Dynamically determine overlay network mtu

Not all clouds will provide us with MTUs of 1500. Instead of assuming a
1500 - 50 byte MTU to accomodate for vxlan overhead we list all
interface MTUs, filter by those that appear to be "real" interfaces (to
avoid those we ourselves may have created), take the smallest one and
subtract it by 50 to accomodate for vxlan overhead. You can still set an
explicitl bridge_mtu value if necessary.

Change-Id: If899a1bee3b4b69df8c2905a219b41e119d8f652

roles/multi-node-bridge/README.rst

@@ -34,9 +34,12 @@ inventory in order to work:
    VXLAN Network Identifier offset (openvswitch key).
 
 .. zuul:rolevar:: bridge_mtu
-   :default: 1450
+   :default: Smallest mtu less 50 bytes for vxlan overhead
 
-   Bridge interface MTU.
+   Bridge interface MTU. By default we determine this value by checking
+   all interfaces on host, taking the smallest MTU and subtracting by
+   50 for vxlan overhead. Can be overridden explicitly if this does not
+   work.
 
 .. zuul:rolevar:: bridge_name
    :default: br-infra

roles/multi-node-bridge/defaults/main.yaml

@@ -1,5 +1,4 @@
 bridge_vni_offset: 1000000
-bridge_mtu: 1450
 bridge_name: br-infra
 
 bridge_authorize_internal_traffic: false

roles/multi-node-bridge/tasks/common.yaml

@@ -52,3 +52,37 @@
   when:
     - bridge_configure_address | bool
     - bridge_authorize_internal_traffic | bool
+
+- when: bridge_mtu is not defined
+  block:
+    - name: Determine bridge mtu
+      shell: |
+        # Find all interfaces with a permanent mac address type.
+        # Permanent mac addrs imply "real" hardware and not interfaces we have
+        # created through this system. This makes our MTU determination mostly
+        # idempotent allowing us to create multiple overlays without
+        # perpetually smaller MTUs.
+        SMALLEST_MTU=""
+        for X in $(ls /sys/class/net) ; do
+            MAC_TYPE=$(cat "/sys/class/net/${X}/addr_assign_type")
+            if [ "$MAC_TYPE" -ne "0" ] ; then
+                # Type 0 is a permanent address implying a "real"
+                # interface. We ignore other interfaces as that is what we
+                # create here
+                continue
+            fi
+            MTU=$(cat "/sys/class/net/${X}/mtu")
+            if [ -z "$SMALLEST_MTU" ] || [ "$SMALLEST_MTU" -gt "$MTU" ] ; then
+                SMALLEST_MTU=$MTU
+            fi
+        done
+        # 50 byte overhead for vxlan
+        echo $(( SMALLEST_MTU - 50 ))
+      args:
+        executable: /bin/bash
+      environment:
+        PATH: '{{ ansible_env.PATH }}:/bin:/sbin:/usr/sbin'
+      register: mtu_output
+    - name: Set bridge_mtu
+      set_fact:
+        bridge_mtu: "{{ mtu_output.stdout }}"

roles/multi-node-bridge/tasks/main.yaml

@@ -1,8 +1,10 @@
-- include: common.yaml
-
 # Note (dmsimard)
 # We explicitely declare a PATH environment variable because '/sbin' is not in
 # PATH when using 'become: yes' on some distributions
+- include: common.yaml
+  environment:
+    PATH: "{{ ansible_env.PATH }}:/sbin:/usr/sbin"
+
 - include: switch.yaml
   environment:
     PATH: "{{ ansible_env.PATH }}:/sbin:/usr/sbin"