[PATCH 2/5] dm-mpath: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE

[Ming Lei]

If .queue_rq() returns BLK_STS_RESOURCE, blk-mq will rerun
the queue in the 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) BLK_MQ_S_TAG_WAITING is set
- queue is rerun after one tag is freed

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

This random dealy of running hw queue is introduced by commit
6077c2d706097c0(dm rq: Avoid that request processing stalls sporadically),
which claimed one request processing stalling is fixed,
but never explained the behind idea, and it is a workaound at most.
Even the question isn't explained by anyone in recent discussion.

Also calling blk_mq_delay_run_hw_queue() inside .queue_rq() is
a horrible hack because it makes BLK_MQ_S_SCHED_RESTART not
working, and will degrade I/O peformance a lot.

Finally this patch makes sure that dm-rq returns
BLK_STS_RESOURCE to blk-mq only when underlying queue is
out of resource, so we switch to return DM_MAPIO_DELAY_REQUEU
if either MPATHF_QUEUE_IO or MPATHF_PG_INIT_REQUIRED is set in
multipath_clone_and_map().

Signed-off-by: Ming Lei <ming....@redhat.com>
---
 drivers/md/dm-mpath.c | 4 +---
 drivers/md/dm-rq.c    | 1 -
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index e8094d8fbe0d..97e4bd100fa1 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -484,9 +484,7 @@ 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;
+               return DM_MAPIO_DELAY_REQUEUE;
        }
 
        memset(mpio, 0, sizeof(*mpio));
diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index f5e2b6967357..46f012185b43 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;
        }
 
-- 
2.9.5