Re: [PATCH] x86/paravirt: Add kernel parameter to choose paravirt lock type

[Boris Ostrovsky]

On 11/01/2017 12:28 PM, Waiman Long wrote:
> On 11/01/2017 11:51 AM, Juergen Gross wrote:
>> On 01/11/17 16:32, Waiman Long wrote:
>>> Currently, there are 3 different lock types that can be chosen for
>>> the x86 architecture:
>>>
>>>  - qspinlock
>>>  - pvqspinlock
>>>  - unfair lock
>>>
>>> One of the above lock types will be chosen at boot time depending on
>>> a number of different factors.
>>>
>>> Ideally, the hypervisors should be able to pick the best performing
>>> lock type for the current VM configuration. That is not currently
>>> the case as the performance of each lock type are affected by many
>>> different factors like the number of vCPUs in the VM, the amount vCPU
>>> overcommitment, the CPU type and so on.
>>>
>>> Generally speaking, unfair lock performs well for VMs with a small
>>> number of vCPUs. Native qspinlock may perform better than pvqspinlock
>>> if there is vCPU pinning and there is no vCPU over-commitment.
>>>
>>> This patch adds a new kernel parameter to allow administrator to
>>> choose the paravirt spinlock type to be used. VM administrators can
>>> experiment with the different lock types and choose one that can best
>>> suit their need, if they want to. Hypervisor developers can also use
>>> that to experiment with different lock types so that they can come
>>> up with a better algorithm to pick the best lock type.
>>>
>>> The hypervisor paravirt spinlock code will override this new parameter
>>> in determining if pvqspinlock should be used. The parameter, however,
>>> will override Xen's xen_nopvspin in term of disabling unfair lock.
>> Hmm, I'm not sure we need pvlock_type _and_ xen_nopvspin. What do others
>> think?
> I don't think we need xen_nopvspin, but I don't want to remove that
> without agreement from the community.

I also don't think xen_nopvspin will be needed after pvlock_type is
introduced.

-boris