dm mpath: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCENotice: this object is not reachable from any branch.

If .queue_rq() returns BLK_STS_RESOURCE, blk-mq will rerun the queue in
these three situations:

1) if BLK_MQ_S_SCHED_RESTART is set
- queue is rerun after one rq is completed, see blk_mq_sched_restart()
which is run from blk_mq_free_request()

2) run out of driver tag
- queue is rerun after one tag is freed

3) otherwise
- queue is run immediately in blk_mq_dispatch_rq_list()

dm-mpath's random (100ms) delayed running of hw queue was introduced by
commit 6077c2d706097c0 ("dm rq: Avoid that request processing stalls
sporadically"), but it never explained the reasoning behind it.  At best
it was a workaround.  And calling blk_mq_delay_run_hw_queue() inside
.queue_rq() is a horrible hack because it breaks BLK_MQ_S_SCHED_RESTART,
which degrades I/O peformance a lot.  So remove it.

Also, avoid using DM_MAPIO_REQUEUE unless absolutely necessary because
it results in dm-rq.c:dm_mq_queue_rq() returning BLK_STS_RESOURCE to
blk-mq -- doing so should only ever be done if the underlying queue is
out of resources.  So switch to returning DM_MAPIO_DELAY_REQUEUE if
either MPATHF_QUEUE_IO or MPATHF_PG_INIT_REQUIRED is set in
multipath_clone_and_map().

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Notice: this object is not reachable from any branch.

2 files changed, 2 insertions, 4 deletions

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index be581765edd1f..3ab5f7a81e5fa 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -517,9 +517,8 @@ static int multipath_clone_and_map(struct dm_target *ti, struct request *rq,
 		return DM_MAPIO_KILL;
 	} else if (test_bit(MPATHF_QUEUE_IO, &m->flags) ||
 		   test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags)) {
-		if (pg_init_all_paths(m))
-			return DM_MAPIO_DELAY_REQUEUE;
-		return DM_MAPIO_REQUEUE;
+		pg_init_all_paths(m);
+		return DM_MAPIO_DELAY_REQUEUE;
 	}
 
 	mpio->pgpath = pgpath;
diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index 9d32f25489c27..cbe8a06ef8b01 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -758,7 +758,6 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
 		/* Undo dm_start_request() before requeuing */
 		rq_end_stats(md, rq);
 		rq_completed(md, rq_data_dir(rq), false);
-		blk_mq_delay_run_hw_queue(hctx, 100/*ms*/);
 		return BLK_STS_RESOURCE;
 	}