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.
