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

[Marcelo Tosatti]

On Fri, Sep 22, 2017 at 02:31:07PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 22, 2017 at 09:16:40AM -0300, Marcelo Tosatti wrote:
> > On Fri, Sep 22, 2017 at 12:00:05PM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 21, 2017 at 10:10:41PM -0300, Marcelo Tosatti wrote:
> > > > When executing guest vcpu-0 with FIFO:1 priority, which is necessary
> > > > to
> > > > deal with the following situation:
> > > > 
> > > > VCPU-0 (housekeeping VCPU)              VCPU-1 (realtime VCPU)
> > > > 
> > > > raw_spin_lock(A)
> > > > interrupted, schedule task T-1          raw_spin_lock(A) (spin)
> > > > 
> > > > raw_spin_unlock(A)
> > > > 
> > > > Certain operations must interrupt guest vcpu-0 (see trace below).
> > > 
> > > Those traces don't make any sense. All they include is kvm_exit and you
> > > can't tell anything from that.
> > 
> > Hi Peter,
> > 
> > OK lets describe whats happening:
> > 
> > With QEMU emulator thread and vcpu-0 sharing a physical CPU
> > (which is a request from several NFV customers, to improve
> > guest packing), the following occurs when the guest generates 
> > the following pattern:
> > 
> > 		1. submit IO.
> > 		2. busy spin.
> 
> User-space spinning is a bad idea in general and terminally broken in
> a RT setup. Sounds like you need to go fix qemu to not suck.

Are you arguing its invalid for the following application to execute on 
housekeeping vcpu of a realtime system:

void main(void)
{

    submit_IO();
    do {
       computation();
    } while (!interrupted());
}

Really?

Replace "busy spin" by "useful computation until interrupted".