Re: [PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

[Peter Zijlstra]

On Tue, Sep 05, 2017 at 08:28:10AM +0200, Juergen Gross wrote:
> On 05/09/17 00:21, Davidlohr Bueso wrote:
> > On Mon, 04 Sep 2017, Peter Zijlstra wrote:
> > 
> >> For testing its trivial to hack your kernel and I don't feel this is
> >> something an Admin can make reasonable decisions about.
> >>
> >> So why? In general less knobs is better.
> > 
> > +1.
> > 
> > Also, note how b8fa70b51aa (xen, pvticketlocks: Add xen_nopvspin parameter
> > to disable xen pv ticketlocks) has no justification as to why its wanted
> > in the first place. The only thing I could find was from 15a3eac0784
> > (xen/spinlock: Document the xen_nopvspin parameter):
> > 
> > "Useful for diagnosing issues and comparing benchmarks in over-commit
> > CPU scenarios."
> 
> Hmm, I think I should clarify the Xen knob, as I was the one requesting
> it:
> 
> In my previous employment we had a configuration where dom0 ran
> exclusively on a dedicated set of physical cpus. We experienced
> scalability problems when doing I/O performance tests: with a decent
> number of dom0 cpus we achieved throughput of 700 MB/s with only 20%
> cpu load in dom0. A higher dom0 cpu count let the throughput drop to
> about 150 MB/s and cpu load was up to 100%. Reason was the additional
> load due to hypervisor interactions on a high frequency lock.
> 
> So in special configurations at least for Xen the knob is useful for
> production environment.

So the problem with qspinlock is that it will revert to a classic
test-and-set spinlock if you don't do paravirt but are running a HV.

And test-and-set is unfair and has all kinds of ugly starvation cases,
esp on slightly bigger hardware.

So if we'd want to cater to the 1:1 virt case, we'll need to come up
with something else. _IF_ it is an issue of course.