ovn-controller: Only rebuild if something changed.

Most of the work done in the ovn-controller main loop only needs to
happen if something has changed either in either the ovs or ovn-sb
database.  This patch makes most of the work get skipped if neither
database changed.

In a large scale environment, ovn-controller was using a large amouint
of CPU in what should be an idle state.  This was partially due to
ovn-controller recalculating the full state of the system every time it
woke up, due to some sort of keepalive (an ovsdb or openflow echo
request).

Reported-at: https://bugs.launchpad.net/networking-ovn/+bug/1536003
Signed-off-by: Russell Bryant <russell@ovn.org>

ovn/controller/ovn-controller.c

@@ -273,6 +273,8 @@ main(int argc, char *argv[])
     struct hmap local_datapaths = HMAP_INITIALIZER(&local_datapaths);
 
     /* Main loop. */
+    unsigned int ovs_seqno = 0;
+    unsigned int ovnsb_seqno = 0;
     exiting = false;
     while (!exiting) {
         struct controller_ctx ctx = {
@@ -281,32 +283,45 @@ main(int argc, char *argv[])
             .ovnsb_idl = ovnsb_idl_loop.idl,
             .ovnsb_idl_txn = ovsdb_idl_loop_run(&ovnsb_idl_loop),
         };
+        bool ovsdb_changed = false;
+
+        if (ovs_seqno != ovsdb_idl_get_seqno(ovs_idl_loop.idl)) {
+            ovs_seqno = ovsdb_idl_get_seqno(ovs_idl_loop.idl);
+            ovsdb_changed = true;
+        }
+        if (ovnsb_seqno != ovsdb_idl_get_seqno(ovnsb_idl_loop.idl)) {
+            ovnsb_seqno = ovsdb_idl_get_seqno(ovnsb_idl_loop.idl);
+            ovsdb_changed = true;
+        }
 
         const struct ovsrec_bridge *br_int = get_br_int(&ctx);
-        const char *chassis_id = get_chassis_id(ctx.ovs_idl);
 
-        if (chassis_id) {
-            chassis_run(&ctx, chassis_id);
-            encaps_run(&ctx, br_int, chassis_id);
-            binding_run(&ctx, br_int, chassis_id, &ct_zones, ct_zone_bitmap,
-                    &local_datapaths);
-        }
+        if (ovsdb_changed) {
+            const char *chassis_id = get_chassis_id(ctx.ovs_idl);
 
-        if (br_int) {
-            patch_run(&ctx, br_int, &local_datapaths);
+            if (chassis_id) {
+                chassis_run(&ctx, chassis_id);
+                encaps_run(&ctx, br_int, chassis_id);
+                binding_run(&ctx, br_int, chassis_id, &ct_zones, ct_zone_bitmap,
+                        &local_datapaths);
+            }
 
-            enum mf_field_id mff_ovn_geneve = ofctrl_run(br_int);
+            if (br_int) {
+                patch_run(&ctx, br_int, &local_datapaths);
 
-            pinctrl_run(&ctx, br_int);
+                enum mf_field_id mff_ovn_geneve = ofctrl_run(br_int);
 
-            struct hmap flow_table = HMAP_INITIALIZER(&flow_table);
-            lflow_run(&ctx, &flow_table, &ct_zones, &local_datapaths);
-            if (chassis_id) {
-                physical_run(&ctx, mff_ovn_geneve,
-                             br_int, chassis_id, &ct_zones, &flow_table);
+                pinctrl_run(&ctx, br_int);
+
+                struct hmap flow_table = HMAP_INITIALIZER(&flow_table);
+                lflow_run(&ctx, &flow_table, &ct_zones, &local_datapaths);
+                if (chassis_id) {
+                    physical_run(&ctx, mff_ovn_geneve,
+                                 br_int, chassis_id, &ct_zones, &flow_table);
+                }
+                ofctrl_put(&flow_table);
+                hmap_destroy(&flow_table);
             }
-            ofctrl_put(&flow_table);
-            hmap_destroy(&flow_table);
         }
 
         unixctl_server_run(unixctl);