[ovs-discuss] Source Code - Multiple Controllers Round Robin Load Balancing

[Brian Perry]

Summary: OvS last commit 52f793b
I am trying to modify the switch's code to use a round robin scheduler for
sending asynchronous messages (especially PACKET_IN messages) to one of the
controllers within a multiple controller SDN setup. After traversing the
code I think I found where I need to insert this round robin scheduler,
line 4766 and 6225 of ofproto-dpif-xlate.c.

I plan on modifying this part of the code and testing it to see if I was
correct but it would be great to have some feedback from the community on
this.
1) Am I on the right path, do I only need to modify the code around line
4766? Or do I need to modify another part of the code?
2) What is OFPACT_CONTROLLER case for? From what I can gather macro
OFPACT_FOR_EACH uses a macro that goes through a list that deal with table
entry actions but it could call the execute_controller_action() function
and I don't know why it would.
3) What does emit_continuation() function do? It looked like it had to do
with table lookups instead of sending asynchronous messages to the
controller. If it only does table lookups why would it call the
execute_controller_action() function.


Details:
I setup a simple Mininet topology with a single switch, 3 user computers,
and 2 controllers, where everything is connected to the switch. Currently
when the switch receives a packet with no corresponding forwarding rule it
sends a request to both controllers. I would like it so that it sends the
forwarding rule request in a round robin method. So the first forwarding
rule request will be sent to only controller 1, then the next forwarding
rule request will be sent to only controller 2, then the next one to only
controller 1, then the next one only to controller 2, etc.

Along side other documents I've read a description of the Open vSwitch
architecture (https://www.slideshare.net/rajdeep/openvswitch-deep-dive) and
pages 25-28 of Pavel's master thesis (https://mail.openvswitch.org/
pipermail/ovs-discuss/2017-February/043763.html) to get a better
understanding of the internals of OvS. This information paired with the
following post (https://mail.openvswitch.org/pipermail/ovs-discuss/2016-
March/040236.html) informed me that the source code I am looking for is
used within the Userspace and located within the ofproto directory. I
started looking in the ofproto directory for the handle_openflow()
function. This eventually lead me to look at the structure and usage of
ofconn which lead me to line 1741 of connmgr.c where I found the structure
ofproto_async_msg and function connmgr_send_async_msg. After following the
function I noticed that ofproto_async_msg.controller_id is assigned in only
2 different places; within the execute_controller_action() and
emit_continuation() functions. I continued following the
execute_controller_action() function and noticed that it's called in only 5
different locations all within the file ofproto-dpif-xlate.c. Within those
5 locations only lines 4766 and 6225 use some sort of loop to craft and
send asynchronous messages to multiple controllers.

So my questions become:
1) Am I on the right path, do I only need to modify the code around line
4766? Or do I need to modify another part of the code?
2) What is OFPACT_CONTROLLER case for? From what I can gather macro
OFPACT_FOR_EACH uses a macro that goes through a list that deal with table
entry actions but it could call the execute_controller_action() function
and I don't know why it would.
3) What does emit_continuation() function do? It looked like it had to do
with table lookups instead of sending asynchronous messages to the
controller. If it only does table lookups why would it call the
execute_controller_action() function.
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