io_uring: flag SQPOLL busy condition to userspace

Now that we have backpressure, for SQPOLL, we have one more condition
that warrants flagging that the application needs to enter the kernel:
we failed to submit IO due to backpressure. Make sure we catch that
and flag it appropriately.

If we run into backpressure issues with the SQPOLL thread, flag it
as such to the application by setting IORING_SQ_NEED_WAKEUP. This will
cause the application to enter the kernel, and that will flush the
backlog and clear the condition.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3118,16 +3118,16 @@ static int io_sq_thread(void *data)
 	DEFINE_WAIT(wait);
 	unsigned inflight;
 	unsigned long timeout;
+	int ret;
 
 	complete(&ctx->completions[1]);
 
 	old_fs = get_fs();
 	set_fs(USER_DS);
 
-	timeout = inflight = 0;
+	ret = timeout = inflight = 0;
 	while (!kthread_should_park()) {
 		unsigned int to_submit;
-		int ret;
 
 		if (inflight) {
 			unsigned nr_events = 0;
@@ -3161,13 +3161,21 @@ static int io_sq_thread(void *data)
 		}
 
 		to_submit = io_sqring_entries(ctx);
-		if (!to_submit) {
+
+		/*
+		 * If submit got -EBUSY, flag us as needing the application
+		 * to enter the kernel to reap and flush events.
+		 */
+		if (!to_submit || ret == -EBUSY) {
 			/*
 			 * We're polling. If we're within the defined idle
 			 * period, then let us spin without work before going
-			 * to sleep.
+			 * to sleep. The exception is if we got EBUSY doing
+			 * more IO, we should wait for the application to
+			 * reap events and wake us up.
 			 */
-			if (inflight || !time_after(jiffies, timeout)) {
+			if (inflight ||
+			    (!time_after(jiffies, timeout) && ret != -EBUSY)) {
 				cond_resched();
 				continue;
 			}
@@ -3193,7 +3201,7 @@ static int io_sq_thread(void *data)
 			smp_mb();
 
 			to_submit = io_sqring_entries(ctx);
-			if (!to_submit) {
+			if (!to_submit || ret == -EBUSY) {
 				if (kthread_should_park()) {
 					finish_wait(&ctx->sqo_wait, &wait);
 					break;
@@ -4351,6 +4359,8 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
 	 */
 	ret = 0;
 	if (ctx->flags & IORING_SETUP_SQPOLL) {
+		if (!list_empty_careful(&ctx->cq_overflow_list))
+			io_cqring_overflow_flush(ctx, false);
 		if (flags & IORING_ENTER_SQ_WAKEUP)
 			wake_up(&ctx->sqo_wait);
 		submitted = to_submit;