Use AZLeastRoutersScheduler by default

Neutron uses an AZ-unaware scheduler (LeastRoutersScheduler) by default
in its configuration and the neutron-api charm does not override it.

AZLeastRoutersScheduler inherits from LeastRoutersScheduler and does the
same, plus respects AZ hints when scheduling HA routers.

For --distributed --ha routers using AZLeastRoutersScheduler means that
snat namespaces will be scheduled with respect to the AZ hints specified
during router creation by an operator.

For --ha but not distributed routers using AZLeastRoutersScheduler means
that qrouter namespaces will be scheduled with respect to the AZ hints.

snat namespaces (--ha & --distributed) and qrouter namespaces (--ha
only) are placed by the scheduler to l3 agents that run in the dvr_snat
mode only so the scheduler change will affect both the deployments with
neutron-gateway units and the ones with neutron-openvswitch running with
use-dvr-snat=True.

Change-Id: I98cd67ff0cf5418a9699acc7aff96c3edb9b2341
Closes-Bug: #1886195

config.yaml

@@ -376,6 +376,12 @@ options:
       namespace will be rescheduled only to L3 agents running in the 'dvr_snat'
       mode, while l3 agents in the 'dvr' mode will only host qrouter and fip
       namespaces.
+  router-scheduler-driver:
+    type: string
+    default: 'neutron.scheduler.l3_agent_scheduler.AZLeastRoutersScheduler'
+    description: |
+      The driver to use for router scheduling. In case of distributed routers
+      this will affect snat namespace scheduling.
   allow-automatic-dhcp-failover:
     type: boolean
     default: True

hooks/neutron_api_context.py

@@ -522,6 +522,9 @@ class NeutronCCContext(context.NeutronContext):
                 'neutron.scheduler.dhcp_agent_scheduler.AZAwareWeightScheduler'
             )
             ctxt['dhcp_load_type'] = config('dhcp-load-type')
+            # AZLeastRoutersScheduler inherits from LeastRoutersScheduler and
+            # is available as of mitaka.
+            ctxt['router_scheduler_driver'] = config('router-scheduler-driver')
 
         extension_drivers = []
         if config('enable-ml2-port-security'):

templates/mitaka/neutron.conf

@@ -32,6 +32,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/newton/neutron.conf

@@ -32,6 +32,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/ocata/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/pike/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/queens/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/rocky/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/train/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

templates/ussuri/neutron.conf

@@ -35,6 +35,9 @@ allow_automatic_dhcp_failover = {{ allow_automatic_dhcp_failover }}
 network_scheduler_driver = {{ network_scheduler_driver }}
 dhcp_load_type = {{ dhcp_load_type }}
 {% endif -%}
+{% if router_scheduler_driver -%}
+router_scheduler_driver = {{ router_scheduler_driver }}
+{% endif -%}
 
 {% if neutron_bind_port -%}
 bind_port = {{ neutron_bind_port }}

unit_tests/test_neutron_api_context.py

@@ -527,6 +527,9 @@ class NeutronCCContextTest(CharmTestCase):
                 'neutron.scheduler.dhcp_agent_scheduler'
                 '.AZAwareWeightScheduler'),
             'dhcp_load_type': 'networks',
+            'router_scheduler_driver': (
+                'neutron.scheduler.l3_agent_scheduler'
+                '.AZLeastRoutersScheduler'),
         }
         napi_ctxt = context.NeutronCCContext()
         self.maxDiff = None