless of what heuristic we use to determine whether or
> > > not a CPU is likely to enter a KVM guest.
> >
> > Please allow me to start over. Are interrupts disabled at this point,
>
> Nope, IRQs are enabled.
>
> Oof, I'm glad you asked, because I was going to say that there's one exception,
> kvm_sched_in(), which is KVM's notifier for when a preempted task/vCPU is scheduled
> back in. But I forgot that kvm_sched_{in,out}() don't use vcpu_{load,put}(),
> i.e. would need explicit calls to context_tracking_guest_{stop,start}_run_loop().
>
> > and, if so, will they remain disabled until the transfer of control to
> > the guest has become visible to RCU via the context-tracking code?
> >
> > Or has the context-tracking code already made the transfer of control
> > to the guest visible to RCU?
>
> Nope. The call to __ct_user_enter(CONTEXT_GUEST) or rcu_virt_note_context_switch()
> happens later, just before the actual VM-Enter. And that call does happen with
> IRQs disabled (and IRQs stay disabled until the CPU enters the guest).

OK, then we can have difficulties with long-running interrupts hitting
this range of code. It is unfortunately not unheard-of for interrupts
plus trailing softirqs to run for tens of seconds, even minutes.

One counter-argument is that that softirq would take scheduling-clock
interrupts, and would eventually make rcu_core() run.

But does a rcu_sched_clock_irq() from a guest OS have its "user"
argument set?

> > > > > __this_cpu_write(kvm_running_vcpu, vcpu);
> > > > > preempt_notifier_register(&vcpu->preempt_notifier);
> > > > > kvm_arch_vcpu_load(vcpu, cpu);
> > > > > @@ -222,6 +225,10 @@ void vcpu_put(struct kvm_vcpu *vcpu)
> > > > > kvm_arch_vcpu_put(vcpu);
> > > > > preempt_notifier_unregister(&vcpu->preempt_notifier);
> > > > > __this_cpu_write(kvm_running_vcpu, NULL);
> > > > > +
> > > >
> > > > And also at this point, if this is a nohz_full CPU, it will no longer
> > > > report quiescent states until the grace period is at least one second old.
> >
> > And here, are interrupts disabled at this point, and if so, have they
> > been disabled since the time that the exit from the guest become
> > visible to RCU via the context-tracking code?
>
> IRQs are enabled.
>
> The gist of my suggestion is:
>
> ioctl(KVM_RUN) {
>
> context_tracking_guest_start_run_loop();
>
> for (;;) {
>
> vcpu_run();
>
> if (<need to return to userspace>)
> break;
> }
>
> context_tracking_guest_stop_run_loop();
> }
>
> where vcpu_run() encompasses a fairly huge amount of code and functionality,
> including the logic to do world switches between host and guest.
>
> E.g. if a vCPU triggers a VM-Exit because it tried to access memory that has been
> swapped out by the host, KVM could end up way down in mm/ doing I/O to bring a
> page back into memory for the guest. Immediately after VM-Exit, before enabling
> IRQs, KVM will notify RCU that the CPU has exited the extended quiescent state
> (this is what happens today). But the "in KVM run loop" flag would stay set, and
> RCU would rely on rcu_nohz_full_cpu() for protection, e.g. in case faulting in
> memory somehow takes more than a second.
>
> But, barring something that triggers a return to userspace, KVM _will_ re-enter
> the guest as quickly as possible. So it's still a heuristic in the sense that
> the CPU isn't guaranteed to enter the guest, nor are there any enforceable SLOs
> on how quickly the CPU will enter the guest, but I think it's the best tradeoff
> between simplicity and functionality, especially since rcu_nohz_full_cpu() has
> a one second timeout to safeguard against some unforeseen hiccup that prevents
> KVM from re-entering the guest in a timely manner.
>
> Note, as above, my intent is that there would also be hooks in kvm_sched_{in,out}()
> to note that the guest run loop is starting/stopping if the vCPU task yields or
> is preempted.

Very good, same responses as for the context_tracking_guest_start_run_loop()
case.

Thanx, Paul