io_uring/napi: check napi_enabled in io_napi_add() before proceeding

doing so avoids the overhead of adding napi ids to all the rings that do
not enable napi.

if no id is added to napi_list because napi is disabled,
__io_napi_busy_loop() will not be called.
Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Fixes: b4ccc4dd ("io_uring/napi: enable even with a timeout of 0")
Link: https://lore.kernel.org/r/bd989ccef5fda14f5fd9888faf4fefcf66bd0369.1723400131.git.olivier@trillion01.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/napi.c

@@ -302,7 +302,7 @@ void __io_napi_busy_loop(struct io_ring_ctx *ctx, struct io_wait_queue *iowq)
 {
 	iowq->napi_prefer_busy_poll = READ_ONCE(ctx->napi_prefer_busy_poll);
 
-	if (!(ctx->flags & IORING_SETUP_SQPOLL) && ctx->napi_enabled)
+	if (!(ctx->flags & IORING_SETUP_SQPOLL))
 		io_napi_blocking_busy_loop(ctx, iowq);
 }
 

io_uring/napi.h

@@ -55,7 +55,7 @@ static inline void io_napi_add(struct io_kiocb *req)
 	struct io_ring_ctx *ctx = req->ctx;
 	struct socket *sock;
 
-	if (!READ_ONCE(ctx->napi_busy_poll_dt))
+	if (!READ_ONCE(ctx->napi_enabled))
 		return;
 
 	sock = sock_from_file(req->file);