winbind: Improve child selection

This improves the situation when a client request blocks a winbind
child. This might be a slow samlogon or lookupnames to a domain that's
far away. With random selection of the child for new request coming in
we could end up with a long queue when other, non-blocked children
could serve those new requests. Choose the shortest queue.

This is an immediate and simple fix. Step two will be to have a
per-domain and not a per-child queue. Right now we're pre-selecting
the check-out queue at Fry's randomly without looking at the queue
length. With this change we're picking the shortest queue. The better
change will be what Fry's really does: One central queue and red/green
lights on the busy/free checkout counters.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Feb 12 19:51:35 CET 2018 on sn-devel-144

diff --git a/source3/winbindd/winbindd_dual.c b/source3/winbindd/winbindd_dual.c
index 33f1393e08203a7d0fd374bd8104895f476aa00e..993166d82d91991646effe62e11a625b027eaefb 100644 (file)
--- a/source3/winbindd/winbindd_dual.c
+++ b/source3/winbindd/winbindd_dual.c
@@ -248,33 +248,31 @@ static void wb_child_request_cleanup(struct tevent_req *req,
        DLIST_REMOVE(winbindd_children, state->child);
 }
 
-static bool winbindd_child_busy(struct winbindd_child *child)
-{
-       return tevent_queue_length(child->queue) > 0;
-}
-
-static struct winbindd_child *find_idle_child(struct winbindd_domain *domain)
+struct winbindd_child *choose_domain_child(struct winbindd_domain *domain)
 {
+       struct winbindd_child *shortest = &domain->children[0];
+       struct winbindd_child *current;
        int i;
 
        for (i=0; i<lp_winbind_max_domain_connections(); i++) {
-               if (!winbindd_child_busy(&domain->children[i])) {
-                       return &domain->children[i];
-               }
-       }
+               size_t shortest_len, current_len;
 
-       return NULL;
-}
+               current = &domain->children[i];
+               current_len = tevent_queue_length(current->queue);
 
-struct winbindd_child *choose_domain_child(struct winbindd_domain *domain)
-{
-       struct winbindd_child *result;
+               if (current_len == 0) {
+                       /* idle child */
+                       return current;
+               }
 
-       result = find_idle_child(domain);
-       if (result != NULL) {
-               return result;
+               shortest_len = tevent_queue_length(shortest->queue);
+
+               if (current_len < shortest_len) {
+                       shortest = current;
+               }
        }
-       return &domain->children[rand() % lp_winbind_max_domain_connections()];
+
+       return shortest;
 }
 
 struct dcerpc_binding_handle *dom_child_handle(struct winbindd_domain *domain)