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

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

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

    Paolo

    > (the one exception where it works is where you have a single thread per
    > cpu, because then there's effectively no scheduling).

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