kernel: msgq: Allow for fast returns

1. Fixes a performance issue in k_msgq_put() to allow for a fast return
path when handling a poll event does nothing.

2. Allows for a fast return path in k_msgq_purge() when no threads were
awakened.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

kernel/msg_q.c

@@ -132,6 +132,7 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 	struct k_thread *pending_thread;
 	k_spinlock_key_t key;
 	int result;
+	bool resched = false;
 
 	key = k_spin_lock(&msgq->lock);
 
@@ -141,16 +142,13 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 		/* message queue isn't full */
 		pending_thread = z_unpend_first_thread(&msgq->wait_q);
 		if (unlikely(pending_thread != NULL)) {
-			SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, 0);
+			resched = true;
 
 			/* give message to waiting thread */
-			(void)memcpy(pending_thread->base.swap_data, data,
-			       msgq->msg_size);
+			(void)memcpy(pending_thread->base.swap_data, data, msgq->msg_size);
 			/* wake up waiting thread */
 			arch_thread_return_value_set(pending_thread, 0);
 			z_ready_thread(pending_thread);
-			z_reschedule(&msgq->lock, key);
-			return 0;
 		} else {
 			/* put message in queue */
 			__ASSERT_NO_MSG(msgq->write_ptr >= msgq->buffer_start &&
@@ -161,7 +159,7 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 				msgq->write_ptr = msgq->buffer_start;
 			}
 			msgq->used_msgs++;
-			(void)handle_poll_events(msgq);
+			resched = handle_poll_events(msgq);
 		}
 		result = 0;
 	} else if (K_TIMEOUT_EQ(timeout, K_NO_WAIT)) {
@@ -180,7 +178,11 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 
 	SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, result);
 
-	z_reschedule(&msgq->lock, key);
+	if (resched) {
+		z_reschedule(&msgq->lock, key);
+	} else {
+		k_spin_unlock(&msgq->lock, key);
+	}
 
 	return result;
 }
@@ -222,6 +224,7 @@ int z_impl_k_msgq_get(struct k_msgq *msgq, void *data, k_timeout_t timeout)
 	k_spinlock_key_t key;
 	struct k_thread *pending_thread;
 	int result;
+	bool resched = false;
 
 	key = k_spin_lock(&msgq->lock);
 
@@ -255,11 +258,7 @@ int z_impl_k_msgq_get(struct k_msgq *msgq, void *data, k_timeout_t timeout)
 			/* wake up waiting thread */
 			arch_thread_return_value_set(pending_thread, 0);
 			z_ready_thread(pending_thread);
-			z_reschedule(&msgq->lock, key);
-
-			SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, get, msgq, timeout, 0);
-
-			return 0;
+			resched = true;
 		}
 		result = 0;
 	} else if (K_TIMEOUT_EQ(timeout, K_NO_WAIT)) {
@@ -278,7 +277,11 @@ int z_impl_k_msgq_get(struct k_msgq *msgq, void *data, k_timeout_t timeout)
 
 	SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, get, msgq, timeout, result);
 
-	k_spin_unlock(&msgq->lock, key);
+	if (resched) {
+		z_reschedule(&msgq->lock, key);
+	} else {
+		k_spin_unlock(&msgq->lock, key);
+	}
 
 	return result;
 }
@@ -379,22 +382,29 @@ void z_impl_k_msgq_purge(struct k_msgq *msgq)
 {
 	k_spinlock_key_t key;
 	struct k_thread *pending_thread;
+	bool resched = false;
 
 	key = k_spin_lock(&msgq->lock);
 
 	SYS_PORT_TRACING_OBJ_FUNC(k_msgq, purge, msgq);
 
 	/* wake up any threads that are waiting to write */
-	for (pending_thread = z_unpend_first_thread(&msgq->wait_q); pending_thread != NULL;
-		 pending_thread = z_unpend_first_thread(&msgq->wait_q)) {
+	for (pending_thread = z_unpend_first_thread(&msgq->wait_q);
+	     pending_thread != NULL;
+	     pending_thread = z_unpend_first_thread(&msgq->wait_q)) {
 		arch_thread_return_value_set(pending_thread, -ENOMSG);
 		z_ready_thread(pending_thread);
+		resched = true;
 	}
 
 	msgq->used_msgs = 0;
 	msgq->read_ptr = msgq->write_ptr;
 
-	z_reschedule(&msgq->lock, key);
+	if (resched) {
+		z_reschedule(&msgq->lock, key);
+	} else {
+		k_spin_unlock(&msgq->lock, key);
+	}
 }
 
 #ifdef CONFIG_USERSPACE