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

[Paolo Bonzini]

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).