nimble/ll: Optimize sched restart

When sched_q head changed we only should stop sched timer to make sure
it does not fire after we leave critical section and we can restart it
then without need for additional locking.

We do not need to change rfmgmt because in case new sched_q head is
later then previous one, rfmgmt will fire too early and we'll either
disable it immediately on restart or keep it enabled for upcoming item,
depending on timing.

nimble/controller/src/ble_ll_sched.c

@@ -37,7 +37,6 @@
 #define BLE_LL_SCHED_MAX_DELAY_ANY      (0x7fffffff)
 
 static struct hal_timer g_ble_ll_sched_timer;
-static uint8_t g_ble_ll_sched_timer_running;
 
 uint8_t g_ble_ll_sched_offset_ticks;
 
@@ -63,6 +62,7 @@ typedef int (* ble_ll_sched_preempt_cb_t)(struct ble_ll_sched_item *sch,
 
 /* Queue for timers */
 static TAILQ_HEAD(ll_sched_qhead, ble_ll_sched_item) g_ble_ll_sched_q;
+static uint8_t g_ble_ll_sched_q_head_changed;
 
 #if MYNEWT_VAL(BLE_LL_STRICT_CONN_SCHEDULING)
 struct ble_ll_sched_obj g_ble_ll_sched_data;
@@ -155,13 +155,13 @@ ble_ll_sched_preempt(struct ble_ll_sched_item *sch,
 static inline void
 ble_ll_sched_q_head_changed(void)
 {
-    struct ble_ll_sched_item *first;
+    if (g_ble_ll_sched_q_head_changed) {
+        return;
+    }
+
+    g_ble_ll_sched_q_head_changed = 1;
 
-    g_ble_ll_sched_timer_running = 0;
     os_cputime_timer_stop(&g_ble_ll_sched_timer);
-
-    first = TAILQ_FIRST(&g_ble_ll_sched_q);
-    ble_ll_rfmgmt_sched_changed(first);
 }
 
 static inline void
@@ -169,10 +169,17 @@ ble_ll_sched_restart(void)
 {
     struct ble_ll_sched_item *first;
 
+    if (!g_ble_ll_sched_q_head_changed) {
+        return;
+    }
+
+    g_ble_ll_sched_q_head_changed = 0;
+
     first = TAILQ_FIRST(&g_ble_ll_sched_q);
 
-    if (!g_ble_ll_sched_timer_running && first) {
-        g_ble_ll_sched_timer_running = 1;
+    ble_ll_rfmgmt_sched_changed(first);
+
+    if (first) {
         os_cputime_timer_start(&g_ble_ll_sched_timer, first->start_time);
     }
 }
@@ -1182,8 +1189,6 @@ ble_ll_sched_run(void *arg)
 
     BLE_LL_DEBUG_GPIO(SCHED_RUN, 1);
 
-    g_ble_ll_sched_timer_running = 0;
-
     /* Look through schedule queue */
     sch = TAILQ_FIRST(&g_ble_ll_sched_q);
     if (sch) {
@@ -1203,7 +1208,10 @@ ble_ll_sched_run(void *arg)
         /* Remove schedule item and execute the callback */
         TAILQ_REMOVE(&g_ble_ll_sched_q, sch, link);
         sch->enqueued = 0;
+        g_ble_ll_sched_q_head_changed = 1;
+
         ble_ll_sched_execute_item(sch);
+
         ble_ll_sched_restart();
     }
 
@@ -1295,7 +1303,6 @@ int ble_ll_sched_dtm(struct ble_ll_sched_item *sch)
 void
 ble_ll_sched_stop(void)
 {
-    g_ble_ll_sched_timer_running = 0;
     os_cputime_timer_stop(&g_ble_ll_sched_timer);
 }
 
@@ -1338,5 +1345,7 @@ ble_ll_sched_init(void)
         g_ble_ll_sched_data.sch_ticks_per_period;
 #endif
 
+    g_ble_ll_sched_q_head_changed = 0;
+
     return 0;
 }