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Subject[PATCH V2 1/5] dm-mpath: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE
Date
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) 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()

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 degrades 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.lei@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 c8faa2b85842..8fe3f45407ce 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 9d32f25489c2..cbe8a06ef8b0 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
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