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.
