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

Leonardo Bras
2024-05-03T22:00:17Z

On Fri, May 03, 2024 at 02:29:57PM -0700, Sean Christopherson wrote:
> On Fri, May 03, 2024, Leonardo Bras wrote:
> > > KVM can provide that information with much better precision, e.g. KVM
> > > knows when when it's in the core vCPU run loop.
> >
> > That would not be enough.
> > I need to present the application/problem to make a point:
> >
> > - There is multiple isolated physical CPU (nohz_full) on which we want to
> > run KVM_RT vcpus, which will be running a real-time (low latency) task.
> > - This task should not miss deadlines (RT), so we test the VM to make sure
> > the maximum latency on a long run does not exceed the latency requirement
> > - This vcpu will run on SCHED_FIFO, but has to run on lower priority than
> > rcuc, so we can avoid stalling other cpus.
> > - There may be some scenarios where the vcpu will go back to userspace
> > (from KVM_RUN ioctl), and that does not mean it's good to interrupt the
> > this to run other stuff (like rcuc).
> >
> > Now, I understand it will cover most of our issues if we have a context
> > tracking around the vcpu_run loop, since we can use that to decide not to
> > run rcuc on the cpu if the interruption hapenned inside the loop.
> >
> > But IIUC we can have a thread that "just got out of the loop" getting
> > interrupted by the timer, and asked to run rcu_core which will be bad for
> > latency.
> >
> > I understand that the chance may be statistically low, but happening once
> > may be enough to crush the latency numbers.
> >
> > Now, I can't think on a place to put this context trackers in kvm code that
> > would avoid the chance of rcuc running improperly, that's why the suggested
> > timeout, even though its ugly.
> >
> > About the false-positive, IIUC we could reduce it if we reset the per-cpu
> > last_guest_exit on kvm_put.
>
> Which then opens up the window that you're trying to avoid (IRQ arriving just
> after the vCPU is put, before the CPU exits to userspace).
>
> If you want the "entry to guest is imminent" status to be preserved across an exit
> to userspace, then it seems liek the flag really should be a property of the task,
> not a property of the physical CPU. Similar to how rcu_is_cpu_rrupt_from_idle()
> detects that an idle task was interrupted, that goal is to detect if a vCPU task
> was interrupted.
>
> PF_VCPU is already "taken" for similar tracking, but if we want to track "this
> task will soon enter an extended quiescent state", I don't see any reason to make
> it specific to vCPU tasks. Unless the kernel/KVM dynamically manages the flag,
> which as above will create windows for false negatives, the kernel needs to
> trust userspace to a certaine extent no matter what. E.g. even if KVM sets a
> PF_xxx flag on the first KVM_RUN, nothing would prevent userspace from calling
> into KVM to get KVM to set the flag, and then doing something else entirely with
> the task.
>
> So if we're comfortable relying on the 1 second timeout to guard against a
> misbehaving userspace, IMO we might as well fully rely on that guardrail. I.e.
> add a generic PF_xxx flag (or whatever flag location is most appropriate) to let
> userspace communicate to the kernel that it's a real-time task that spends the
> overwhelming majority of its time in userspace or guest context, i.e. should be
> given extra leniency with respect to rcuc if the task happens to be interrupted
> while it's in kernel context.
>

I think I understand what you propose here.

But I am not sure what would happen in this case:

- RT guest task calls short HLT
- Host schedule another kernel thread (other task)
- Timer interruption, rcu_pending will() check the task which is not set
with above flag.
- rcuc runs, introducing latency
- Goes back to previous kernel thread, finishes running with rcuc latency
- Goes back to vcpu thread

Isn't there any chance that, on an short guest HLT, the latency previously
introduced by rcuc preempting another kernel thread gets to introduce a
latency to the RT task running in the vcpu?

Thanks!
Leo

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