blk-mq: Always insert sequential zoned writes into a software queue

One of the optimizations in the block layer is that the software queues
are bypassed if it is expected that the block driver will accept a
request. This can cause request reordering even for requests submitted
from the same CPU core.  This patch preserves the order for sequential
zoned writes submitted from a given CPU core by always inserting these
requests into the appropriate software queue.

Cc: Damien Le Moal <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Signed-off-by: Bart Van Assche <[email protected]>

Author: bvanassche

block/blk-mq.c

@@ -1537,6 +1537,27 @@ void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list)
 }
 EXPORT_SYMBOL(blk_mq_requeue_request);
 
+/*
+ * Whether the block layer should preserve the order of @rq relative to other
+ * requests submitted to the same software queue.
+ */
+static bool blk_mq_preserve_order(struct request *rq)
+{
+	return rq->q->limits.features & BLK_FEAT_ORDERED_HWQ &&
+	       blk_rq_is_seq_zoned_write(rq);
+}
+
+static bool blk_mq_preserve_order_for_list(struct list_head *list)
+{
+	struct request *rq;
+
+	list_for_each_entry(rq, list, queuelist)
+		if (blk_mq_preserve_order(rq))
+			return true;
+
+	return false;
+}
+
 static void blk_mq_requeue_work(struct work_struct *work)
 {
 	struct request_queue *q =
@@ -2566,7 +2587,8 @@ static void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx,
 	 * Try to issue requests directly if the hw queue isn't busy to save an
 	 * extra enqueue & dequeue to the sw queue.
 	 */
-	if (!hctx->dispatch_busy && !run_queue_async) {
+	if (!hctx->dispatch_busy && !run_queue_async &&
+	    !blk_mq_preserve_order_for_list(list)) {
 		blk_mq_run_dispatch_ops(hctx->queue,
 			blk_mq_try_issue_list_directly(hctx, list));
 		if (list_empty(list))
@@ -3215,7 +3237,8 @@ void blk_mq_submit_bio(struct bio *bio)
 
 	hctx = rq->mq_hctx;
 	if ((rq->rq_flags & RQF_USE_SCHED) ||
-	    (hctx->dispatch_busy && (q->nr_hw_queues == 1 || !is_sync))) {
+	    (hctx->dispatch_busy && (q->nr_hw_queues == 1 || !is_sync)) ||
+	    blk_mq_preserve_order(rq)) {
 		blk_mq_insert_request(rq, 0);
 		blk_mq_run_hw_queue(hctx, true);
 	} else {

include/linux/blk-mq.h

@@ -1191,4 +1191,15 @@ static inline int blk_rq_map_sg(struct request *rq, struct scatterlist *sglist)
 }
 void blk_dump_rq_flags(struct request *, char *);
 
+static inline bool blk_rq_is_seq_zoned_write(struct request *rq)
+{
+	switch (req_op(rq)) {
+	case REQ_OP_WRITE:
+	case REQ_OP_WRITE_ZEROES:
+		return bdev_zone_is_seq(rq->q->disk->part0, blk_rq_pos(rq));
+	default:
+		return false;
+	}
+}
+
 #endif /* BLK_MQ_H */