[libvirt] [RFC]Add new mdev interface for QoS

Gao, Ping A ping.a.gao at intel.com
Tue Aug 1 05:54:27 UTC 2017

On 2017/7/28 0:00, Gao, Ping A wrote:
> On 2017/7/27 0:43, Alex Williamson wrote:
>> [cc +libvir-list]
>> On Wed, 26 Jul 2017 21:16:59 +0800
>> "Gao, Ping A" <ping.a.gao at intel.com> wrote:
>>> The vfio-mdev provide the capability to let different guest share the
>>> same physical device through mediate sharing, as result it bring a
>>> requirement about how to control the device sharing, we need a QoS
>>> related interface for mdev to management virtual device resource.
>>> E.g. In practical use, vGPUs assigned to different quests almost has
>>> different performance requirements, some guests may need higher priority
>>> for real time usage, some other may need more portion of the GPU
>>> resource to get higher 3D performance, corresponding we can define some
>>> interfaces like weight/cap for overall budget control, priority for
>>> single submission control.
>>> So I suggest to add some common attributes which are vendor agnostic in
>>> mdev core sysfs for QoS purpose.
>> I think what you're asking for is just some standardization of a QoS
>> attribute_group which a vendor can optionally include within the
>> existing mdev_parent_ops.mdev_attr_groups.  The mdev core will
>> transparently enable this, but it really only provides the standard,
>> all of the support code is left for the vendor.  I'm fine with that,
>> but of course the trouble with and sort of standardization is arriving
>> at an agreed upon standard.  Are there QoS knobs that are generic
>> across any mdev device type?  Are there others that are more specific
>> to vGPU?  Are there existing examples of this that we can steal their
>> specification?
> Yes, you are right, standardization QoS knobs are exactly what I wanted.
> Only when it become a part of the mdev framework and libvirt, then QoS
> such critical feature can be leveraged by cloud usage. HW vendor only
> need to focus on the implementation of the corresponding QoS algorithm
> in their back-end driver.
> Vfio-mdev framework provide the capability to share the device that lack
> of HW virtualization support to guests, no matter the device type,
> mediated sharing actually is a time sharing multiplex method, from this
> point of view, QoS can be take as a generic way about how to control the
> time assignment for virtual mdev device that occupy HW. As result we can
> define QoS knob generic across any device type by this way. Even if HW
> has build in with some kind of QoS support, I think it's not a problem
> for back-end driver to convert mdev standard QoS definition to their
> specification to reach the same performance expectation. Seems there are
> no examples for us to follow, we need define it from scratch.
> I proposal universal QoS control interfaces like below:
> Cap: The cap limits the maximum percentage of time a mdev device can own
> physical device. e.g. cap=60, means mdev device cannot take over 60% of
> total physical resource.
> Weight: The weight define proportional control of the mdev device
> resource between guests, it’s orthogonal with Cap, to target load
> balancing. E.g. if guest 1 should take double mdev device resource
> compare with guest 2, need set weight ratio to 2:1.
> Priority: The guest who has higher priority will get execution first,
> target to some real time usage and speeding interactive response.
> Above QoS interfaces cover both overall budget control and single
> submission control. I will sent out detail design later once get aligned.

Hi Alex,
Any comments about the interface mentioned above?

>> Also, mdev devices are not necessarily the exclusive users of the
>> hardware, we can have a native user such as a local X client.  They're
>> not an mdev user, so we can't support them via the mdev_attr_group.
>> Does there need to be a per mdev parent QoS attribute_group standard
>> for somehow defining the QoS of all the child mdev devices, or perhaps
>> representing the remaining host QoS attributes?
> That's really an open, if we don't take host workload into consideration
> for cloud usage, it's not a problem any more, however such assumption is
> not reasonable. Any way if we take mdev devices as clients of host
> driver, and host driver provide the capability to divide out a portion
> HW resource to mdev devices, then it's only need to take care about the
> resource that host assigned for mdev devices. Follow this way QoS for
> mdev focus on the relationship between mdev devices no need to take care
> the host workload.
> -Ping
>> Ultimately libvirt and upper level management tools would be the
>> consumer of these control knobs, so let's immediately get libvirt
>> involved in the discussion.  Thanks,
>> Alex

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