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Subject[PATCH V10 0/8] blk-mq-sched: improve sequential I/O performance
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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
performance degrades a lot.

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

This 8 patches improve this situation, and brings back 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 performance improvement on lpfc/qla2xx was observed with V1.[1]

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

gitweb:
https://github.com/ming1/linux/commits/blk_mq_improve_scsi_mpath_perf_V10_part1

V10:
- fix hang with kyber used
- address comments from Bart

V9:
- change title of patch1
- rename blk_mq_get_budget() as blk_mq_get_dispatch_budget()
- make check on .get_budget/.put_budget cleaner
- all are suggested by Jens, thank for review

V8:
- introduce helper of blk_mq_get_budget
- handle failure path of get_budget

V7:
- introduce .get_budget/.put_budget, and IO merge gets improved
compared with V6, and in theory this approach is better than the way
taken in block legacy

- drop patch of 'blk-mq-sched: don't dequeue request until all in ->dispatch are flushed'

V6:
- address comments from Christoph
- drop the 1st patch which changes blk_mq_request_bypass_insert(),
which should belong to dm-mpath's improvement
- move ' blk-mq-sched: move actual dispatching into one helper'
as 2nd patch, and use the introduced helper to simplify dispatch
logic
- merge two previous patches into one for improving dispatch from sw queue
- make comment/commit log line width as ~70, as suggested by
Christoph

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 (8):
blk-mq-sched: dispatch from scheduler only after progress is made on
->dispatch
blk-mq-sched: move actual dispatching into one helper
sbitmap: introduce __sbitmap_for_each_set()
block: kyber: check if there is request in ctx in kyber_has_work()
blk-mq: introduce .get_budget and .put_budget in blk_mq_ops
blk-mq-sched: improve dispatching from sw queue
SCSI: allow to pass null rq to scsi_prep_state_check()
SCSI: implement .get_budget and .put_budget for blk-mq

block/blk-mq-sched.c | 148 +++++++++++++++++++++++++++++++++++++++++-------
block/blk-mq-sched.h | 2 +-
block/blk-mq.c | 82 +++++++++++++++++++++++++--
block/blk-mq.h | 22 ++++++-
block/kyber-iosched.c | 2 +-
drivers/scsi/scsi_lib.c | 79 ++++++++++++++++++--------
include/linux/blk-mq.h | 13 +++++
include/linux/sbitmap.h | 64 +++++++++++++++------
8 files changed, 342 insertions(+), 70 deletions(-)

--
2.9.5

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