block/mq-deadline: Micro-optimize the batching algorithm

When dispatching the first request of a batch, the deadline_move_request()
call clears .next_rq[] for the opposite data direction. .next_rq[] is not
restored when changing data direction. Fix this by not clearing .next_rq[]
and by keeping track of the data direction of a batch in a variable instead.

This patch is a micro-optimization because:
- The number of deadline_next_request() calls for the read direction is
  halved.
- The number of times that deadline_next_request() returns NULL is reduced.

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Cc: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20210618004456.7280-13-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/mq-deadline.c

@@ -53,6 +53,8 @@ struct deadline_data {
 	struct rb_root sort_list[DD_DIR_COUNT];
 	struct list_head fifo_list[DD_DIR_COUNT];
 
+	/* Data direction of latest dispatched request. */
+	enum dd_data_dir last_dir;
 	/*
 	 * next in sort order. read, write or both are NULL
 	 */
@@ -179,8 +181,6 @@ deadline_move_request(struct deadline_data *dd, struct request *rq)
 {
 	const enum dd_data_dir data_dir = rq_data_dir(rq);
 
-	dd->next_rq[DD_READ] = NULL;
-	dd->next_rq[DD_WRITE] = NULL;
 	dd->next_rq[data_dir] = deadline_latter_request(rq);
 
 	/*
@@ -292,10 +292,7 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd)
 	/*
 	 * batches are currently reads XOR writes
 	 */
-	rq = deadline_next_request(dd, DD_WRITE);
-	if (!rq)
-		rq = deadline_next_request(dd, DD_READ);
-
+	rq = deadline_next_request(dd, dd->last_dir);
 	if (rq && dd->batching < dd->fifo_batch)
 		/* we have a next request are still entitled to batch */
 		goto dispatch_request;
@@ -361,6 +358,7 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd)
 	if (!rq)
 		return NULL;
 
+	dd->last_dir = data_dir;
 	dd->batching = 0;
 
 dispatch_request:
@@ -473,6 +471,7 @@ static int dd_init_sched(struct request_queue *q, struct elevator_type *e)
 	dd->fifo_expire[DD_WRITE] = write_expire;
 	dd->writes_starved = writes_starved;
 	dd->front_merges = 1;
+	dd->last_dir = DD_WRITE;
 	dd->fifo_batch = fifo_batch;
 	spin_lock_init(&dd->lock);
 	spin_lock_init(&dd->zone_lock);