[PATCH RFC v2 0/5] blk-mq/scsi: Provide hostwide shared tags for SCSI HBAs

[John Garry]

This is another stab at solving the problem of hostwide shared tags for SCSI
HBAs.

As discussed previously, Ming Lei's most recent series in [0] to use
hctx[0] tags for all hctx for a host was a bit messy and intrusive, so seen
as a no go. Indeed, blk-mq is designed for separate tags per hctx.

Bart also followed up on my v1 RFC with another implementation along those
same lines, which was neater, but I am concerned that the change in this
approach may cause issues - see [1].

This series introduces a different approach to solve that problem, in
keeping the per-hctx tags but introducing a new separate sbitmap per
tagset. The shared sbitmap is used to generate a unique tag over all hctx per
tagset.

Currently I just fixed up the hisi_sas driver to use the shared tags,
but should not be much trouble to change others over.

Patch #3 is still quite experimental at this point - I added some code
comments on this. I also threw in a minor tidy-up patch.

[0] https://lore.kernel.org/linux-block/20190531022801.10003-1-ming.lei@xxxxxxxxxx/
[1] https://lore.kernel.org/linux-block/ff77beff-5fd9-9f05-12b6-826922bace1f@xxxxxxxxxx/T/#m3db0a602f095cbcbff27e9c884d6b4ae826144be

Differences to v1:
- Use a shared sbitmap, and not a separate shared tags (a big change!)
	- Drop request.shared_tag
- Add RB tags

Hannes Reinecke (1):
  scsi: Add template flag 'host_tagset'

John Garry (3):
  blk-mq: Remove some unused function arguments
  blk-mq: Facilitate a shared sbitmap per tagset
  scsi: hisi_sas: Switch v3 hw to MQ

Ming Lei (1):
  blk-mq: rename BLK_MQ_F_TAG_SHARED as BLK_MQ_F_TAG_QUEUE_SHARED

 block/bfq-iosched.c                    |  4 +-
 block/blk-mq-debugfs.c                 |  6 +-
 block/blk-mq-sched.c                   | 14 +++++
 block/blk-mq-tag.c                     | 80 +++++++++++++++++------
 block/blk-mq-tag.h                     | 18 ++++--
 block/blk-mq.c                         | 87 ++++++++++++++++++++------
 block/blk-mq.h                         |  7 ++-
 block/kyber-iosched.c                  |  4 +-
 drivers/scsi/hisi_sas/hisi_sas.h       |  3 +-
 drivers/scsi/hisi_sas/hisi_sas_main.c  | 36 ++++++-----
 drivers/scsi/hisi_sas/hisi_sas_v3_hw.c | 86 +++++++++++--------------
 drivers/scsi/scsi_lib.c                |  2 +
 include/linux/blk-mq.h                 |  9 ++-
 include/scsi/scsi_host.h               |  3 +
 14 files changed, 239 insertions(+), 120 deletions(-)

-- 
2.17.1