Re: [RFC PATCH 0/2] KVM: Support Heterogeneous RT VCPU Configurations.

[Suleiman Souhlal]

On Wed, Jul 28, 2021 at 7:34 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>
> On Wed, Jul 28, 2021 at 10:10:31AM +0200, Peter Zijlstra wrote:
> > On Wed, Jul 28, 2021 at 04:36:58PM +0900, Suleiman Souhlal wrote:
> > > Hello,
> > >
> > > This series attempts to solve some issues that arise from
> > > having some VCPUs be real-time while others aren't.
> > >
> > > We are trying to play media inside a VM on a desktop environment
> > > (Chromebooks), which requires us to have some tasks in the guest
> > > be serviced at real-time priority on the host so that the media
> > > can be played smoothly.
> > >
> > > To achieve this, we give a VCPU real-time priority on the host
> > > and use isolcpus= to ensure that only designated tasks are allowed
> > > to run on the RT VCPU.
> >
> > WTH do you need isolcpus for that? What's wrong with cpusets?
> >
> > > In order to avoid priority inversions (for example when the RT
> > > VCPU preempts a non-RT that's holding a lock that it wants to
> > > acquire), we dedicate a host core to the RT vcpu: Only the RT
> > > VCPU is allowed to run on that CPU, while all the other non-RT
> > > cores run on all the other host CPUs.
> > >
> > > This approach works on machines that have a large enough number
> > > of CPUs where it's possible to dedicate a whole CPU for this,
> > > but we also have machines that only have 2 CPUs and doing this
> > > on those is too costly.
> > >
> > > This patch series makes it possible to have a RT VCPU without
> > > having to dedicate a whole host core for it.
> > > It does this by making it so that non-RT VCPUs can't be
> > > preempted if they are in a critical section, which we
> > > approximate as having interrupts disabled or non-zero
> > > preempt_count. Once the VCPU is found to not be in a critical
> > > section anymore, it will give up the CPU.
> > > There measures to ensure that preemption isn't delayed too
> > > many times.
> > >
> > > (I realize that the hooks in the scheduler aren't very
> > > tasteful, but I couldn't figure out a better way.
> > > SVM support will be added when sending the patch for
> > > inclusion.)
> > >
> > > Feedback or alternatives are appreciated.
> >
> > This is disguisting and completely wrecks the host scheduling. You're
> > placing guest over host, that's fundamentally wrong.
> >
> > NAK!
> >
> > If you want co-ordinated RT scheduling, look at paravirtualized deadline
> > scheduling.
>
> Peter, not sure what exactly are you thinking of? (to solve this
> particular problem with pv deadline scheduling).
>
> Shouldnt it be possible to, through paravirt locks, boost the priority
> of the non-RT vCPU (when locking fails in the -RT vCPU) ?

Unfortunately paravirt locks doesn't work in this configuration
because sched_yield() doesn't work across scheduling classes (non-RT
vs RT). :-(

-- Suleiman