Comment 0 for bug 1882804

Problem
=======

Neutron and Nova support creating servers with port having QoS minimum bandwidth policy rule. Such server create request results in bandwidth resource allocation in placement and this way ensures that the server is placed to a compute host where enough bandwidth is available.

However the use case to change the minimum bandwidth guarantee of a bound port hasn't been supported yet.

The end user can update the bound port to point to a different QoS policy and Neutron will reflect such change in the resource_request of the port, but the bandwidth allocation in placement has not been updated.

Step to reproduce
=================

# Set up net, subnet, QoS policies with different min bw rules
# and a port with the first policy (qp0)
#
openstack network create net0 \
--provider-network-type vlan \
--provider-physical-network physnet0 \
--provider-segment 100 \
##
openstack subnet create subnet0 \
--network net0 \
--subnet-range 10.0.4.0/24 \
##
openstack network qos policy create qp0
openstack network qos rule create qp0 \
--type minimum-bandwidth \
--min-kbps 1000 \
--egress \
##
openstack network qos rule create qp0 \
--type minimum-bandwidth \
--min-kbps 1000 \
--ingress \
##
openstack port create port-normal-qos \
--network net0 \
--vnic-type normal \
--qos-policy qp0 \
##
openstack network qos policy create qp1
openstack network qos rule create qp1 \
--type minimum-bandwidth \
--min-kbps 2000 \
--egress \
##
openstack network qos rule create qp1 \
--type minimum-bandwidth \
--min-kbps 2000 \
--ingress \
##

# Create a nova server with the port and check the resource_request
# of the port and the resulting allocation in placement
#
openstack --os-compute-api-version 2.72 \
server create vm1 \
--flavor c1 \
--image cirros-0.4.0-x86_64-disk \
--nic port-id=port-normal-qos \
--wait \
##
openstack port show port-normal-qos \
-f table \
-c binding_profile -c resource_request -c status -c device_owner \
##
openstack --os-placement-api-version 1.30 \
resource provider usage show 1110cf59-cabf-526c-bacc-08baabbac692 \
##

# Change the QoS policy from qp0 to qp1 on the bound port
#
openstack port set port-normal-qos \
--qos-policy qp1 \
##

# The resource request of the port is updated according to qp1
#
openstack port show port-normal-qos \
-f table \
-c binding_profile -c resource_request -c status -c device_owner -c device_id \
##
# But the resource allocation in placement does not changed according to qp1
openstack --os-placement-api-version 1.30 \
resource provider usage show 1110cf59-cabf-526c-bacc-08baabbac692 \
##

Proposed Solution
=================

This use case was discussed on the Neutron-Nova cross project session of the Victoria PTG [1]. There the result was to try to implement the placement update on the Neutron side. This could be achieved by the following high level sequence

1) Neutron receives the PUT /ports/{port_id} API request for a bound port where the qos_policy_id is requested to be changed
2) Neutron calculates the difference in the resource_request of the port due to the requested policy change. If no there is no change then port update continues as today. If there is a change then
3) Neutron calls the Placement GET /allocations/{consumer_uuid} API[2] where the consumer_id is the device_id of the port. In the Placement response Neutron updates the requested resources under the "allocations".{rp_uuid} key where the rp_uuid is the value of the binding:profile["allocation"] key of the port. The resources subdictionary is updated by applying the resource request difference calculated at #2). The rest of the structure is left unchanged including the generation keys and the consumer_generation key.
4) Neutron calls the Placement PUT /allocations/{consumer_uuid} API[3] with the updated allocation structure.
4a) If Placement returns HTTP 200 then port update continues as today.
4b) If Placement returns HTTP 409 with error code "placement.concurrent_update" then Neutron needs to repeat step #3) and #4)
4c) If Placement returns HTTP 409 with other error code then there is not enough bandwidth resource available on the device and therefore the QoS policy change should be rejected.

Notes:
* use Placement API version 1.28 or higher as there the GET response and PUT request has a symmetric structure
* ports that are not bound can be skipped as they will not have resource allocation in placement
* if the Nova server is deleted during the QoS policy update then Neutron might query placement with a non-existing consumer_uuid at #3). Placement will return an empty allocation in this case and Neutron needs to fail during #3) when trying to modify the received allocation locally as Placement will blindly allow PUT-ing back a new allocation at #4 without any error resulting in a resource leak.

[1] https://etherpad.opendev.org/p/nova-victoria-ptg
[2] https://docs.openstack.org/api-ref/placement/?expanded=list-allocations-detail#list-allocations
[3] https://docs.openstack.org/api-ref/placement/?expanded=list-resource-provider-allocations-detail#list-resource-provider-allocations