io_uring: optimise plugging

Plugging is only needed with requests that also need a file, so hide
plugging under a ->needs_file check. Also, place ->needs_file and ->plug
bits into the same byte of io_op_defs, it may matter for compilers, e.g.
only with the change a tested one decided to optimise two memory testb
into a more with two register testb.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1600d1287bb7d16451d4ef3343252787a5314927.1633532552.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -894,12 +894,12 @@ struct io_defer_entry {
 struct io_op_def {
 	/* needs req->file assigned */
 	unsigned		needs_file : 1;
+	/* should block plug */
+	unsigned		plug : 1;
 	/* hash wq insertion if file is a regular file */
 	unsigned		hash_reg_file : 1;
 	/* unbound wq insertion if file is a non-regular file */
 	unsigned		unbound_nonreg_file : 1;
-	/* opcode is not supported by this kernel */
-	unsigned		not_supported : 1;
 	/* set if opcode supports polled "wait" */
 	unsigned		pollin : 1;
 	unsigned		pollout : 1;
@@ -907,8 +907,8 @@ struct io_op_def {
 	unsigned		buffer_select : 1;
 	/* do prep async if is going to be punted */
 	unsigned		needs_async_setup : 1;
-	/* should block plug */
-	unsigned		plug : 1;
+	/* opcode is not supported by this kernel */
+	unsigned		not_supported : 1;
 	/* size of async data needed, if any */
 	unsigned short		async_size;
 };
@@ -6978,7 +6978,6 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		       const struct io_uring_sqe *sqe)
 	__must_hold(&ctx->uring_lock)
 {
-	struct io_submit_state *state;
 	unsigned int sqe_flags;
 	int personality;
 
@@ -7025,19 +7024,20 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		get_cred(req->creds);
 		req->flags |= REQ_F_CREDS;
 	}
-	state = &ctx->submit_state;
+
+	if (io_op_defs[req->opcode].needs_file) {
+		struct io_submit_state *state = &ctx->submit_state;
 
 		/*
-	 * Plug now if we have more than 1 IO left after this, and the target
-	 * is potentially a read/write to block based storage.
+		 * Plug now if we have more than 2 IO left after this, and the
+		 * target is potentially a read/write to block based storage.
 		 */
 		if (state->need_plug && io_op_defs[req->opcode].plug) {
-		blk_start_plug(&state->plug);
 			state->plug_started = true;
 			state->need_plug = false;
+			blk_start_plug(&state->plug);
 		}
 
-	if (io_op_defs[req->opcode].needs_file) {
 		req->file = io_file_get(ctx, req, READ_ONCE(sqe->fd),
 					(sqe_flags & IOSQE_FIXED_FILE));
 		if (unlikely(!req->file))