[ovs-dev] [resubmit 4/4] ofproto: Resubmit Statistics.

Ben Pfaff blp at nicira.com
Tue Feb 15 20:49:40 UTC 2011


On Mon, Feb 14, 2011 at 02:06:01PM -0800, Ethan Jackson wrote:
> This patch causes statistics to be updated for rules which are
> resubmitted into. Once per second statistics are queried from the
> datapath and pushed along the resubmit graph (calculated on demand
> from the action list).  This approach is simple, easy to understand,
> and in most cases accurate.  However, when the resubmit graph
> changes, it is possible that some statistics will be accounted to
> the wrong rule for a short period of time.
> 
> Bug #3730.

I like this approach.  Thank you for writing it up.  I see a few
problems with the implementation, though.

The first problem is that the semantics for the in_port in
flow_push_stats() do not match those in xlate_table_action().  In
xlate_table_action(), the resubmitted in_port is used only for the
rule_lookup() and then restored.  In flow_push_stats(), the resubmitted
in_port is not restored until flow_push_stats() returns.  Maybe this
isn't a real problem, because OFPP_NORMAL and OFPP_IN_PORT do not have
any special semantics in flow_push_stats(), but I do not see a reason
for the difference either.  (You could write a helper function
rule_lookup_with_in_port() to encapsulate this lookup.)

The second problem is that flow_push_stats() ignores some actions that
will definitely make a difference in behavior, that is, any action that
modifies the flow.  Some examples are NXAST_SET_TUNNEL and
NXAST_REG_MOVE.  (It's easy enough to find these actions: just
temporarily mark the "flow" member of struct action_xlate_ctx "const"
and recompile and the compiler will point them out for you.)

I hope that we don't have to duplicate a lot of code to fix the latter
problem.




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