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).
