io_uring: optimise task_work submit flushing

tctx_task_work() tries to fetch a next batch of requests, but before it
would flush completions from the previous batch that may be sub-optimal.
E.g. io_req_task_queue() executes a head of the link where all the
linked may be enqueued through the same io_req_task_queue(). And there
are more cases for that.

Do the flushing at the end, so it can cache completions of several waves
of a single tctx_task_work(), and do the flush at the very end.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/3cac83934e4fbce520ff8025c3524398b3ae0270.1623949695.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1890,13 +1890,13 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx)
 
 static void tctx_task_work(struct callback_head *cb)
 {
+	struct io_ring_ctx *ctx = NULL;
 	struct io_uring_task *tctx = container_of(cb, struct io_uring_task,
 						  task_work);
 
 	clear_bit(0, &tctx->task_state);
 
 	while (!wq_list_empty(&tctx->task_list)) {
-		struct io_ring_ctx *ctx = NULL;
 		struct io_wq_work_list list;
 		struct io_wq_work_node *node;
 
@@ -1920,11 +1920,12 @@ static void tctx_task_work(struct callback_head *cb)
 			node = next;
 		}
 
-		ctx_flush_and_put(ctx);
 		if (!list.first)
 			break;
 		cond_resched();
 	}
+
+	ctx_flush_and_put(ctx);
 }
 
 static int io_req_task_work_add(struct io_kiocb *req)