To reduce memory churn during client list transfer (#14068)

When we transfer clients between IO thread and main thread, the creation
and destruction of list nodes also consume some CPU. In this commit, we
reuse list nodes to avoid this issue.

src/iothread.c

@@ -51,17 +51,18 @@ void enqueuePendingClientsToMainThread(client *c, int unbind) {
     if (unbind) connUnbindEventLoop(c->conn);
     /* Just skip if it already is transferred. */
     if (c->io_thread_client_list_node) {
+        IOThread *t = &IOThreads[c->tid];
         /* If there are several clients to process, let the main thread handle them ASAP.
          * Since the client being added to the queue may still need to be processed by
          * the IO thread, we must call this before adding it to the queue to avoid
          * races with the main thread. */
-        sendPendingClientsToMainThreadIfNeeded(&IOThreads[c->tid], 1);
-        /* Remove the client from clients list of IO thread. */
-        listDelNode(IOThreads[c->tid].clients, c->io_thread_client_list_node);
-        c->io_thread_client_list_node = NULL;
+        sendPendingClientsToMainThreadIfNeeded(t, 1);
         /* Disable read and write to avoid race when main thread processes. */
         c->io_flags &= ~(CLIENT_IO_READ_ENABLED | CLIENT_IO_WRITE_ENABLED);
-        listAddNodeTail(IOThreads[c->tid].pending_clients_to_main_thread, c);
+        /* Remove the client from IO thread, add it to main thread's pending list. */
+        listUnlinkNode(t->clients, c->io_thread_client_list_node);
+        listLinkNodeTail(t->pending_clients_to_main_thread, c->io_thread_client_list_node);
+        c->io_thread_client_list_node = NULL;
     }
 }
 
@@ -544,7 +545,8 @@ int processClientsFromMainThread(IOThread *t) {
 
         /* Link client in IO thread clients list first. */
         serverAssert(c->io_thread_client_list_node == NULL);
-        listAddNodeTail(t->clients, c);
+        listUnlinkNode(t->processing_clients, ln);
+        listLinkNodeTail(t->clients, ln);
         c->io_thread_client_list_node = listLast(t->clients);
 
         /* The client now is in the IO thread, let's free deferred objects. */
@@ -575,7 +577,8 @@ int processClientsFromMainThread(IOThread *t) {
             }
         }
     }
-    listEmpty(t->processing_clients);
+    /* All clients must are processed. */
+    serverAssert(listLength(t->processing_clients) == 0);
     return processed;
 }