[PATCH V5 00/14] blk-mq-sched: improve sequential I/O performance(part 1)

From: Ming Lei
Date: Sat Sep 30 2017 - 06:28:00 EST


Hi Jens,

In Red Hat internal storage test wrt. blk-mq scheduler, we
found that I/O performance is much bad with mq-deadline, especially
about sequential I/O on some multi-queue SCSI devcies(lpfc, qla2xxx,
SRP...)

Turns out one big issue causes the performance regression: requests
are still dequeued from sw queue/scheduler queue even when ldd's
queue is busy, so I/O merge becomes quite difficult to make, then
sequential IO degrades a lot.

This issue becomes one of mains reasons for reverting default SCSI_MQ
in V4.13.

The 1st patch takes direct issue in blk_mq_request_bypass_insert(),
then we can improve dm-mpath's performance in part 2, which will
be posted out soon.

The 2nd six patches improve this situation, and brings back
some performance loss.

With this change, SCSI-MQ sequential I/O performance is
improved much, Paolo reported that mq-deadline performance
improved much[2] in his dbench test wrt V2. Also performanc
improvement on lpfc/qla2xx was observed with V1.[1]

Please consider it for V4.15.

[1] http://marc.info/?l=linux-block&m=150151989915776&w=2
[2] https://marc.info/?l=linux-block&m=150217980602843&w=2

V5:
- address some comments from Omar
- add Tested-by & Reveiewed-by tag
- use direct issue for blk_mq_request_bypass_insert(), and
start to consider to improve sequential I/O for dm-mpath
- only include part 1(the original patch 1 ~ 6), as suggested
by Omar

V4:
- add Reviewed-by tag
- some trival change: typo fix in commit log or comment,
variable name, no actual functional change

V3:
- totally round robin for picking req from ctx, as suggested
by Bart
- remove one local variable in __sbitmap_for_each_set()
- drop patches of single dispatch list, which can improve
performance on mq-deadline, but cause a bit degrade on
none because all hctxs need to be checked after ->dispatch
is flushed. Will post it again once it is mature.
- rebase on v4.13-rc6 with block for-next

V2:
- dequeue request from sw queues in round roubin's style
as suggested by Bart, and introduces one helper in sbitmap
for this purpose
- improve bio merge via hash table from sw queue
- add comments about using DISPATCH_BUSY state in lockless way,
simplifying handling on busy state,
- hold ctx->lock when clearing ctx busy bit as suggested
by Bart


Ming Lei (7):
blk-mq: issue rq directly in blk_mq_request_bypass_insert()
blk-mq-sched: fix scheduler bad performance
sbitmap: introduce __sbitmap_for_each_set()
blk-mq: introduce blk_mq_dequeue_from_ctx()
blk-mq-sched: move actual dispatching into one helper
blk-mq-sched: improve dispatching from sw queue
blk-mq-sched: don't dequeue request until all in ->dispatch are
flushed

block/blk-core.c | 3 +-
block/blk-mq-debugfs.c | 1 +
block/blk-mq-sched.c | 104 ++++++++++++++++++++++++++++++++++++-------
block/blk-mq.c | 114 +++++++++++++++++++++++++++++++++++++++---------
block/blk-mq.h | 4 +-
drivers/md/dm-rq.c | 2 +-
include/linux/blk-mq.h | 3 ++
include/linux/sbitmap.h | 64 +++++++++++++++++++--------
8 files changed, 238 insertions(+), 57 deletions(-)

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2.9.5