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

[Daniel P. Berrange]

On Thu, Jul 27, 2017 at 12:01:58PM -0600, Alex Williamson wrote:
> On Thu, 27 Jul 2017 17:17:48 +0100
> "Daniel P. Berrange" <berrange@xxxxxxxxxx> wrote:
> 
> > On Wed, Jul 26, 2017 at 10:43:43AM -0600, Alex Williamson wrote:
> > > [cc +libvir-list]
> > > 
> > > On Wed, 26 Jul 2017 21:16:59 +0800
> > > "Gao, Ping A" <ping.a.gao@xxxxxxxxx> 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?
> > > 
> > > 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?
> > > 
> > > 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,  
> > 
> > My view on this from libvirt side is pretty much unchanged since the
> > last time we discussed this.
> > 
> > We would like the kernel maintainers to define standard sets of properties
> > for mdevs, whether global to all mdevs, or scoped to certain classes of
> > mdev (eg a class=gpu). These properties would be exported in sysfs, with
> > one file per property.
> 
> Yes, I think that much of the mechanics are obvious (standardized
> sysfs layout, one property per file, properties under the device node
> in sysfs, etc).  Are you saying that you don't want to be consulted on
> which properties are exposed and how they operate and therefore won't
> complain regardless of what we implement in the kernel? ;)

Well ultimately the kernel maintainers know what is possible from the
hardware / driver POV, so yeah, I think we can mostly leave it upto
you what individual things need to be exposed - not much different
scenario from all the knobs we already exposed for physical devices.


> I'm hoping that libvirt folks have some experience managing basic
> scheduling level QoS attributes and might have some input as to what
> sorts of things work well vs what seems like a good idea, but falls
> apart or isn't useful in practice.

Sure, happy to give feedback where desired.

Regards,
Daniel
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