io_uring: fold evfd signalling under a slower path

Add ->has_evfd flag, which is true IFF there is an eventfd attached, and
use it to hide io_eventfd_signal() into __io_commit_cqring_flush() and
combine fast checks in a single if. Also, gcc 11.2 wasn't inlining
io_cqring_ev_posted() without this change, so helps with that as well.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f6168471997decded475a063f92915787975a30b.1647481208.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -347,6 +347,7 @@ struct io_ring_ctx {
 		unsigned int		off_timeout_used: 1;
 		unsigned int		drain_active: 1;
 		unsigned int		drain_disabled: 1;
+		unsigned int		has_evfd: 1;
 	} ____cacheline_aligned_in_smp;
 
 	/* submission data */
@@ -1181,6 +1182,7 @@ static int io_install_fixed_file(struct io_kiocb *req, struct file *file,
 static int io_close_fixed(struct io_kiocb *req, unsigned int issue_flags);
 
 static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer);
+static void io_eventfd_signal(struct io_ring_ctx *ctx);
 
 static struct kmem_cache *req_cachep;
 
@@ -1785,15 +1787,19 @@ static inline void io_commit_cqring(struct io_ring_ctx *ctx)
 	smp_store_release(&ctx->rings->cq.tail, ctx->cached_cq_tail);
 }
 
-static __cold void __io_commit_cqring_flush(struct io_ring_ctx *ctx)
+static void __io_commit_cqring_flush(struct io_ring_ctx *ctx)
 {
-	spin_lock(&ctx->completion_lock);
-	if (ctx->off_timeout_used)
-		io_flush_timeouts(ctx);
-	if (ctx->drain_active)
-		io_queue_deferred(ctx);
-	io_commit_cqring(ctx);
-	spin_unlock(&ctx->completion_lock);
+	if (ctx->off_timeout_used || ctx->drain_active) {
+		spin_lock(&ctx->completion_lock);
+		if (ctx->off_timeout_used)
+			io_flush_timeouts(ctx);
+		if (ctx->drain_active)
+			io_queue_deferred(ctx);
+		io_commit_cqring(ctx);
+		spin_unlock(&ctx->completion_lock);
+	}
+	if (ctx->has_evfd)
+		io_eventfd_signal(ctx);
 }
 
 static inline bool io_sqring_full(struct io_ring_ctx *ctx)
@@ -1852,6 +1858,17 @@ out:
 	rcu_read_unlock();
 }
 
+static inline void io_cqring_wake(struct io_ring_ctx *ctx)
+{
+	/*
+	 * wake_up_all() may seem excessive, but io_wake_function() and
+	 * io_should_wake() handle the termination of the loop and only
+	 * wake as many waiters as we need to.
+	 */
+	if (wq_has_sleeper(&ctx->cq_wait))
+		wake_up_all(&ctx->cq_wait);
+}
+
 /*
  * This should only get called when at least one event has been posted.
  * Some applications rely on the eventfd notification count only changing
@@ -1861,31 +1878,21 @@ out:
  */
 static inline void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 {
-	if (unlikely(ctx->off_timeout_used || ctx->drain_active))
+	if (unlikely(ctx->off_timeout_used || ctx->drain_active ||
+		     ctx->has_evfd))
 		__io_commit_cqring_flush(ctx);
 
-	/*
-	 * wake_up_all() may seem excessive, but io_wake_function() and
-	 * io_should_wake() handle the termination of the loop and only
-	 * wake as many waiters as we need to.
-	 */
-	if (wq_has_sleeper(&ctx->cq_wait))
-		wake_up_all(&ctx->cq_wait);
-	if (unlikely(rcu_dereference_raw(ctx->io_ev_fd)))
-		io_eventfd_signal(ctx);
+	io_cqring_wake(ctx);
 }
 
 static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
 {
-	if (unlikely(ctx->off_timeout_used || ctx->drain_active))
+	if (unlikely(ctx->off_timeout_used || ctx->drain_active ||
+		     ctx->has_evfd))
 		__io_commit_cqring_flush(ctx);
 
-	if (ctx->flags & IORING_SETUP_SQPOLL) {
-		if (wq_has_sleeper(&ctx->cq_wait))
-			wake_up_all(&ctx->cq_wait);
-	}
-	if (unlikely(rcu_dereference_raw(ctx->io_ev_fd)))
-		io_eventfd_signal(ctx);
+	if (ctx->flags & IORING_SETUP_SQPOLL)
+		io_cqring_wake(ctx);
 }
 
 /* Returns true if there are no backlogged entries after the flush */
@@ -9898,7 +9905,7 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
 		return ret;
 	}
 	ev_fd->eventfd_async = eventfd_async;
-
+	ctx->has_evfd = true;
 	rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
 	return 0;
 }
@@ -9918,6 +9925,7 @@ static int io_eventfd_unregister(struct io_ring_ctx *ctx)
 	ev_fd = rcu_dereference_protected(ctx->io_ev_fd,
 					lockdep_is_held(&ctx->uring_lock));
 	if (ev_fd) {
+		ctx->has_evfd = false;
 		rcu_assign_pointer(ctx->io_ev_fd, NULL);
 		call_rcu(&ev_fd->rcu, io_eventfd_put);
 		return 0;