tbf/sched: We pick the _last_ entry, rotate the lists

We always pick the _last_ entry from the lists. Let's rotate
so we make it a bit more fair.

src/gprs_rlcmac_sched.cpp

@@ -133,6 +133,7 @@ static struct msgb *sched_select_ctrl_msg(struct gprs_rlcmac_bts *bts,
 	}
 	/* any message */
 	if (msg) {
+		tbf->rotate_in_list();
 		LOGP(DRLCMACSCHED, LOGL_DEBUG, "Scheduling control "
 			"message at RTS for %s (TRX=%d, TS=%d)\n",
 			tbf_name(tbf), trx, ts);

src/tbf.cpp

@@ -1742,3 +1742,12 @@ bool gprs_rlcmac_tbf::dl_window_stalled() const
 {
 	return dir.dl.window.window_stalled();
 }
+
+void gprs_rlcmac_tbf::rotate_in_list()
+{
+	llist_del(&list);
+	if (direction == GPRS_RLCMAC_UL_TBF)
+		llist_add(&list, &bts->bts_data()->ul_tbfs);
+	else
+		llist_add(&list, &bts->bts_data()->dl_tbfs);
+}

src/tbf.h

@@ -135,6 +135,9 @@ struct gprs_rlcmac_tbf {
 
 	uint16_t sns() const;
 
+	/* attempt to make things a bit more fair */
+	void rotate_in_list();
+
 	struct llist_head list;
 	uint32_t state_flags;
 	enum gprs_rlcmac_tbf_direction direction;