Merge github.com:grpc/grpc into thread_pool

6 files changed, 323 insertions, 368 deletions

diff --git a/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.c b/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.c
index b4d1565c45..16035208e3 100644
--- a/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.c
+++ b/src/core/ext/filters/client_channel/lb_policy/round_robin/round_robin.c
@@ -99,26 +99,13 @@ typedef struct pending_pick {
   grpc_closure *on_complete;
 } pending_pick;
 
-/** List of subchannels in a connectivity READY state */
-typedef struct ready_list {
-  grpc_subchannel *subchannel;
-  /* references namesake entry in subchannel_data */
-  void *user_data;
-  struct ready_list *next;
-  struct ready_list *prev;
-} ready_list;
-
 typedef struct {
-  /** index within policy->subchannels */
-  size_t index;
   /** backpointer to owning policy */
   round_robin_lb_policy *policy;
   /** subchannel itself */
   grpc_subchannel *subchannel;
   /** notification that connectivity has changed on subchannel */
   grpc_closure connectivity_changed_closure;
-  /** this subchannels current position in subchannel->ready_list */
-  ready_list *ready_list_node;
   /** last observed connectivity. Not updated by
    * \a grpc_subchannel_notify_on_state_change. Used to determine the previous
    * state while processing the new state in \a rr_connectivity_changed */
@@ -126,6 +113,10 @@ typedef struct {
   /** current connectivity state. Updated by \a
    * grpc_subchannel_notify_on_state_change */
   grpc_connectivity_state curr_connectivity_state;
+  /** connectivity state to be updated by the watcher, not guarded by
+   * the combiner.  Will be moved to curr_connectivity_state inside of
+   * the combiner by rr_connectivity_changed_locked(). */
+  grpc_connectivity_state pending_connectivity_state_unsafe;
   /** the subchannel's target user data */
   void *user_data;
   /** vtable to operate over \a user_data */
@@ -141,182 +132,106 @@ struct round_robin_lb_policy {
 
   /** all our subchannels */
   size_t num_subchannels;
-  subchannel_data **subchannels;
+  subchannel_data *subchannels;
 
-  /** how many subchannels are in TRANSIENT_FAILURE */
+  /** how many subchannels are in state READY */
+  size_t num_ready;
+  /** how many subchannels are in state TRANSIENT_FAILURE */
   size_t num_transient_failures;
-  /** how many subchannels are IDLE */
+  /** how many subchannels are in state IDLE */
   size_t num_idle;
 
   /** have we started picking? */
-  int started_picking;
+  bool started_picking;
   /** are we shutting down? */
-  int shutdown;
+  bool shutdown;
   /** List of picks that are waiting on connectivity */
   pending_pick *pending_picks;
 
   /** our connectivity state tracker */
   grpc_connectivity_state_tracker state_tracker;
 
-  /** (Dummy) root of the doubly linked list containing READY subchannels */
-  ready_list ready_list;
-  /** Last pick from the ready list. */
-  ready_list *ready_list_last_pick;
+  // Index into subchannels for last pick.
+  size_t last_ready_subchannel_index;
 };
 
-/** Returns the next subchannel from the connected list or NULL if the list is
- * empty.
+/** Returns the index into p->subchannels of the next subchannel in
+ * READY state, or p->num_subchannels if no subchannel is READY.
  *
- * Note that this function does *not* advance p->ready_list_last_pick. Use \a
- * advance_last_picked_locked() for that. */
-static ready_list *peek_next_connected_locked(const round_robin_lb_policy *p) {
-  ready_list *selected;
-  selected = p->ready_list_last_pick->next;
-
-  while (selected != NULL) {
-    if (selected == &p->ready_list) {
-      GPR_ASSERT(selected->subchannel == NULL);
-      /* skip dummy root */
-      selected = selected->next;
-    } else {
-      GPR_ASSERT(selected->subchannel != NULL);
-      return selected;
-    }
+ * Note that this function does *not* update p->last_ready_subchannel_index.
+ * The caller must do that if it returns a pick. */
+static size_t get_next_ready_subchannel_index_locked(
+    const round_robin_lb_policy *p) {
+  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+    gpr_log(GPR_INFO,
+            "[RR: %p] getting next ready subchannel, "
+            "last_ready_subchannel_index=%lu",
+            p, (unsigned long)p->last_ready_subchannel_index);
   }
-  return NULL;
-}
-
-/** Advance the \a ready_list picking head. */
-static void advance_last_picked_locked(round_robin_lb_policy *p) {
-  if (p->ready_list_last_pick->next != NULL) { /* non-empty list */
-    p->ready_list_last_pick = p->ready_list_last_pick->next;
-    if (p->ready_list_last_pick == &p->ready_list) {
-      /* skip dummy root */
-      p->ready_list_last_pick = p->ready_list_last_pick->next;
+  for (size_t i = 0; i < p->num_subchannels; ++i) {
+    const size_t index =
+        (i + p->last_ready_subchannel_index + 1) % p->num_subchannels;
+    if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+      gpr_log(GPR_DEBUG, "[RR %p] checking index %lu: state=%d", p,
+              (unsigned long)index,
+              p->subchannels[index].curr_connectivity_state);
+    }
+    if (p->subchannels[index].curr_connectivity_state == GRPC_CHANNEL_READY) {
+      if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+        gpr_log(GPR_DEBUG, "[RR %p] found next ready subchannel at index %lu",
+                p, (unsigned long)index);
+      }
+      return index;
     }
-  } else { /* should be an empty list */
-    GPR_ASSERT(p->ready_list_last_pick == &p->ready_list);
   }
-
   if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
-    gpr_log(GPR_DEBUG,
-            "[READYLIST, RR: %p] ADVANCED LAST PICK. NOW AT NODE %p (SC %p, "
-            "CSC %p)",
-            (void *)p, (void *)p->ready_list_last_pick,
-            (void *)p->ready_list_last_pick->subchannel,
-            (void *)grpc_subchannel_get_connected_subchannel(
-                p->ready_list_last_pick->subchannel));
+    gpr_log(GPR_DEBUG, "[RR %p] no subchannels in ready state", p);
   }
+  return p->num_subchannels;
 }
 
-/** Prepends (relative to the root at p->ready_list) the connected subchannel \a
- * csc to the list of ready subchannels. */
-static ready_list *add_connected_sc_locked(round_robin_lb_policy *p,
-                                           subchannel_data *sd) {
-  ready_list *new_elem = gpr_zalloc(sizeof(ready_list));
-  new_elem->subchannel = sd->subchannel;
-  new_elem->user_data = sd->user_data;
-  if (p->ready_list.prev == NULL) {
-    /* first element */
-    new_elem->next = &p->ready_list;
-    new_elem->prev = &p->ready_list;
-    p->ready_list.next = new_elem;
-    p->ready_list.prev = new_elem;
-  } else {
-    new_elem->next = &p->ready_list;
-    new_elem->prev = p->ready_list.prev;
-    p->ready_list.prev->next = new_elem;
-    p->ready_list.prev = new_elem;
-  }
+// Sets p->last_ready_subchannel_index to last_ready_index.
+static void update_last_ready_subchannel_index_locked(round_robin_lb_policy *p,
+                                                      size_t last_ready_index) {
+  GPR_ASSERT(last_ready_index < p->num_subchannels);
+  p->last_ready_subchannel_index = last_ready_index;
   if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
-    gpr_log(GPR_DEBUG, "[READYLIST] ADDING NODE %p (Conn. SC %p)",
-            (void *)new_elem, (void *)sd->subchannel);
-  }
-  return new_elem;
-}
-
-/** Removes \a node from the list of connected subchannels */
-static void remove_disconnected_sc_locked(round_robin_lb_policy *p,
-                                          ready_list *node) {
-  if (node == NULL) {
-    return;
-  }
-  if (node == p->ready_list_last_pick) {
-    p->ready_list_last_pick = p->ready_list_last_pick->prev;
-  }
-
-  /* removing last item */
-  if (node->next == &p->ready_list && node->prev == &p->ready_list) {
-    GPR_ASSERT(p->ready_list.next == node);
-    GPR_ASSERT(p->ready_list.prev == node);
-    p->ready_list.next = NULL;
-    p->ready_list.prev = NULL;
-  } else {
-    node->prev->next = node->next;
-    node->next->prev = node->prev;
-  }
-
-  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
-    gpr_log(GPR_DEBUG, "[READYLIST] REMOVED NODE %p (SC %p)", (void *)node,
-            (void *)node->subchannel);
+    gpr_log(GPR_DEBUG,
+            "[RR: %p] setting last_ready_subchannel_index=%lu (SC %p, CSC %p)",
+            (void *)p, (unsigned long)last_ready_index,
+            (void *)p->subchannels[last_ready_index].subchannel,
+            (void *)grpc_subchannel_get_connected_subchannel(
+                p->subchannels[last_ready_index].subchannel));
   }
-
-  node->next = NULL;
-  node->prev = NULL;
-  node->subchannel = NULL;
-
-  gpr_free(node);
-}
-
-static bool is_ready_list_empty(round_robin_lb_policy *p) {
-  return p->ready_list.prev == NULL;
 }
 
 static void rr_destroy(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  ready_list *elem;
-
   if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
     gpr_log(GPR_DEBUG, "Destroying Round Robin policy at %p", (void *)pol);
   }
-
   for (size_t i = 0; i < p->num_subchannels; i++) {
-    subchannel_data *sd = p->subchannels[i];
-    GRPC_SUBCHANNEL_UNREF(exec_ctx, sd->subchannel, "rr_destroy");
-    if (sd->user_data != NULL) {
-      GPR_ASSERT(sd->user_data_vtable != NULL);
-      sd->user_data_vtable->destroy(exec_ctx, sd->user_data);
+    subchannel_data *sd = &p->subchannels[i];
+    if (sd->subchannel != NULL) {
+      GRPC_SUBCHANNEL_UNREF(exec_ctx, sd->subchannel, "rr_destroy");
+      if (sd->user_data != NULL) {
+        GPR_ASSERT(sd->user_data_vtable != NULL);
+        sd->user_data_vtable->destroy(exec_ctx, sd->user_data);
+      }
     }
-    gpr_free(sd);
   }
-
   grpc_connectivity_state_destroy(exec_ctx, &p->state_tracker);
   gpr_free(p->subchannels);
-
-  elem = p->ready_list.next;
-  while (elem != NULL && elem != &p->ready_list) {
-    ready_list *tmp;
-    tmp = elem->next;
-    elem->next = NULL;
-    elem->prev = NULL;
-    elem->subchannel = NULL;
-    gpr_free(elem);
-    elem = tmp;
-  }
-
   gpr_free(p);
 }
 
 static void rr_shutdown_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  pending_pick *pp;
-  size_t i;
-
   if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
     gpr_log(GPR_DEBUG, "Shutting down Round Robin policy at %p", (void *)pol);
   }
-
-  p->shutdown = 1;
+  p->shutdown = true;
+  pending_pick *pp;
   while ((pp = p->pending_picks)) {
     p->pending_picks = pp->next;
     *pp->target = NULL;
@@ -328,10 +243,13 @@ static void rr_shutdown_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol) {
   grpc_connectivity_state_set(
       exec_ctx, &p->state_tracker, GRPC_CHANNEL_SHUTDOWN,
       GRPC_ERROR_CREATE_FROM_STATIC_STRING("Channel Shutdown"), "rr_shutdown");
-  for (i = 0; i < p->num_subchannels; i++) {
-    subchannel_data *sd = p->subchannels[i];
-    grpc_subchannel_notify_on_state_change(exec_ctx, sd->subchannel, NULL, NULL,
-                                           &sd->connectivity_changed_closure);
+  for (size_t i = 0; i < p->num_subchannels; i++) {
+    subchannel_data *sd = &p->subchannels[i];
+    if (sd->subchannel != NULL) {
+      grpc_subchannel_notify_on_state_change(exec_ctx, sd->subchannel, NULL,
+                                             NULL,
+                                             &sd->connectivity_changed_closure);
+    }
   }
 }
 
@@ -339,8 +257,7 @@ static void rr_cancel_pick_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
                                   grpc_connected_subchannel **target,
                                   grpc_error *error) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  pending_pick *pp;
-  pp = p->pending_picks;
+  pending_pick *pp = p->pending_picks;
   p->pending_picks = NULL;
   while (pp != NULL) {
     pending_pick *next = pp->next;
@@ -364,8 +281,7 @@ static void rr_cancel_picks_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
                                    uint32_t initial_metadata_flags_eq,
                                    grpc_error *error) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  pending_pick *pp;
-  pp = p->pending_picks;
+  pending_pick *pp = p->pending_picks;
   p->pending_picks = NULL;
   while (pp != NULL) {
     pending_pick *next = pp->next;
@@ -387,21 +303,16 @@ static void rr_cancel_picks_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
 
 static void start_picking_locked(grpc_exec_ctx *exec_ctx,
                                  round_robin_lb_policy *p) {
-  size_t i;
-  p->started_picking = 1;
-
-  for (i = 0; i < p->num_subchannels; i++) {
-    subchannel_data *sd = p->subchannels[i];
-    /* use some sentinel value outside of the range of grpc_connectivity_state
-     * to signal an undefined previous state. We won't be referring to this
-     * value again and it'll be overwritten after the first call to
-     * rr_connectivity_changed */
-    sd->prev_connectivity_state = GRPC_CHANNEL_INIT;
-    sd->curr_connectivity_state = GRPC_CHANNEL_IDLE;
-    GRPC_LB_POLICY_WEAK_REF(&p->base, "rr_connectivity");
-    grpc_subchannel_notify_on_state_change(
-        exec_ctx, sd->subchannel, p->base.interested_parties,
-        &sd->curr_connectivity_state, &sd->connectivity_changed_closure);
+  p->started_picking = true;
+  for (size_t i = 0; i < p->num_subchannels; i++) {
+    subchannel_data *sd = &p->subchannels[i];
+    if (sd->subchannel != NULL) {
+      GRPC_LB_POLICY_WEAK_REF(&p->base, "rr_connectivity");
+      grpc_subchannel_notify_on_state_change(
+          exec_ctx, sd->subchannel, p->base.interested_parties,
+          &sd->pending_connectivity_state_unsafe,
+          &sd->connectivity_changed_closure);
+    }
   }
 }
 
@@ -418,36 +329,32 @@ static int rr_pick_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
                           grpc_call_context_element *context, void **user_data,
                           grpc_closure *on_complete) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  pending_pick *pp;
-  ready_list *selected;
-
   if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
     gpr_log(GPR_INFO, "Round Robin %p trying to pick", (void *)pol);
   }
-
-  if ((selected = peek_next_connected_locked(p))) {
+  const size_t next_ready_index = get_next_ready_subchannel_index_locked(p);
+  if (next_ready_index < p->num_subchannels) {
     /* readily available, report right away */
+    subchannel_data *sd = &p->subchannels[next_ready_index];
     *target = GRPC_CONNECTED_SUBCHANNEL_REF(
-        grpc_subchannel_get_connected_subchannel(selected->subchannel),
-        "rr_picked");
-
+        grpc_subchannel_get_connected_subchannel(sd->subchannel), "rr_picked");
     if (user_data != NULL) {
-      *user_data = selected->user_data;
+      *user_data = sd->user_data;
     }
     if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
       gpr_log(GPR_DEBUG,
-              "[RR PICK] TARGET <-- CONNECTED SUBCHANNEL %p (NODE %p)",
-              (void *)*target, (void *)selected);
+              "[RR PICK] TARGET <-- CONNECTED SUBCHANNEL %p (INDEX %lu)",
+              (void *)*target, (unsigned long)next_ready_index);
     }
     /* only advance the last picked pointer if the selection was used */
-    advance_last_picked_locked(p);
+    update_last_ready_subchannel_index_locked(p, next_ready_index);
     return 1;
   } else {
     /* no pick currently available. Save for later in list of pending picks */
     if (!p->started_picking) {
       start_picking_locked(exec_ctx, p);
     }
-    pp = gpr_malloc(sizeof(*pp));
+    pending_pick *pp = gpr_malloc(sizeof(*pp));
     pp->next = p->pending_picks;
     pp->target = target;
     pp->on_complete = on_complete;
@@ -458,25 +365,31 @@ static int rr_pick_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
   }
 }
 
-static void update_state_counters(subchannel_data *sd) {
+static void update_state_counters_locked(subchannel_data *sd) {
   round_robin_lb_policy *p = sd->policy;
-
-  /* update p->num_transient_failures (resp. p->num_idle): if the previous
-   * state was TRANSIENT_FAILURE (resp. IDLE), decrement
-   * p->num_transient_failures (resp. p->num_idle). */
-  if (sd->prev_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
+  if (sd->prev_connectivity_state == GRPC_CHANNEL_READY) {
+    GPR_ASSERT(p->num_ready > 0);
+    --p->num_ready;
+  } else if (sd->prev_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
     GPR_ASSERT(p->num_transient_failures > 0);
     --p->num_transient_failures;
   } else if (sd->prev_connectivity_state == GRPC_CHANNEL_IDLE) {
     GPR_ASSERT(p->num_idle > 0);
     --p->num_idle;
   }
+  if (sd->curr_connectivity_state == GRPC_CHANNEL_READY) {
+    ++p->num_ready;
+  } else if (sd->curr_connectivity_state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
+    ++p->num_transient_failures;
+  } else if (sd->curr_connectivity_state == GRPC_CHANNEL_IDLE) {
+    ++p->num_idle;
+  }
 }
 
 /* sd is the subchannel_data associted with the updated subchannel.
  * shutdown_error will only be used upon policy transition to TRANSIENT_FAILURE
  * or SHUTDOWN */
-static grpc_connectivity_state update_lb_connectivity_status(
+static grpc_connectivity_state update_lb_connectivity_status_locked(
     grpc_exec_ctx *exec_ctx, subchannel_data *sd, grpc_error *error) {
   /* In priority order. The first rule to match terminates the search (ie, if we
    * are on rule n, all previous rules were unfulfilled).
@@ -498,7 +411,7 @@ static grpc_connectivity_state update_lb_connectivity_status(
    *    CHECK: p->num_idle == p->num_subchannels.
    */
   round_robin_lb_policy *p = sd->policy;
-  if (!is_ready_list_empty(p)) { /* 1) READY */
+  if (p->num_ready > 0) { /* 1) READY */
     grpc_connectivity_state_set(exec_ctx, &p->state_tracker, GRPC_CHANNEL_READY,
                                 GRPC_ERROR_NONE, "rr_ready");
     return GRPC_CHANNEL_READY;
@@ -532,32 +445,62 @@ static void rr_connectivity_changed_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                            grpc_error *error) {
   subchannel_data *sd = arg;
   round_robin_lb_policy *p = sd->policy;
-  pending_pick *pp;
-
-  GRPC_ERROR_REF(error);
-
+  // Now that we're inside the combiner, copy the pending connectivity
+  // state (which was set by the connectivity state watcher) to
+  // curr_connectivity_state, which is what we use inside of the combiner.
+  sd->curr_connectivity_state = sd->pending_connectivity_state_unsafe;
+  if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
+    gpr_log(GPR_DEBUG,
+            "[RR %p] connectivity changed for subchannel %p: "
+            "prev_state=%d new_state=%d",
+            p, sd->subchannel, sd->prev_connectivity_state,
+            sd->curr_connectivity_state);
+  }
+  // If we're shutting down, unref and return.
   if (p->shutdown) {
     GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, &p->base, "rr_connectivity");
-    GRPC_ERROR_UNREF(error);
     return;
   }
-  switch (sd->curr_connectivity_state) {
-    case GRPC_CHANNEL_INIT:
-      GPR_UNREACHABLE_CODE(return );
-    case GRPC_CHANNEL_READY:
-      /* add the newly connected subchannel to the list of connected ones.
-       * Note that it goes to the "end of the line". */
-      sd->ready_list_node = add_connected_sc_locked(p, sd);
+  // Update state counters and determine new overall state.
+  update_state_counters_locked(sd);
+  sd->prev_connectivity_state = sd->curr_connectivity_state;
+  grpc_connectivity_state new_connectivity_state =
+      update_lb_connectivity_status_locked(exec_ctx, sd, GRPC_ERROR_REF(error));
+  // If the new state is SHUTDOWN, unref the subchannel, and if the new
+  // overall state is SHUTDOWN, clean up.
+  if (sd->curr_connectivity_state == GRPC_CHANNEL_SHUTDOWN) {
+    GRPC_SUBCHANNEL_UNREF(exec_ctx, sd->subchannel, "rr_subchannel_shutdown");
+    sd->subchannel = NULL;
+    if (sd->user_data != NULL) {
+      GPR_ASSERT(sd->user_data_vtable != NULL);
+      sd->user_data_vtable->destroy(exec_ctx, sd->user_data);
+    }
+    if (new_connectivity_state == GRPC_CHANNEL_SHUTDOWN) {
+      /* the policy is shutting down. Flush all the pending picks... */
+      pending_pick *pp;
+      while ((pp = p->pending_picks)) {
+        p->pending_picks = pp->next;
+        *pp->target = NULL;
+        grpc_closure_sched(exec_ctx, pp->on_complete, GRPC_ERROR_NONE);
+        gpr_free(pp);
+      }
+    }
+    /* unref the "rr_connectivity" weak ref from start_picking */
+    GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, &p->base, "rr_connectivity");
+  } else {
+    if (sd->curr_connectivity_state == GRPC_CHANNEL_READY) {
       /* at this point we know there's at least one suitable subchannel. Go
        * ahead and pick one and notify the pending suitors in
        * p->pending_picks. This preemtively replicates rr_pick()'s actions. */
-      ready_list *selected = peek_next_connected_locked(p);
-      GPR_ASSERT(selected != NULL);
+      const size_t next_ready_index = get_next_ready_subchannel_index_locked(p);
+      GPR_ASSERT(next_ready_index < p->num_subchannels);
+      subchannel_data *selected = &p->subchannels[next_ready_index];
       if (p->pending_picks != NULL) {
         /* if the selected subchannel is going to be used for the pending
          * picks, update the last picked pointer */
-        advance_last_picked_locked(p);
+        update_last_ready_subchannel_index_locked(p, next_ready_index);
       }
+      pending_pick *pp;
       while ((pp = p->pending_picks)) {
         p->pending_picks = pp->next;
         *pp->target = GRPC_CONNECTED_SUBCHANNEL_REF(
@@ -568,72 +511,20 @@ static void rr_connectivity_changed_locked(grpc_exec_ctx *exec_ctx, void *arg,
         }
         if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
           gpr_log(GPR_DEBUG,
-                  "[RR CONN CHANGED] TARGET <-- SUBCHANNEL %p (NODE %p)",
-                  (void *)selected->subchannel, (void *)selected);
+                  "[RR CONN CHANGED] TARGET <-- SUBCHANNEL %p (INDEX %lu)",
+                  (void *)selected->subchannel,
+                  (unsigned long)next_ready_index);
         }
         grpc_closure_sched(exec_ctx, pp->on_complete, GRPC_ERROR_NONE);
         gpr_free(pp);
       }
-      update_lb_connectivity_status(exec_ctx, sd, error);
-      sd->prev_connectivity_state = sd->curr_connectivity_state;
-      /* renew notification: reuses the "rr_connectivity" weak ref */
-      grpc_subchannel_notify_on_state_change(
-          exec_ctx, sd->subchannel, p->base.interested_parties,
-          &sd->curr_connectivity_state, &sd->connectivity_changed_closure);
-      break;
-    case GRPC_CHANNEL_IDLE:
-      ++p->num_idle;
-    /* fallthrough */
-    case GRPC_CHANNEL_CONNECTING:
-      update_state_counters(sd);
-      update_lb_connectivity_status(exec_ctx, sd, error);
-      sd->prev_connectivity_state = sd->curr_connectivity_state;
-      /* renew notification: reuses the "rr_connectivity" weak ref */
-      grpc_subchannel_notify_on_state_change(
-          exec_ctx, sd->subchannel, p->base.interested_parties,
-          &sd->curr_connectivity_state, &sd->connectivity_changed_closure);
-      break;
-    case GRPC_CHANNEL_TRANSIENT_FAILURE:
-      ++p->num_transient_failures;
-      /* remove from ready list if still present */
-      if (sd->ready_list_node != NULL) {
-        remove_disconnected_sc_locked(p, sd->ready_list_node);
-        sd->ready_list_node = NULL;
-      }
-      update_lb_connectivity_status(exec_ctx, sd, error);
-      sd->prev_connectivity_state = sd->curr_connectivity_state;
-      /* renew notification: reuses the "rr_connectivity" weak ref */
-      grpc_subchannel_notify_on_state_change(
-          exec_ctx, sd->subchannel, p->base.interested_parties,
-          &sd->curr_connectivity_state, &sd->connectivity_changed_closure);
-      break;
-    case GRPC_CHANNEL_SHUTDOWN:
-      update_state_counters(sd);
-      if (sd->ready_list_node != NULL) {
-        remove_disconnected_sc_locked(p, sd->ready_list_node);
-        sd->ready_list_node = NULL;
-      }
-      --p->num_subchannels;
-      GPR_SWAP(subchannel_data *, p->subchannels[sd->index],
-               p->subchannels[p->num_subchannels]);
-      GRPC_SUBCHANNEL_UNREF(exec_ctx, sd->subchannel, "rr_subchannel_shutdown");
-      p->subchannels[sd->index]->index = sd->index;
-      if (update_lb_connectivity_status(exec_ctx, sd, error) ==
-          GRPC_CHANNEL_SHUTDOWN) {
-        /* the policy is shutting down. Flush all the pending picks... */
-        while ((pp = p->pending_picks)) {
-          p->pending_picks = pp->next;
-          *pp->target = NULL;
-          grpc_closure_sched(exec_ctx, pp->on_complete, GRPC_ERROR_NONE);
-          gpr_free(pp);
-        }
-      }
-      gpr_free(sd);
-      /* unref the "rr_connectivity" weak ref from start_picking */
-      GRPC_LB_POLICY_WEAK_UNREF(exec_ctx, &p->base, "rr_connectivity");
-      break;
+    }
+    /* renew notification: reuses the "rr_connectivity" weak ref */
+    grpc_subchannel_notify_on_state_change(
+        exec_ctx, sd->subchannel, p->base.interested_parties,
+        &sd->pending_connectivity_state_unsafe,
+        &sd->connectivity_changed_closure);
   }
-  GRPC_ERROR_UNREF(error);
 }
 
 static grpc_connectivity_state rr_check_connectivity_locked(
@@ -654,10 +545,10 @@ static void rr_notify_on_state_change_locked(grpc_exec_ctx *exec_ctx,
 static void rr_ping_one_locked(grpc_exec_ctx *exec_ctx, grpc_lb_policy *pol,
                                grpc_closure *closure) {
   round_robin_lb_policy *p = (round_robin_lb_policy *)pol;
-  ready_list *selected;
-  grpc_connected_subchannel *target;
-  if ((selected = peek_next_connected_locked(p))) {
-    target = GRPC_CONNECTED_SUBCHANNEL_REF(
+  const size_t next_ready_index = get_next_ready_subchannel_index_locked(p);
+  if (next_ready_index < p->num_subchannels) {
+    subchannel_data *selected = &p->subchannels[next_ready_index];
+    grpc_connected_subchannel *target = GRPC_CONNECTED_SUBCHANNEL_REF(
         grpc_subchannel_get_connected_subchannel(selected->subchannel),
         "rr_picked");
     grpc_connected_subchannel_ping(exec_ctx, target, closure);
@@ -708,7 +599,7 @@ static grpc_lb_policy *round_robin_create(grpc_exec_ctx *exec_ctx,
   p->subchannels = gpr_zalloc(sizeof(*p->subchannels) * num_addrs);
 
   grpc_subchannel_args sc_args;
-  size_t subchannel_idx = 0;
+  size_t subchannel_index = 0;
   for (size_t i = 0; i < addresses->num_addresses; i++) {
     /* Skip balancer addresses, since we only know how to handle backends. */
     if (addresses->addresses[i].is_balancer) continue;
@@ -727,42 +618,44 @@ static grpc_lb_policy *round_robin_create(grpc_exec_ctx *exec_ctx,
     if (GRPC_TRACER_ON(grpc_lb_round_robin_trace)) {
       char *address_uri =
           grpc_sockaddr_to_uri(&addresses->addresses[i].address);
-      gpr_log(GPR_DEBUG, "Created subchannel %p for address uri %s",
-              (void *)subchannel, address_uri);
+      gpr_log(GPR_DEBUG, "index %lu: Created subchannel %p for address uri %s",
+              (unsigned long)subchannel_index, (void *)subchannel, address_uri);
       gpr_free(address_uri);
     }
     grpc_channel_args_destroy(exec_ctx, new_args);
 
     if (subchannel != NULL) {
-      subchannel_data *sd = gpr_zalloc(sizeof(*sd));
-      p->subchannels[subchannel_idx] = sd;
+      subchannel_data *sd = &p->subchannels[subchannel_index];
       sd->policy = p;
-      sd->index = subchannel_idx;
       sd->subchannel = subchannel;
+      /* use some sentinel value outside of the range of grpc_connectivity_state
+       * to signal an undefined previous state. We won't be referring to this
+       * value again and it'll be overwritten after the first call to
+       * rr_connectivity_changed */
+      sd->prev_connectivity_state = GRPC_CHANNEL_INIT;
+      sd->curr_connectivity_state = GRPC_CHANNEL_IDLE;
       sd->user_data_vtable = addresses->user_data_vtable;
       if (sd->user_data_vtable != NULL) {
         sd->user_data =
             sd->user_data_vtable->copy(addresses->addresses[i].user_data);
       }
-      ++subchannel_idx;
       grpc_closure_init(&sd->connectivity_changed_closure,
                         rr_connectivity_changed_locked, sd,
                         grpc_combiner_scheduler(args->combiner));
+      ++subchannel_index;
     }
   }
-  if (subchannel_idx == 0) {
+  if (subchannel_index == 0) {
     /* couldn't create any subchannel. Bail out */
     gpr_free(p->subchannels);
     gpr_free(p);
     return NULL;
   }
-  p->num_subchannels = subchannel_idx;
+  p->num_subchannels = subchannel_index;
 
-  /* The (dummy node) root of the ready list */
-  p->ready_list.subchannel = NULL;
-  p->ready_list.prev = NULL;
-  p->ready_list.next = NULL;
-  p->ready_list_last_pick = &p->ready_list;
+  // Initialize the last pick index to the last subchannel, so that the
+  // first pick will start at the beginning of the list.
+  p->last_ready_subchannel_index = subchannel_index - 1;
 
   grpc_lb_policy_init(&p->base, &round_robin_lb_policy_vtable, args->combiner);
   grpc_connectivity_state_init(&p->state_tracker, GRPC_CHANNEL_IDLE,
diff --git a/src/core/ext/filters/client_channel/parse_address.c b/src/core/ext/filters/client_channel/parse_address.c
index edc6ce697d..18381eec55 100644
--- a/src/core/ext/filters/client_channel/parse_address.c
+++ b/src/core/ext/filters/client_channel/parse_address.c
@@ -57,11 +57,11 @@ bool grpc_parse_unix(const grpc_uri *uri,
   struct sockaddr_un *un = (struct sockaddr_un *)resolved_addr->addr;
   const size_t maxlen = sizeof(un->sun_path);
   const size_t path_len = strnlen(uri->path, maxlen);
-  if (path_len == maxlen) return 0;
+  if (path_len == maxlen) return false;
   un->sun_family = AF_UNIX;
   strcpy(un->sun_path, uri->path);
   resolved_addr->len = sizeof(*un);
-  return 1;
+  return true;
 }
 
 #else /* GRPC_HAVE_UNIX_SOCKET */
@@ -73,74 +73,65 @@ bool grpc_parse_unix(const grpc_uri *uri,
 
 #endif /* GRPC_HAVE_UNIX_SOCKET */
 
-bool grpc_parse_ipv4(const grpc_uri *uri,
-                     grpc_resolved_address *resolved_addr) {
-  if (strcmp("ipv4", uri->scheme) != 0) {
-    gpr_log(GPR_ERROR, "Expected 'ipv4' scheme, got '%s'", uri->scheme);
-    return false;
-  }
-  const char *host_port = uri->path;
+bool grpc_parse_ipv4_hostport(const char *hostport, grpc_resolved_address *addr,
+                              bool log_errors) {
+  bool success = false;
+  // Split host and port.
   char *host;
   char *port;
-  int port_num;
-  bool result = false;
-  struct sockaddr_in *in = (struct sockaddr_in *)resolved_addr->addr;
-
-  if (*host_port == '/') ++host_port;
-  if (!gpr_split_host_port(host_port, &host, &port)) {
-    return false;
-  }
-
-  memset(resolved_addr, 0, sizeof(grpc_resolved_address));
-  resolved_addr->len = sizeof(struct sockaddr_in);
+  if (!gpr_split_host_port(hostport, &host, &port)) return false;
+  // Parse IP address.
+  memset(addr, 0, sizeof(*addr));
+  addr->len = sizeof(struct sockaddr_in);
+  struct sockaddr_in *in = (struct sockaddr_in *)addr->addr;
   in->sin_family = AF_INET;
   if (inet_pton(AF_INET, host, &in->sin_addr) == 0) {
-    gpr_log(GPR_ERROR, "invalid ipv4 address: '%s'", host);
+    if (log_errors) gpr_log(GPR_ERROR, "invalid ipv4 address: '%s'", host);
     goto done;
   }
-
-  if (port != NULL) {
-    if (sscanf(port, "%d", &port_num) != 1 || port_num < 0 ||
-        port_num > 65535) {
-      gpr_log(GPR_ERROR, "invalid ipv4 port: '%s'", port);
-      goto done;
-    }
-    in->sin_port = htons((uint16_t)port_num);
-  } else {
-    gpr_log(GPR_ERROR, "no port given for ipv4 scheme");
+  // Parse port.
+  if (port == NULL) {
+    if (log_errors) gpr_log(GPR_ERROR, "no port given for ipv4 scheme");
     goto done;
   }
-
-  result = true;
+  int port_num;
+  if (sscanf(port, "%d", &port_num) != 1 || port_num < 0 || port_num > 65535) {
+    if (log_errors) gpr_log(GPR_ERROR, "invalid ipv4 port: '%s'", port);
+    goto done;
+  }
+  in->sin_port = htons((uint16_t)port_num);
+  success = true;
 done:
   gpr_free(host);
   gpr_free(port);
-  return result;
+  return success;
 }
 
-bool grpc_parse_ipv6(const grpc_uri *uri,
+bool grpc_parse_ipv4(const grpc_uri *uri,
                      grpc_resolved_address *resolved_addr) {
-  if (strcmp("ipv6", uri->scheme) != 0) {
-    gpr_log(GPR_ERROR, "Expected 'ipv6' scheme, got '%s'", uri->scheme);
+  if (strcmp("ipv4", uri->scheme) != 0) {
+    gpr_log(GPR_ERROR, "Expected 'ipv4' scheme, got '%s'", uri->scheme);
     return false;
   }
   const char *host_port = uri->path;
-  char *host;
-  char *port;
-  int port_num;
-  int result = 0;
-  struct sockaddr_in6 *in6 = (struct sockaddr_in6 *)resolved_addr->addr;
-
   if (*host_port == '/') ++host_port;
-  if (!gpr_split_host_port(host_port, &host, &port)) {
-    return 0;
-  }
+  return grpc_parse_ipv4_hostport(host_port, resolved_addr,
+                                  true /* log_errors */);
+}
 
-  memset(in6, 0, sizeof(*in6));
-  resolved_addr->len = sizeof(*in6);
+bool grpc_parse_ipv6_hostport(const char *hostport, grpc_resolved_address *addr,
+                              bool log_errors) {
+  bool success = false;
+  // Split host and port.
+  char *host;
+  char *port;
+  if (!gpr_split_host_port(hostport, &host, &port)) return false;
+  // Parse IP address.
+  memset(addr, 0, sizeof(*addr));
+  addr->len = sizeof(struct sockaddr_in6);
+  struct sockaddr_in6 *in6 = (struct sockaddr_in6 *)addr->addr;
   in6->sin6_family = AF_INET6;
-
-  /* Handle the RFC6874 syntax for IPv6 zone identifiers. */
+  // Handle the RFC6874 syntax for IPv6 zone identifiers.
   char *host_end = (char *)gpr_memrchr(host, '%', strlen(host));
   if (host_end != NULL) {
     GPR_ASSERT(host_end >= host);
@@ -159,7 +150,7 @@ bool grpc_parse_ipv6(const grpc_uri *uri,
       gpr_log(GPR_ERROR, "invalid ipv6 scope id: '%s'", host_end + 1);
       goto done;
     }
-    // Handle "sin6_scope_id" being type "u_long". See grpc issue ##10027.
+    // Handle "sin6_scope_id" being type "u_long". See grpc issue #10027.
     in6->sin6_scope_id = sin6_scope_id;
   } else {
     if (inet_pton(AF_INET6, host, &in6->sin6_addr) == 0) {
@@ -167,24 +158,34 @@ bool grpc_parse_ipv6(const grpc_uri *uri,
       goto done;
     }
   }
-
-  if (port != NULL) {
-    if (sscanf(port, "%d", &port_num) != 1 || port_num < 0 ||
-        port_num > 65535) {
-      gpr_log(GPR_ERROR, "invalid ipv6 port: '%s'", port);
-      goto done;
-    }
-    in6->sin6_port = htons((uint16_t)port_num);
-  } else {
-    gpr_log(GPR_ERROR, "no port given for ipv6 scheme");
+  // Parse port.
+  if (port == NULL) {
+    if (log_errors) gpr_log(GPR_ERROR, "no port given for ipv6 scheme");
     goto done;
   }
-
-  result = 1;
+  int port_num;
+  if (sscanf(port, "%d", &port_num) != 1 || port_num < 0 || port_num > 65535) {
+    if (log_errors) gpr_log(GPR_ERROR, "invalid ipv6 port: '%s'", port);
+    goto done;
+  }
+  in6->sin6_port = htons((uint16_t)port_num);
+  success = true;
 done:
   gpr_free(host);
   gpr_free(port);
-  return result;
+  return success;
+}
+
+bool grpc_parse_ipv6(const grpc_uri *uri,
+                     grpc_resolved_address *resolved_addr) {
+  if (strcmp("ipv6", uri->scheme) != 0) {
+    gpr_log(GPR_ERROR, "Expected 'ipv6' scheme, got '%s'", uri->scheme);
+    return false;
+  }
+  const char *host_port = uri->path;
+  if (*host_port == '/') ++host_port;
+  return grpc_parse_ipv6_hostport(host_port, resolved_addr,
+                                  true /* log_errors */);
 }
 
 bool grpc_parse_uri(const grpc_uri *uri, grpc_resolved_address *resolved_addr) {
diff --git a/src/core/ext/filters/client_channel/parse_address.h b/src/core/ext/filters/client_channel/parse_address.h
index fa7ea33a00..1a203a3b26 100644
--- a/src/core/ext/filters/client_channel/parse_address.h
+++ b/src/core/ext/filters/client_channel/parse_address.h
@@ -54,4 +54,10 @@ bool grpc_parse_ipv6(const grpc_uri *uri, grpc_resolved_address *resolved_addr);
 /** Populate \a resolved_addr from \a uri. Returns true upon success. */
 bool grpc_parse_uri(const grpc_uri *uri, grpc_resolved_address *resolved_addr);
 
+/** Parse bare IPv4 or IPv6 "IP:port" strings. */
+bool grpc_parse_ipv4_hostport(const char *hostport, grpc_resolved_address *addr,
+                              bool log_errors);
+bool grpc_parse_ipv6_hostport(const char *hostport, grpc_resolved_address *addr,
+                              bool log_errors);
+
 #endif /* GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_PARSE_ADDRESS_H */
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
index 656c3a5860..861b809b19 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.c
@@ -61,6 +61,8 @@
 typedef struct {
   /** base class: must be first */
   grpc_resolver base;
+  /** DNS server to use (if not system default) */
+  char *dns_server;
   /** name to resolve (usually the same as target_name) */
   char *name_to_resolve;
   /** default port to use */
@@ -172,6 +174,8 @@ static void dns_ares_on_resolved_locked(grpc_exec_ctx *exec_ctx, void *arg,
     grpc_resolved_addresses_destroy(r->addresses);
     grpc_lb_addresses_destroy(exec_ctx, addresses);
   } else {
+    const char *msg = grpc_error_string(error);
+    gpr_log(GPR_DEBUG, "dns resolution failed: %s", msg);
     gpr_timespec now = gpr_now(GPR_CLOCK_MONOTONIC);
     gpr_timespec next_try = gpr_backoff_step(&r->backoff_state, now);
     gpr_timespec timeout = gpr_time_sub(next_try, now);
@@ -221,9 +225,9 @@ static void dns_ares_start_resolving_locked(grpc_exec_ctx *exec_ctx,
   GPR_ASSERT(!r->resolving);
   r->resolving = true;
   r->addresses = NULL;
-  grpc_resolve_address(exec_ctx, r->name_to_resolve, r->default_port,
-                       r->interested_parties, &r->dns_ares_on_resolved_locked,
-                       &r->addresses);
+  grpc_dns_lookup_ares(exec_ctx, r->dns_server, r->name_to_resolve,
+                       r->default_port, r->interested_parties,
+                       &r->dns_ares_on_resolved_locked, &r->addresses);
 }
 
 static void dns_ares_maybe_finish_next_locked(grpc_exec_ctx *exec_ctx,
@@ -246,6 +250,7 @@ static void dns_ares_destroy(grpc_exec_ctx *exec_ctx, grpc_resolver *gr) {
     grpc_channel_args_destroy(exec_ctx, r->resolved_result);
   }
   grpc_pollset_set_destroy(exec_ctx, r->interested_parties);
+  gpr_free(r->dns_server);
   gpr_free(r->name_to_resolve);
   gpr_free(r->default_port);
   grpc_channel_args_destroy(exec_ctx, r->channel_args);
@@ -257,14 +262,13 @@ static grpc_resolver *dns_ares_create(grpc_exec_ctx *exec_ctx,
                                       const char *default_port) {
   // Get name from args.
   const char *path = args->uri->path;
-  if (0 != strcmp(args->uri->authority, "")) {
-    gpr_log(GPR_ERROR, "authority based dns uri's not supported");
-    return NULL;
-  }
   if (path[0] == '/') ++path;
   // Create resolver.
   ares_dns_resolver *r = gpr_zalloc(sizeof(ares_dns_resolver));
   grpc_resolver_init(&r->base, &dns_ares_resolver_vtable, args->combiner);
+  if (0 != strcmp(args->uri->authority, "")) {
+    r->dns_server = gpr_strdup(args->uri->authority);
+  }
   r->name_to_resolve = gpr_strdup(path);
   r->default_port = gpr_strdup(default_port);
   r->channel_args = grpc_channel_args_copy(args->args);
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
index 09c46a66e0..e0cfd8b629 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.c
@@ -48,7 +48,10 @@
 #include <grpc/support/string_util.h>
 #include <grpc/support/time.h>
 #include <grpc/support/useful.h>
+
+#include "src/core/ext/filters/client_channel/parse_address.h"
 #include "src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_ev_driver.h"
+#include "src/core/lib/iomgr/error.h"
 #include "src/core/lib/iomgr/executor.h"
 #include "src/core/lib/iomgr/iomgr_internal.h"
 #include "src/core/lib/iomgr/sockaddr_utils.h"
@@ -58,6 +61,8 @@ static gpr_once g_basic_init = GPR_ONCE_INIT;
 static gpr_mu g_init_mu;
 
 typedef struct grpc_ares_request {
+  /** indicates the DNS server to use, if specified */
+  struct ares_addr_port_node dns_server_addr;
   /** following members are set in grpc_resolve_address_ares_impl */
   /** host to resolve, parsed from the name to resolve */
   char *host;
@@ -192,11 +197,12 @@ static void on_done_cb(void *arg, int status, int timeouts,
   grpc_ares_request_unref(NULL, r);
 }
 
-void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx, const char *name,
-                                    const char *default_port,
-                                    grpc_pollset_set *interested_parties,
-                                    grpc_closure *on_done,
-                                    grpc_resolved_addresses **addrs) {
+void grpc_dns_lookup_ares(grpc_exec_ctx *exec_ctx, const char *dns_server,
+                          const char *name, const char *default_port,
+                          grpc_pollset_set *interested_parties,
+                          grpc_closure *on_done,
+                          grpc_resolved_addresses **addrs) {
+  grpc_error *error = GRPC_ERROR_NONE;
   /* TODO(zyc): Enable tracing after #9603 is checked in */
   /* if (grpc_dns_trace) {
       gpr_log(GPR_DEBUG, "resolve_address (blocking): name=%s, default_port=%s",
@@ -208,28 +214,23 @@ void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx, const char *name,
   char *port;
   gpr_split_host_port(name, &host, &port);
   if (host == NULL) {
-    grpc_error *err = grpc_error_set_str(
+    error = grpc_error_set_str(
         GRPC_ERROR_CREATE_FROM_STATIC_STRING("unparseable host:port"),
         GRPC_ERROR_STR_TARGET_ADDRESS, grpc_slice_from_copied_string(name));
-    grpc_closure_sched(exec_ctx, on_done, err);
     goto error_cleanup;
   } else if (port == NULL) {
     if (default_port == NULL) {
-      grpc_error *err = grpc_error_set_str(
+      error = grpc_error_set_str(
           GRPC_ERROR_CREATE_FROM_STATIC_STRING("no port in name"),
           GRPC_ERROR_STR_TARGET_ADDRESS, grpc_slice_from_copied_string(name));
-      grpc_closure_sched(exec_ctx, on_done, err);
       goto error_cleanup;
     }
     port = gpr_strdup(default_port);
   }
 
   grpc_ares_ev_driver *ev_driver;
-  grpc_error *err = grpc_ares_ev_driver_create(&ev_driver, interested_parties);
-  if (err != GRPC_ERROR_NONE) {
-    GRPC_LOG_IF_ERROR("grpc_ares_ev_driver_create() failed", err);
-    goto error_cleanup;
-  }
+  error = grpc_ares_ev_driver_create(&ev_driver, interested_parties);
+  if (error != GRPC_ERROR_NONE) goto error_cleanup;
 
   grpc_ares_request *r = gpr_malloc(sizeof(grpc_ares_request));
   gpr_mu_init(&r->mu);
@@ -242,6 +243,40 @@ void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx, const char *name,
   r->success = false;
   r->error = GRPC_ERROR_NONE;
   ares_channel *channel = grpc_ares_ev_driver_get_channel(r->ev_driver);
+
+  // If dns_server is specified, use it.
+  if (dns_server != NULL) {
+    gpr_log(GPR_INFO, "Using DNS server %s", dns_server);
+    grpc_resolved_address addr;
+    if (grpc_parse_ipv4_hostport(dns_server, &addr, false /* log_errors */)) {
+      r->dns_server_addr.family = AF_INET;
+      memcpy(&r->dns_server_addr.addr.addr4, addr.addr, addr.len);
+      r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
+      r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
+    } else if (grpc_parse_ipv6_hostport(dns_server, &addr,
+                                        false /* log_errors */)) {
+      r->dns_server_addr.family = AF_INET6;
+      memcpy(&r->dns_server_addr.addr.addr6, addr.addr, addr.len);
+      r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
+      r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
+    } else {
+      error = grpc_error_set_str(
+          GRPC_ERROR_CREATE_FROM_STATIC_STRING("cannot parse authority"),
+          GRPC_ERROR_STR_TARGET_ADDRESS, grpc_slice_from_copied_string(name));
+      goto error_cleanup;
+    }
+    int status = ares_set_servers_ports(*channel, &r->dns_server_addr);
+    if (status != ARES_SUCCESS) {
+      char *error_msg;
+      gpr_asprintf(&error_msg, "C-ares status is not ARES_SUCCESS: %s",
+                   ares_strerror(status));
+      error = GRPC_ERROR_CREATE_FROM_COPIED_STRING(error_msg);
+      gpr_free(error_msg);
+      goto error_cleanup;
+    }
+  }
+  // An extra reference is put here to avoid destroying the request in
+  // on_done_cb before calling grpc_ares_ev_driver_start.
   gpr_ref_init(&r->pending_queries, 2);
   if (grpc_ipv6_loopback_available()) {
     gpr_ref(&r->pending_queries);
@@ -254,10 +289,20 @@ void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx, const char *name,
   return;
 
 error_cleanup:
+  grpc_closure_sched(exec_ctx, on_done, error);
   gpr_free(host);
   gpr_free(port);
 }
 
+void grpc_resolve_address_ares_impl(grpc_exec_ctx *exec_ctx, const char *name,
+                                    const char *default_port,
+                                    grpc_pollset_set *interested_parties,
+                                    grpc_closure *on_done,
+                                    grpc_resolved_addresses **addrs) {
+  grpc_dns_lookup_ares(exec_ctx, NULL /* dns_server */, name, default_port,
+                       interested_parties, on_done, addrs);
+}
+
 void (*grpc_resolve_address_ares)(
     grpc_exec_ctx *exec_ctx, const char *name, const char *default_port,
     grpc_pollset_set *interested_parties, grpc_closure *on_done,
diff --git a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
index 3dd40ea268..84fd7fcbd6 100644
--- a/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
+++ b/src/core/ext/filters/client_channel/resolver/dns/c_ares/grpc_ares_wrapper.h
@@ -51,6 +51,12 @@ extern void (*grpc_resolve_address_ares)(grpc_exec_ctx *exec_ctx,
                                          grpc_closure *on_done,
                                          grpc_resolved_addresses **addresses);
 
+void grpc_dns_lookup_ares(grpc_exec_ctx *exec_ctx, const char *dns_server,
+                          const char *addr, const char *default_port,
+                          grpc_pollset_set *interested_parties,
+                          grpc_closure *on_done,
+                          grpc_resolved_addresses **addresses);
+
 /* Initialize gRPC ares wrapper. Must be called at least once before
    grpc_resolve_address_ares(). */
 grpc_error *grpc_ares_init(void);