io-wq: briefly spin for new work after finishing work

To avoid going to sleep only to get woken shortly thereafter, spin
briefly for new work upon completion of work.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -491,26 +491,46 @@ static void io_worker_handle_work(struct io_worker *worker)
 	} while (1);
 }
 
+static inline void io_worker_spin_for_work(struct io_wqe *wqe)
+{
+	int i = 0;
+
+	while (++i < 1000) {
+		if (io_wqe_run_queue(wqe))
+			break;
+		if (need_resched())
+			break;
+		cpu_relax();
+	}
+}
+
 static int io_wqe_worker(void *data)
 {
 	struct io_worker *worker = data;
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
+	bool did_work;
 
 	io_worker_start(wqe, worker);
 
+	did_work = false;
 	while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
 		set_current_state(TASK_INTERRUPTIBLE);
+loop:
+		if (did_work)
+			io_worker_spin_for_work(wqe);
 		spin_lock_irq(&wqe->lock);
 		if (io_wqe_run_queue(wqe)) {
 			__set_current_state(TASK_RUNNING);
 			io_worker_handle_work(worker);
-			continue;
+			did_work = true;
+			goto loop;
 		}
+		did_work = false;
 		/* drops the lock on success, retry */
 		if (__io_worker_idle(wqe, worker)) {
 			__release(&wqe->lock);
-			continue;
+			goto loop;
 		}
 		spin_unlock_irq(&wqe->lock);
 		if (signal_pending(current))

fs/io-wq.h

@@ -35,7 +35,8 @@ static inline void wq_list_add_tail(struct io_wq_work_node *node,
 				    struct io_wq_work_list *list)
 {
 	if (!list->first) {
-		list->first = list->last = node;
+		list->last = node;
+		WRITE_ONCE(list->first, node);
 	} else {
 		list->last->next = node;
 		list->last = node;
@@ -47,7 +48,7 @@ static inline void wq_node_del(struct io_wq_work_list *list,
 			       struct io_wq_work_node *prev)
 {
 	if (node == list->first)
-		list->first = node->next;
+		WRITE_ONCE(list->first, node->next);
 	if (node == list->last)
 		list->last = prev;
 	if (prev)
@@ -58,7 +59,7 @@ static inline void wq_node_del(struct io_wq_work_list *list,
 #define wq_list_for_each(pos, prv, head)			\
 	for (pos = (head)->first, prv = NULL; pos; prv = pos, pos = (pos)->next)
 
-#define wq_list_empty(list)	((list)->first == NULL)
+#define wq_list_empty(list)	(READ_ONCE((list)->first) == NULL)
 #define INIT_WQ_LIST(list)	do {				\
 	(list)->first = NULL;					\
 	(list)->last = NULL;					\