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Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall

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On Sat, Sep 23, 2017 at 12:56:12PM +0200, Paolo Bonzini wrote:
> On 22/09/2017 14:55, Peter Zijlstra wrote:
> > You just explained it yourself. If the thread that needs to complete
> > what you're waiting on has lower priority, it will _never_ get to run if
> > you're busy waiting on it.
> > 
> > This is _trivial_.
> > 
> > And even for !RT it can be quite costly, because you can end up having
> > to burn your entire slot of CPU time before you run the other task.
> > 
> > Userspace spinning is _bad_, do not do this.
> 
> This is not userspace spinning, it is guest spinning---which has
> effectively the same effect but you cannot quite avoid.

So I'm virt illiterate and have no clue on how all this works; but
wasn't this a vmexit ? (that's what marcelo traced). And once you've
done a vmexit you're a regular task again, not a vcpu.

> But I agree that the solution is properly prioritizing threads that can
> interrupt the VCPU, and using PI mutexes.

Right, if you want to run RT VCPUs the whole emulator/vcpu interaction
needs to be designed for RT.

> I'm not a priori opposed to paravirt scheduling primitives, but I am not
> at all sure that it's required.

Problem is that the proposed thing doesn't solve anything. There is
nothing that prohibits the guest from triggering a vmexit while holding
a spinlock and landing in the self-same problems.

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[patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-21 13:50 +0200
  Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2017-09-21 15:40 +0200
    Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-21 16:10 +0200
      Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 03:20 +0200
        Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 12:10 +0200
          Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 13:00 +0200
            Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-23 13:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-23 15:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-24 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 05:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-25 11:20 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Paolo Bonzini <pbonzini@redhat.com> - 2017-09-25 17:20 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> - 2017-09-25 18:30 +0200
          Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:20 +0200
            Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 14:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:40 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 04:30 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall Peter Zijlstra <peterz@infradead.org> - 2017-09-25 10:40 +0200
              Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-22 14:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Peter Zijlstra <peterz@infradead.org> - 2017-09-22 15:10 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Marcelo Tosatti <mtosatti@redhat.com> - 2017-09-25 04:30 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Peter Zijlstra <peterz@infradead.org> - 2017-09-25 11:00 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Thomas Gleixner <tglx@linutronix.de> - 2017-09-25 12:50 +0200
                Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO  hypercall\ Jan Kiszka <jan.kiszka@siemens.com> - 2017-09-25 20:40 +0200

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