[ovs-dev] [PATCH v3 3/3] ofp-actions: Add limit to learn action.

[Ben Pfaff]

On Fri, Mar 10, 2017 at 03:44:40PM -0800, Daniele Di Proietto wrote:
> This commit adds a new feature to the learn actions: the possibility to
> limit the number of learned flows.
> 
> To be compatible with users of the old learn action, a new structure is
> introduced as well as a new OpenFlow raw action number.
> 
> There's a small corner case when we have to delete the ukey.  This
> happens when:
> * The learned rule has expired (or has been deleted).
> * The ukey that learned the rule is still in the datapath.
> * No packets hit the datapath flow recently.
> In this case we cannot relearn the rule (because there are no new
> packets), and the actions might depend on the learn execution, so the
> only option is to delete the ukey.  I don't think this has big
> performance implications since it's done only for ukey with no traffic.
> 
> We could also slowpath it, but that will cause an action upcall and the
> correct datapath actions will be installed later by a revalidator.  If
> we delete the ukey, the next upcall will be a miss upcall and that will
> immediatedly install the correct datapath flow.
> 
> Signed-off-by: Daniele Di Proietto <diproiettod at vmware.com>

Thanks!

>From looking at the code I had questions about the difference between
'slow' and 'avoid_caching', but the commit message answered them.
Therefore I'm folding in the following explanation, and then I'll apply
this series to master:

--- a/ofproto/ofproto-dpif-xlate.h
+++ b/ofproto/ofproto-dpif-xlate.h
@@ -38,13 +38,28 @@ struct mcast_snooping;
 struct xlate_cache;
 
 struct xlate_out {
-    enum slow_path_reason slow; /* 0 if fast path may be used. */
-
-    bool avoid_caching; /* If true, the result of this translation shouldn't
-                           be cached. */
+    /* Caching exceptions:
+     *
+     *   - If 'slow' is nonzero, the translation needs to be slow-pathed for
+     *     one reason or another.  (The particular value is only important for
+     *     explaining to an administrator why the flow is slow-pathed.)  This
+     *     makes OVS install a datapath flow with a send-to-userspace action.
+     *     Only on revalidation will the flow be replaced, if appropriate, by
+     *     one that does something else with the traffic.
+     *
+     *   - If 'avoid_caching' is true, then OVS won't install a datapath flow
+     *     at all.  If the reason to avoid caching goes away, the next upcall
+     *     will immediately install a correct datapath flow.
+     *
+     *   - Otherwise a datapath flow can be installed in the usual way.
+     *
+     * If 'avoid_caching' is true then 'slow' doesn't matter.
+     */
+    enum slow_path_reason slow;
+    bool avoid_caching;
 
-    struct recirc_refs recircs; /* Recirc action IDs on which references are
-                                 * held. */
+    /* Recirc action IDs on which references are held. */
+    struct recirc_refs recircs;
 };
 
 struct xlate_in {