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

[Eduardo Habkost]

On Tue, Oct 10, 2017 at 02:07:25PM -0400, Waiman Long wrote:
> On 10/10/2017 11:50 AM, Eduardo Habkost wrote:
> >> 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.
> > Can't the guest make this decision, instead of the host?
> 
> By guest, do you mean the guest OS itself or the admin of the guest VM?

It could be either.  But even if action is required from the
guest admin to get better performance in some cases, I'd argue
that the default behavior of a Linux guest shouldn't cause a
performance regression if the host stops hiding a feature in
CPUID.

> 
> I am thinking about maybe adding kernel boot command line option like
> "unfair_pvspinlock_cpu_threshold=4" which will instruct the OS to use
> unfair spinlock if the number of CPUs is 4 or less, for example. The
> default value of 0 will have the same behavior as it is today. Please
> let me know what you guys think about that.

If that's implemented, can't Linux choose a reasonable default
for unfair_pvspinlock_cpu_threshold that won't require the admin
to manually configure it on most cases?

-- 
Eduardo