kernel: Apply 'unlikely' attribute

Applies the 'unlikely' attribute to various kernel objects that
use z_unpend_first_thread() to optimize for the non-blocking path.

This boosts the thread_metric synchronization benchmark numbers
on the frdm_k64f board by about 10%.

(cherry picked from commit cc415bc)

Original-Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>
GitOrigin-RevId: cc415bc
Cr-Build-Id: 8733565196572304993
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8733565196572304993
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I275af4be4c682e18201421c2e021d5ead8cbac65
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5934669
Reviewed-by: Jeremy Bettis <jbettis@chromium.org>
Reviewed-by: Dawid Niedźwiecki <dawidn@google.com>
Commit-Queue: Dawid Niedźwiecki <dawidn@google.com>
Tested-by: Dawid Niedźwiecki <dawidn@google.com>

kernel/condvar.c

@@ -49,7 +49,7 @@ int z_impl_k_condvar_signal(struct k_condvar *condvar)
 
 	struct k_thread *thread = z_unpend_first_thread(&condvar->wait_q);
 
-	if (thread != NULL) {
+	if (unlikely(thread != NULL)) {
 		SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_condvar, signal, condvar, K_FOREVER);
 
 		arch_thread_return_value_set(thread, 0);

kernel/mem_slab.c

@@ -275,7 +275,7 @@ void k_mem_slab_free(struct k_mem_slab *slab, void *mem)
 	if ((slab->free_list == NULL) && IS_ENABLED(CONFIG_MULTITHREADING)) {
 		struct k_thread *pending_thread = z_unpend_first_thread(&slab->wait_q);
 
-		if (pending_thread != NULL) {
+		if (unlikely(pending_thread != NULL)) {
 			SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_mem_slab, free, slab);
 
 			z_thread_return_value_set_with_data(pending_thread, 0, mem);

kernel/msg_q.c

@@ -142,7 +142,7 @@ int z_impl_k_msgq_put(struct k_msgq *msgq, const void *data, k_timeout_t timeout
 	if (msgq->used_msgs < msgq->max_msgs) {
 		/* message queue isn't full */
 		pending_thread = z_unpend_first_thread(&msgq->wait_q);
-		if (pending_thread != NULL) {
+		if (unlikely(pending_thread != NULL)) {
 			SYS_PORT_TRACING_OBJ_FUNC_EXIT(k_msgq, put, msgq, timeout, 0);
 
 			/* give message to waiting thread */
@@ -242,7 +242,7 @@ int z_impl_k_msgq_get(struct k_msgq *msgq, void *data, k_timeout_t timeout)
 
 		/* handle first thread waiting to write (if any) */
 		pending_thread = z_unpend_first_thread(&msgq->wait_q);
-		if (pending_thread != NULL) {
+		if (unlikely(pending_thread != NULL)) {
 			SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_msgq, get, msgq, timeout);
 
 			/* add thread's message to queue */

kernel/mutex.c

@@ -261,7 +261,7 @@ int z_impl_k_mutex_unlock(struct k_mutex *mutex)
 	LOG_DBG("new owner of mutex %p: %p (prio: %d)",
 		mutex, new_owner, new_owner ? new_owner->base.prio : -1000);
 
-	if (new_owner != NULL) {
+	if (unlikely(new_owner != NULL)) {
 		/*
 		 * new owner is already of higher or equal prio than first
 		 * waiter since the wait queue is priority-based: no need to

kernel/queue.c

@@ -139,7 +139,7 @@ static int32_t queue_insert(struct k_queue *queue, void *prev, void *data,
 	}
 	first_pending_thread = z_unpend_first_thread(&queue->wait_q);
 
-	if (first_pending_thread != NULL) {
+	if (unlikely(first_pending_thread != NULL)) {
 		SYS_PORT_TRACING_OBJ_FUNC_BLOCKING(k_queue, queue_insert, queue, alloc, K_FOREVER);
 
 		prepare_thread_to_run(first_pending_thread, data);

kernel/sched.c

@@ -725,7 +725,7 @@ struct k_thread *z_unpend_first_thread(_wait_q_t *wait_q)
 	K_SPINLOCK(&_sched_spinlock) {
 		thread = _priq_wait_best(&wait_q->waitq);
 
-		if (thread != NULL) {
+		if (unlikely(thread != NULL)) {
 			unpend_thread_no_timeout(thread);
 			(void)z_abort_thread_timeout(thread);
 		}

kernel/sem.c

@@ -103,15 +103,15 @@ void z_impl_k_sem_give(struct k_sem *sem)
 
 	thread = z_unpend_first_thread(&sem->wait_q);
 
-	if (thread != NULL) {
+	if (unlikely(thread != NULL)) {
 		arch_thread_return_value_set(thread, 0);
 		z_ready_thread(thread);
 	} else {
 		sem->count += (sem->count != sem->limit) ? 1U : 0U;
 		resched = handle_poll_events(sem);
 	}
 
-	if (resched) {
+	if (unlikely(resched)) {
 		z_reschedule(&lock, key);
 	} else {
 		k_spin_unlock(&lock, key);

kernel/stack.c

@@ -119,7 +119,7 @@ int z_impl_k_stack_push(struct k_stack *stack, stack_data_t data)
 
 	first_pending_thread = z_unpend_first_thread(&stack->wait_q);
 
-	if (first_pending_thread != NULL) {
+	if (unlikely(first_pending_thread != NULL)) {
 		z_thread_return_value_set_with_data(first_pending_thread,
 						   0, (void *)data);