[openstack-dev] vGPUs support for Nova

Matt Riedemann mriedemos at gmail.com
Mon Sep 25 14:29:25 UTC 2017

On 9/25/2017 5:40 AM, Jay Pipes wrote:
> On 09/25/2017 05:39 AM, Sahid Orentino Ferdjaoui wrote:
>> There is a desire to expose the vGPUs resources on top of Resource
>> Provider which is probably the path we should be going in the long
>> term. I was not there for the last PTG and you probably already made a
>> decision about moving in that direction anyway. My personal feeling is
>> that it is premature.
>> The nested Resource Provider work is not yet feature-complete and
>> requires more reviewer attention. If we continue in the direction of
>> Resource Provider, it will need at least 2 more releases to expose the
>> vGPUs feature and that without the support of NUMA, and with the
>> feeling of pushing something which is not stable/production-ready.
>> It's seems safer to first have the Resource Provider work well
>> finalized/stabilized to be production-ready. Then on top of something
>> stable we could start to migrate our current virt specific features
>> like NUMA, CPU Pinning, Huge Pages and finally PCI devices.
>> I'm talking about PCI devices in general because I think we should
>> implement the vGPU on top of our /pci framework which is production
>> ready and provides the support of NUMA.
>> The hardware vendors building their drivers using mdev and the /pci
>> framework currently understand only SRIOV but on a quick glance it
>> does not seem complicated to make it support mdev.
>> In the /pci framework we will have to:
>> * Update the PciDevice object fields to accept NULL value for
>>    'address' and add new field 'uuid'
>> * Update PciRequest to handle a new tag like 'vgpu_types'
>> * Update PciDeviceStats to also maintain pool of vGPUs
>> The operators will have to create alias(-es) and configure
>> flavors. Basically most of the logic is already implemented and the
>> method 'consume_request' is going to select the right vGPUs according
>> the request.
>> In /virt we will have to:
>> * Update the field 'pci_passthrough_devices' to also include GPUs
>>    devices.
>> * Update attach/detach PCI device to handle vGPUs
>> We have a few people interested in working on it, so we could
>> certainly make this feature available for Queen.
>> I can take the lead updating/implementing the PCI and libvirt driver
>> part, I'm sure Jianghua Wang will be happy to take the lead for the
>> virt XenServer part.
>> And I trust Jay, Stephen and Sylvain to follow the developments.
> I understand the desire to get something in to Nova to support vGPUs, 
> and I understand that the existing /pci modules represent the 
> fastest/cheapest way to get there.
> I won't block you from making any of the above changes, Sahid. I'll even 
> do my best to review them. However, I will be primarily focusing this 
> cycle on getting the nested resource providers work feature-complete for 
> (at least) SR-IOV PF/VF devices.
> The decision of whether to allow an approach that adds more to the 
> existing /pci module is ultimately Matt's.
> Best,
> -jay
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Nested resource providers is not merged or production ready because we 
haven't made it a priority. We've certainly talked about it and Jay has 
had patches proposed for several releases now though.

Building vGPU support into the existing framework, which only a couple 
of people understand - certainly not me, might be a short-term gain but 
is just more technical debt we have to pay off later, and delays any 
focus on nested resource providers for the wider team.

At the Queens PTG it was abundantly clear that many features are 
dependent on nested resource providers, including several 
networking-related features like bandwidth-based scheduling.

The priorities for placement/scheduler in Queens are:

1. Dan Smith's migration allocations cleanup.
2. Alternative hosts for reschedules with cells v2.
3. Nested resource providers.

All of these are in progress and need review.

I personally don't think we should abandon the plan to implement vGPU 
support with nested resource providers without first seeing any code 
changes for it as a proof of concept. It also sounds like we have a 
pretty simple staggered plan for rolling out vGPU support so it's not 
very detailed to start. The virt driver reports vGPU inventory and we 
decorate the details later with traits (which Alex Xu is working on and 
needs review).

Sahid, you could certainly implement a separate proof of concept and 
make that available if the nested resource providers-based change hits 
major issues or goes far too long and has too much risk, then we have a 
contingency plan at least. But I don't expect that to get review 
priority and you'd have to accept that it might not get merged since we 
want to use nested resource providers.

Either way we are going to need solid functional testing and that 
functional testing should be written against the API as much as possible 
so that it works regardless of the backend implementation of the 
feature. One of the big things we failed at in Pike was not doing enough 
functional testing of move operations with claims in the scheduler 
earlier in the cycle. That all came in late and we're still fixing bugs 
as a result.

If we can get started early on the functional testing for vGPUs, then 
work both implementations in parallel, we should be able to retain the 
functional tests and determine which implementation we ultimately need 
to go with probably sometime in the second milestone.




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