block: mq-deadline: Track the dispatch position

Track the position (sector_t) of the most recently dispatched request
instead of tracking a pointer to the next request to dispatch. This
patch is the basis for patch "Handle requeued requests correctly".
Without this patch it would be significantly more complicated to make
sure that zoned writes are dispatched in LBA order per zone.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230517174230.897144-10-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/mq-deadline.c

@@ -74,8 +74,8 @@ struct dd_per_prio {
 	struct list_head dispatch;
 	struct rb_root sort_list[DD_DIR_COUNT];
 	struct list_head fifo_list[DD_DIR_COUNT];
-	/* Next request in FIFO order. Read, write or both are NULL. */
-	struct request *next_rq[DD_DIR_COUNT];
+	/* Position of the most recently dispatched request. */
+	sector_t latest_pos[DD_DIR_COUNT];
 	struct io_stats_per_prio stats;
 };
 
@@ -156,6 +156,25 @@ deadline_latter_request(struct request *rq)
 	return NULL;
 }
 
+/* Return the first request for which blk_rq_pos() >= pos. */
+static inline struct request *deadline_from_pos(struct dd_per_prio *per_prio,
+				enum dd_data_dir data_dir, sector_t pos)
+{
+	struct rb_node *node = per_prio->sort_list[data_dir].rb_node;
+	struct request *rq, *res = NULL;
+
+	while (node) {
+		rq = rb_entry_rq(node);
+		if (blk_rq_pos(rq) >= pos) {
+			res = rq;
+			node = node->rb_left;
+		} else {
+			node = node->rb_right;
+		}
+	}
+	return res;
+}
+
 static void
 deadline_add_rq_rb(struct dd_per_prio *per_prio, struct request *rq)
 {
@@ -167,11 +186,6 @@ deadline_add_rq_rb(struct dd_per_prio *per_prio, struct request *rq)
 static inline void
 deadline_del_rq_rb(struct dd_per_prio *per_prio, struct request *rq)
 {
-	const enum dd_data_dir data_dir = rq_data_dir(rq);
-
-	if (per_prio->next_rq[data_dir] == rq)
-		per_prio->next_rq[data_dir] = deadline_latter_request(rq);
-
 	elv_rb_del(deadline_rb_root(per_prio, rq), rq);
 }
 
@@ -251,10 +265,6 @@ static void
 deadline_move_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 		      struct request *rq)
 {
-	const enum dd_data_dir data_dir = rq_data_dir(rq);
-
-	per_prio->next_rq[data_dir] = deadline_latter_request(rq);
-
 	/*
 	 * take it off the sort and fifo list
 	 */
@@ -363,7 +373,8 @@ deadline_next_request(struct deadline_data *dd, struct dd_per_prio *per_prio,
 	struct request *rq;
 	unsigned long flags;
 
-	rq = per_prio->next_rq[data_dir];
+	rq = deadline_from_pos(per_prio, data_dir,
+			       per_prio->latest_pos[data_dir]);
 	if (!rq)
 		return NULL;
 
@@ -426,16 +437,19 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
 		if (started_after(dd, rq, latest_start))
 			return NULL;
 		list_del_init(&rq->queuelist);
+		data_dir = rq_data_dir(rq);
 		goto done;
 	}
 
 	/*
 	 * batches are currently reads XOR writes
 	 */
 	rq = deadline_next_request(dd, per_prio, dd->last_dir);
-	if (rq && dd->batching < dd->fifo_batch)
+	if (rq && dd->batching < dd->fifo_batch) {
 		/* we have a next request and are still entitled to batch */
+		data_dir = rq_data_dir(rq);
 		goto dispatch_request;
+	}
 
 	/*
 	 * at this point we are not running a batch. select the appropriate
@@ -513,6 +527,7 @@ static struct request *__dd_dispatch_request(struct deadline_data *dd,
 done:
 	ioprio_class = dd_rq_ioclass(rq);
 	prio = ioprio_class_to_prio[ioprio_class];
+	dd->per_prio[prio].latest_pos[data_dir] = blk_rq_pos(rq);
 	dd->per_prio[prio].stats.dispatched++;
 	/*
 	 * If the request needs its target zone locked, do it.
@@ -1026,8 +1041,10 @@ static int deadline_##name##_next_rq_show(void *data,			\
 	struct request_queue *q = data;					\
 	struct deadline_data *dd = q->elevator->elevator_data;		\
 	struct dd_per_prio *per_prio = &dd->per_prio[prio];		\
-	struct request *rq = per_prio->next_rq[data_dir];		\
+	struct request *rq;						\
 									\
+	rq = deadline_from_pos(per_prio, data_dir,			\
+			       per_prio->latest_pos[data_dir]);		\
 	if (rq)								\
 		__blk_mq_debugfs_rq_show(m, rq);			\
 	return 0;							\