Add new config option to set host_subset_size

A new config option called scheduler-host-subset-size was added
to configure FilterScheduler's host_subset_size property, which
size of the subset of best hosts selected by the scheduler.

Change-Id: Ic61917732d50806f74be0b4e7c87a51d738afbe7
Closes-bug: #1846203

config.yaml

@@ -591,3 +591,17 @@ options:
       override YAML files in the service's policy.d directory.  The resource
       file should be a ZIP file containing at least one yaml file with a .yaml
       or .yml extension.  If False then remove the overrides.
+  scheduler-host-subset-size:
+    type: int
+    default:
+    description: |
+      The value to be configured for the host_subset_size property on
+      FilterScheduler. This property sets the size of the subset of best hosts
+      selected by the scheduler.
+      .
+      When a new instance is created, it will be scheduled on a host chosen
+      randomly from a subset of the best hosts with the size set by this
+      property.
+      .
+      Possible Values are positive integers. Any value less than 1 will be
+      treated as 1.

hooks/nova_cc_context.py

@@ -387,6 +387,8 @@ class NovaConfigContext(ch_context.WorkerConfigContext):
         ctxt['console_access_protocol'] = hookenv.config(
             'console-access-protocol')
         ctxt['console_access_port'] = hookenv.config('console-access-port')
+        ctxt['scheduler_host_subset_size'] = hookenv.config(
+            'scheduler-host-subset-size')
         return ctxt
 
 

templates/ocata/nova.conf

@@ -193,6 +193,10 @@ enabled_filters = {{ scheduler_default_filters }},{{ additional_neutron_filters
 enabled_filters = {{ scheduler_default_filters }}
 {% endif %}
 
+{%- if scheduler_host_subset_size %}
+host_subset_size = {{ scheduler_host_subset_size }}
+{%- endif %}
+
 [api]
 auth_strategy=keystone
 {% if vendor_data or vendor_data_url -%}

templates/pike/nova.conf

@@ -208,6 +208,10 @@ enabled_filters = {{ scheduler_default_filters }}
 # https://bugs.launchpad.net/charm-nova-cloud-controller/+bug/1818239
 build_failure_weight_multiplier = 0.0
 
+{%- if scheduler_host_subset_size %}
+host_subset_size = {{ scheduler_host_subset_size }}
+{%- endif %}
+
 [api]
 auth_strategy=keystone
 {% if vendor_data or vendor_data_url -%}

templates/rocky/nova.conf

@@ -214,6 +214,10 @@ enabled_filters = {{ scheduler_default_filters }}
 # https://bugs.launchpad.net/charm-nova-cloud-controller/+bug/1818239
 build_failure_weight_multiplier = 0.0
 
+{%- if scheduler_host_subset_size %}
+host_subset_size = {{ scheduler_host_subset_size }}
+{%- endif %}
+
 [api]
 auth_strategy=keystone
 {% if vendor_data or vendor_data_url -%}