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[PATCH 0/5] dm-rq: improve sequential I/O performance

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  [PATCH 0/5] dm-rq: improve sequential I/O performance Ming Lei <ming.lei@redhat.com> - 2017-09-30 13:50 +0200
    [PATCH 2/5] dm-mpath: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE Ming Lei <ming.lei@redhat.com> - 2017-09-30 13:50 +0200

#1742620 — [PATCH 0/5] dm-rq: improve sequential I/O performance

Hi,

This 1st one patch removes one log message which can be triggered
very easily.

The 2nd patch removes the workaround of blk_mq_delay_run_hw_queue()
in case of requeue, this way isn't necessary, and more worse, it
makes BLK_MQ_S_SCHED_RESTART not working, and degarde I/O performance.

The 3rd patch return DM_MAPIO_REQUEUE to dm-rq if underlying request
allocation fails, then we can return BLK_STS_RESOURCE from dm-rq
to blk-mq, so that blk-mq can hold the requests to be dequeued.

The 4th patch is a pre-patch for the 5th one, becasue even though
underlying request allocation succeeds, its queue may be
busy and we can get this feedback from blk_insert_cloned_request()
now. This patch trys to cache the allocated request so that
it may be reused in next dispatch to underlying queue.

The 5th patch improves sequential I/O performance by returning
STS_RESOURCE if underlying queue is busy.

In the commit log of the 5th patch, I/O IOPS data is provided and
we can see sequential I/O performance is improved a lot with this
patchset.

This patchset depends on the following two patchset:

[1] [PATCH V5 0/7] blk-mq-sched: improve sequential I/O performance(part 1)
    
	https://marc.info/?l=linux-block&m=150676854821077&w=2
    
[2] [PATCH V5 0/8] blk-mq: improve bio merge for none scheduler
    
	https://marc.info/?l=linux-block&m=150677085521416&w=2

Any comments are welcome! 

Thanks,
Ming

Ming Lei (5):
  dm-mpath: remove annoying message of 'blk_get_request() returned -11'
  dm-mpath: don't call blk_mq_delay_run_hw_queue() in case of
    BLK_STS_RESOURCE
  dm-mpath: return DM_MAPIO_REQUEUE in case of rq allocation failure
  dm-mpath: cache ti->clone during requeue
  dm-rq: improve I/O merge by dealing with underlying STS_RESOURCE

 block/blk-mq.c        | 17 +---------------
 drivers/md/dm-mpath.c | 51 ++++++++++++++++++++++++++++++++++------------
 drivers/md/dm-rq.c    | 56 +++++++++++++++++++++++++++++++++++++--------------
 3 files changed, 80 insertions(+), 44 deletions(-)

-- 
2.9.5

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#1742621 — [PATCH 2/5] dm-mpath: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE

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.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 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

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