Linux Virtualization - Mar 2014 - [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest

On 03/18/2014 04:14 AM, Paolo Bonzini wrote:
> Il 17/03/2014 20:05, Konrad Rzeszutek Wilk ha scritto:
>>> > Measurements were done by Gleb for two guests running 2.6.32
with 16
>>> > vcpus each, on a 16-core system.  One guest ran with unfair
locks,
>>> > one guest ran with fair locks.  Two kernel compilations
("time make
>> And when you say fair locks are you saying PV ticketlocks or generic
>> ticketlocks?
>
> Generic, of course.
>
>> You should see the same values with the PV ticketlock. It is not clear
>> to me if this testing did include that variant of locks?
>
> Yes, PV is fine.  But up to this point of the series, we are concerned 
> about spinlock performance when running on an overcommitted hypervisor 
> that doesn't support PV spinlocks.
The unfair queue lock is designed in such a way that it will only be 
activated when running in a PV guest or it won't be mergeable upstream. 
So there must be some way to determine if it is running under a hypervisor.

-Longman

Il 19/03/2014 04:15, Waiman Long ha scritto:
>>> You should see the same values with the PV ticketlock. It is not
clear
>>> to me if this testing did include that variant of locks?
>>
>> Yes, PV is fine.  But up to this point of the series, we are concerned
>> about spinlock performance when running on an overcommitted hypervisor
>> that doesn't support PV spinlocks.
>
> The unfair queue lock is designed in such a way that it will only be
> activated when running in a PV guest or it won't be mergeable upstream.
> So there must be some way to determine if it is running under a hypervisor.
Exactly.  What you want is boot_cpu_has(X86_FEATURE_HYPERVISOR).

Paolo

On 03/19/2014 06:07 AM, Paolo Bonzini wrote:
> Il 19/03/2014 04:15, Waiman Long ha scritto:
>>>> You should see the same values with the PV ticketlock. It is
not clear
>>>> to me if this testing did include that variant of locks?
>>>
>>> Yes, PV is fine.  But up to this point of the series, we are
concerned
>>> about spinlock performance when running on an overcommitted
hypervisor
>>> that doesn't support PV spinlocks.
>>
>> The unfair queue lock is designed in such a way that it will only be
>> activated when running in a PV guest or it won't be mergeable
upstream.
>> So there must be some way to determine if it is running under a 
>> hypervisor.
>
> Exactly.  What you want is boot_cpu_has(X86_FEATURE_HYPERVISOR).
>
> Paolo
The unfair lock is to be enabled by boot time check, not just by the 
presence of a configuration macro during the build process in order to 
avoid using unfair lock on bare metal. Of course, Linux distros can 
modify this if that suits their need.

-Longman