io_uring: don't burn CPU for iopoll on exit

First of all don't spin in io_ring_ctx_wait_and_kill() on iopoll.
Requests won't complete faster because of that, but only lengthen
io_uring_release().

The same goes for offloaded cleanup in io_ring_exit_work() -- it
already has waiting loop, don't do blocking active spinning.

For that, pass min=0 into io_iopoll_[try_]reap_events(), so it won't
actively spin. Leave the function if io_do_iopoll() there can't
complete a request to sleep in io_ring_exit_work().
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2055,7 +2055,7 @@ static int io_iopoll_getevents(struct io_ring_ctx *ctx, unsigned int *nr_events,
  * We can't just wait for polled events to come to us, we have to actively
  * find and complete them.
  */
-static void io_iopoll_reap_events(struct io_ring_ctx *ctx)
+static void io_iopoll_try_reap_events(struct io_ring_ctx *ctx)
 {
 	if (!(ctx->flags & IORING_SETUP_IOPOLL))
 		return;
@@ -2064,8 +2064,11 @@ static void io_iopoll_reap_events(struct io_ring_ctx *ctx)
 	while (!list_empty(&ctx->poll_list)) {
 		unsigned int nr_events = 0;
 
-		io_do_iopoll(ctx, &nr_events, 1);
+		io_do_iopoll(ctx, &nr_events, 0);
 
+		/* let it sleep and repeat later if can't complete a request */
+		if (nr_events == 0)
+			break;
 		/*
 		 * Ensure we allow local-to-the-cpu processing to take place,
 		 * in this case we need to ensure that we reap all events.
@@ -7648,7 +7651,6 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
 		ctx->sqo_mm = NULL;
 	}
 
-	io_iopoll_reap_events(ctx);
 	io_sqe_buffer_unregister(ctx);
 	io_sqe_files_unregister(ctx);
 	io_eventfd_unregister(ctx);
@@ -7715,11 +7717,8 @@ static int io_remove_personalities(int id, void *p, void *data)
 
 static void io_ring_exit_work(struct work_struct *work)
 {
-	struct io_ring_ctx *ctx;
-
-	ctx = container_of(work, struct io_ring_ctx, exit_work);
-	if (ctx->rings)
-		io_cqring_overflow_flush(ctx, true);
+	struct io_ring_ctx *ctx = container_of(work, struct io_ring_ctx,
+					       exit_work);
 
 	/*
 	 * If we're doing polled IO and end up having requests being
@@ -7727,11 +7726,11 @@ static void io_ring_exit_work(struct work_struct *work)
 	 * we're waiting for refs to drop. We need to reap these manually,
 	 * as nobody else will be looking for them.
 	 */
-	while (!wait_for_completion_timeout(&ctx->ref_comp, HZ/20)) {
-		io_iopoll_reap_events(ctx);
+	do {
 		if (ctx->rings)
 			io_cqring_overflow_flush(ctx, true);
-	}
+		io_iopoll_try_reap_events(ctx);
+	} while (!wait_for_completion_timeout(&ctx->ref_comp, HZ/20));
 	io_ring_ctx_free(ctx);
 }
 
@@ -7747,10 +7746,10 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 	if (ctx->io_wq)
 		io_wq_cancel_all(ctx->io_wq);
 
-	io_iopoll_reap_events(ctx);
 	/* if we failed setting up the ctx, we might not have any rings */
 	if (ctx->rings)
 		io_cqring_overflow_flush(ctx, true);
+	io_iopoll_try_reap_events(ctx);
 	idr_for_each(&ctx->personality_idr, io_remove_personalities, ctx);
 	INIT_WORK(&ctx->exit_work, io_ring_exit_work);
 	queue_work(system_wq, &ctx->exit_work);