core/sched: add runq_callback hook and runq inspection functions

core/include/sched.h

@@ -298,6 +298,62 @@ static inline void sched_runq_advance(uint8_t prio)
     clist_lpoprpush(&sched_runqueues[prio]);
 }
 
+#if (IS_USED(MODULE_SCHED_RUNQ_CALLBACK)) || defined(DOXYGEN)
+/**
+ * @brief   Scheduler runqueue (change) callback
+ *
+ * @details Function has to be provided by the user of this API.
+ *          It will be called:
+ *          - when the scheduler is run,
+ *          - when a thread enters the active queue or
+ *          - when the last thread leaves a queue
+ *
+ * @warning This API is not intended for out of tree users.
+ *          Breaking API changes will be done without notice and
+ *          without deprecation. Consider yourself warned!
+ *
+ * @param   prio      the priority of the runqueue that changed
+ *
+ */
+extern void sched_runq_callback(uint8_t prio);
+#endif
+
+/**
+ * @brief   Tell if the number of threads in a runqueue is 0
+ *
+ * @param[in]   prio      The priority of the runqueue to get information of
+ * @return      Truth value for that information
+ * @warning     This API is not intended for out of tree users.
+ */
+static inline int sched_runq_is_empty(uint8_t prio)
+{
+    return clist_is_empty(&sched_runqueues[prio]);
+}
+
+/**
+ * @brief   Tell if the number of threads in a runqueue is 1
+ *
+ * @param[in]   prio      The priority of the runqueue to get information of
+ * @return      Truth value for that information
+ * @warning     This API is not intended for out of tree users.
+ */
+static inline int sched_runq_is_one(uint8_t prio)
+{
+    return clist_exactly_one(&sched_runqueues[prio]);
+}
+
+/**
+ * @brief   Tell if the number of threads in a runqueue greater than 1
+ *
+ * @param[in]   prio      The priority of the runqueue to get information of
+ * @return      Truth value for that information
+ * @warning     This API is not intended for out of tree users.
+ */
+static inline int sched_runq_greater_one(uint8_t prio)
+{
+    return clist_more_than_one(&sched_runqueues[prio]);
+}
+
 #ifdef __cplusplus
 }
 #endif

core/sched.c

@@ -156,6 +156,10 @@ thread_t *__attribute__((used)) sched_run(void)
     thread_t *next_thread = container_of(sched_runqueues[nextrq].next->next,
                                          thread_t, rq_entry);
 
+#if (IS_USED(MODULE_SCHED_RUNQ_CALLBACK))
+    sched_runq_callback(nextrq);
+#endif
+
     DEBUG(
         "sched_run: active thread: %" PRIkernel_pid ", next thread: %" PRIkernel_pid "\n",
         (kernel_pid_t)((active_thread == NULL)
@@ -220,6 +224,16 @@ void sched_set_status(thread_t *process, thread_status_t status)
             clist_rpush(&sched_runqueues[process->priority],
                         &(process->rq_entry));
             _set_runqueue_bit(process);
+
+            /* some thread entered a runqueue
+             * if it is the active runqueue
+             * inform the runqueue_change callback */
+#if (IS_USED(MODULE_SCHED_RUNQ_CALLBACK))
+            thread_t *active_thread = thread_get_active();
+            if (active_thread && active_thread->priority == process->priority) {
+                sched_runq_callback(process->priority);
+            }
+#endif
         }
     }
     else {
@@ -231,6 +245,9 @@ void sched_set_status(thread_t *process, thread_status_t status)
 
             if (!sched_runqueues[process->priority].next) {
                 _clear_runqueue_bit(process);
+#if (IS_USED(MODULE_SCHED_RUNQ_CALLBACK))
+                sched_runq_callback(process->priority);
+#endif
             }
         }
     }

makefiles/pseudomodules.inc.mk

@@ -123,6 +123,7 @@ PSEUDOMODULES += saul_nrf_temperature
 PSEUDOMODULES += saul_pwm
 PSEUDOMODULES += scanf_float
 PSEUDOMODULES += sched_cb
+PSEUDOMODULES += sched_runq_callback
 PSEUDOMODULES += semtech_loramac_rx
 PSEUDOMODULES += shell_hooks
 PSEUDOMODULES += slipdev_stdio