Re: [RFC PATCH v2 0/5] Paravirt Scheduling (Dynamic vcpu priority management)

[Sean Christopherson <seanjc@google.com>]

On Wed, Jul 17, 2024, Joel Fernandes wrote:
> On Tue, Jul 16, 2024 at 7:44 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Fri, Jul 12, 2024, Steven Rostedt wrote:
> > > On Fri, 12 Jul 2024 09:44:16 -0700
> > > Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > > > All we need is a notifier that gets called at every VMEXIT.
> > > >
> > > > Why?  The only argument I've seen for needing to hook VM-Exit is so that the
> > > > host can speculatively boost the priority of the vCPU when deliverying an IRQ,
> > > > but (a) I'm unconvinced that is necessary, i.e. that the vCPU needs to be boosted
> > > > _before_ the guest IRQ handler is invoked and (b) it has almost no benefit on
> > > > modern hardware that supports posted interrupts and IPI virtualization, i.e. for
> > > > which there will be no VM-Exit.
> > >
> > > No. The speculatively boost was for something else, but slightly
> > > related. I guess the ideal there was to have the interrupt coming in
> > > boost the vCPU because the interrupt could be waking an RT task. It may
> > > still be something needed, but that's not what I'm talking about here.
> > >
> > > The idea here is when an RT task is scheduled in on the guest, we want
> > > to lazily boost it. As long as the vCPU is running on the CPU, we do
> > > not need to do anything. If the RT task is scheduled for a very short
> > > time, it should not need to call any hypercall. It would set the shared
> > > memory to the new priority when the RT task is scheduled, and then put
> > > back the lower priority when it is scheduled out and a SCHED_OTHER task
> > > is scheduled in.
> > >
> > > Now if the vCPU gets preempted, it is this moment that we need the host
> > > kernel to look at the current priority of the task thread running on
> > > the vCPU. If it is an RT task, we need to boost the vCPU to that
> > > priority, so that a lower priority host thread does not interrupt it.
> >
> > I got all that, but I still don't see any need to hook VM-Exit.  If the vCPU gets
> > preempted, the host scheduler is already getting "notified", otherwise the vCPU
> > would still be scheduled in, i.e. wouldn't have been preempted.
> 
> What you're saying is the scheduler should change the priority of the
> vCPU thread dynamically. That's really not the job of the scheduler.
> The user of the scheduler is what changes the priority of threads, not
> the scheduler itself.

No.  If we go the proposed route[*] of adding a data structure that lets userspace
and/or the guest express/adjust the task's priority, then the scheduler simply
checks that data structure when querying the priority of a task.

[*] https://lore.kernel.org/all/ZpFWfInsXQdPJC0V@google.com