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

[Peter Zijlstra]

On Fri, Sep 22, 2017 at 09:36:39AM -0300, Marcelo Tosatti wrote:
> 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.
> 
> One can run whatever application they want on the housekeeping
> vcpus. This is why rteval exists.

Nobody cares about other tasks. The problem is between the VCPU and
emulator thread. They get a priority inversion and live-lock because of
spin-waiting.

> This is not the realtime vcpu we are talking about.

You're being confused, its a RT _guest_, all VCPUs _must_ be RT.
Because, as you ran into, the guest functions as a whole, not as a bunch
of individual CPUs.

> We can fix the BIOS, which is hanging now, but userspace can 
> do whatever it wants, on non realtime vcpus (again, this is why
> rteval test exists and is used by the -RT community as 
> a testcase).

But nobody cares what other tasks on the system do, all you care about
is that the VCPUs make deterministic forward progress.

> I haven't understood what is the wrong with the patch? Are you trying
> to avoid pollution of the spinlock codepath to keep it simple?

Your patch is voodoo programming. You don't solve the actual problem,
you try and paper over it.