io_uring: kill off ios_left

->ios_left is only used to decide whether to plug or not, kill it to
avoid this extra accounting, just use the initial submission number.
There is no much difference in regards of enabling plugging, where this
one does it in a few more cases, but all major ones should be covered
well.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f13993bcf5b477f9a7d52881fc49f9457ea9870a.1631115443.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -315,6 +315,7 @@ struct io_submit_state {
 	unsigned int		free_reqs;
 
 	bool			plug_started;
+	bool			need_plug;
 
 	/*
 	 * Batch completion logic
@@ -323,8 +324,6 @@ struct io_submit_state {
 	unsigned int		compl_nr;
 	/* inline/task_work completion list, under ->uring_lock */
 	struct list_head	free_list;
-
-	unsigned int		ios_left;
 };
 
 struct io_ring_ctx {
@@ -7090,10 +7089,10 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	 * Plug now if we have more than 1 IO left after this, and the target
 	 * is potentially a read/write to block based storage.
 	 */
-	if (!state->plug_started && state->ios_left > 1 &&
-	    io_op_defs[req->opcode].plug) {
+	if (state->need_plug && io_op_defs[req->opcode].plug) {
 		blk_start_plug(&state->plug);
 		state->plug_started = true;
+		state->need_plug = false;
 	}
 
 	if (io_op_defs[req->opcode].needs_file) {
@@ -7102,8 +7101,6 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		if (unlikely(!req->file))
 			ret = -EBADF;
 	}
-
-	state->ios_left--;
 	return ret;
 }
 
@@ -7212,7 +7209,7 @@ static void io_submit_state_start(struct io_submit_state *state,
 				  unsigned int max_ios)
 {
 	state->plug_started = false;
-	state->ios_left = max_ios;
+	state->need_plug = max_ios > 2;
 	/* set only head, no need to init link_last in advance */
 	state->link.head = NULL;
 }