Re: [RFC][PATCH 0/4] arm64:kvm: teach guest sched that VCPUs can be preempted

[Joel Fernandes]

Hi Marc, nice to hear from you.

On Wed, Dec 9, 2020 at 4:43 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> Hi all,
>
> On 2020-12-08 20:02, Joel Fernandes wrote:
> > On Fri, Sep 11, 2020 at 4:58 AM Sergey Senozhatsky
> > <sergey.senozhatsky@xxxxxxxxx> wrote:
> >>
> >> My apologies for the slow reply.
> >>
> >> On (20/08/17 13:25), Marc Zyngier wrote:
> >> >
> >> > It really isn't the same thing at all. You are exposing PV spinlocks,
> >> > while Sergey exposes preemption to vcpus.
> >> >
> >>
> >> Correct, we see vcpu preemption as a "fundamental" feature, with
> >> consequences that affect scheduling, which is a core feature :)
> >>
> >> Marc, is there anything in particular that you dislike about this RFC
> >> patch set? Joel has some ideas, which we may discuss offline if that
> >> works for you.
> >
> > Hi Marc, Sergey, Just checking what is the latest on this series?
>
> I was planning to give it a go, but obviously got sidetracked. :-(

Ah, that happens.

> > About the idea me and Sergey discussed, at a high level we discussed
> > being able to share information similar to "Is the vCPU preempted?"
> > using a more arch-independent infrastructure. I do not believe this
> > needs to be arch-specific. Maybe the speciifc mechanism about how to
> > share a page of information needs to be arch-specific, but the actual
> > information shared need not be.
>
> We already have some information sharing in the form of steal time
> accounting, and I believe this "vcpu preempted" falls in the same
> bucket. It looks like we could implement the feature as an extension
> of the steal-time accounting, as the two concepts are linked
> (one describes the accumulation of non-running time, the other is
> instantaneous).

Yeah I noticed the steal stuff. Will go look more into that.

> > This could open the door to sharing
> > more such information in an arch-independent way (for example, if the
> > scheduler needs to know other information such as the capacity of the
> > CPU that the vCPU is on).
>
> Quentin and I have discussed potential ways of improving guest
> scheduling
> on terminally broken systems (otherwise known as big-little), in the
> form of a capacity request from the guest to the host. I'm not really
> keen on the host exposing its own capacity, as that doesn't tell the
> host what the guest actually needs.

I am not sure how a capacity request could work well. It seems the
cost of a repeated hypercall could be prohibitive. In this case, a
lighter approach might be for KVM to restrict vCPU threads to run on
certain types of cores, and pass the capacity information to the guest
at guest's boot time. This would be a one-time cost to pay. And then,
then the guest scheduler can handle the scheduling appropriately
without any more hypercalls. Thoughts?

- Joel