Re: [Qemu-devel] [PATCH 0/7] x86: Rework KVM-defaults compat code, enable kvm_pv_unhalt by default

[Paolo Bonzini <pbonzini@redhat.com>]

On 09/10/2017 17:15, Waiman Long wrote:
> On 10/09/2017 09:39 AM, Paolo Bonzini wrote:
>> On 06/10/2017 23:52, Eduardo Habkost wrote:
>>> This series enables kvm_pv_unhalt by default on pc-*-2.11 and
>>> newer.
>>
>> I've discussed PV spinlocks with some folks at Microsoft for a few weeks
>> now, and I'm not 100% sure about enabling kvm_pv_unhalt by default.
>> It's probably a good idea overall, but it does come with some caveats.
>>
>> The problem is that there were two different implementations of fair
>> spinlocks in Linux, ticket spinlocks and queued spinlocks.  When
>> kvm_pv_unhalt is disabled, ticket spinlocks sucked badly indeed; queued
>> spinlocks however simply revert to unfair spinlocks, which loses the
>> fairness but has the best performance.  See virt_spin_lock in
>> arch/x86/include/asm/qspinlock.h.
>>
>> Now, fair spinlocks are only really needed for large NUMA machines.
>> With a single NUMA node, for example, test-and-set spinlocks work well
>> enough; there's not _much_ need for fairness in practice, and the
>> paravirtualization does introduce some overhead.
>>
>> Therefore, the best performance would be achieved with kvm_pv_unhalt
>> disabled on small VMs, and enabled on large VMs spanning multiple host
>> NUMA nodes.
>>
>> Waiman, Davidlohr, do you have an opinion on this as well?
> 
> I agree. Using unfair lock in a small VM is good for performance. The
> only problem I see is how do we define what is small. Now, even a
> machine with a single socket can have up to 28 cores, 56 threads. If a
> VM has up to 56 vCPUs, we may still want pvqspinlock to be used.

Yes.  Another possibility is to enable it when there is >1 NUMA node in
the guest.  We generally don't do this kind of magic but higher layers
(oVirt/OpenStack) do.

It also depends on how worse performance is with PV qspinlocks,
especially since now there's also vcpu_is_preempted that has to be
thrown in the mix.  A lot more testing is needed.

> Is the kvm_pv_unhalt flag a global one for all VMs within a machine? Or
> can it be different for each VM? We may want to have this flag
> dynamically determined depending on the configuration of the VM.

It's per-VM, so that's easy.

Paolo