[PATCH RFC v2 0/8] blk-mq: improve tag fair sharing

[Yu Kuai]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Current implementation:
 - a counter active_queues record how many queue/hctx is sharing tags,
 and it's updated while issue new IO, and cleared in
 blk_mq_timeout_work().
 - if active_queues is more than 1, then tags is fair shared to each
 node;

New implementation:
 - a new field 'available_tags' is added to each node, and it's
 calculate in slow path, hence fast path won't be affected, patch 5;
 - a new counter 'busy_queues' is added to blk_mq_tags, and it's updated
 while fail to get driver tag, and it's also cleared in
 blk_mq_timeout_work(), and tag sharing will based on 'busy_queues'
 instead of 'active_queues', patch 6,7;
 - a new counter 'busy_count' is added to each node to record how many
 times a node failed to get driver tag, and it's used to judge if a node
 is busy and need more tags, patch 8;
 - a new timer is added to blk_mq_tags, it will start if any node failed
 to get driver tag, and timer function will be used to borrow tags and
 return borrowed tags, patch 8;

A simple test, 32 tags with two shared node:
[global]
ioengine=libaio
iodepth=2
bs=4k
direct=1
rw=randrw
group_reporting

[sda]
numjobs=32
filename=/dev/sda

[sdb]
numjobs=1
filename=/dev/sdb

Test result(monitor new debugfs entry):

time    active          available
        sda     sdb     sda     sdb
0       0       0       32      32
1       16      2       16      16      -> start fair sharing
2       19      2       20      16
3       24      2       24      16
4       26      2       28      16      -> borrow 32/8=4 tags each round
5       28      2       28      16      -> save at lease 4 tags for sdb

Yu Kuai (8):
  blk-mq: factor out a structure from blk_mq_tags
  blk-mq: factor out a structure to store information for tag sharing
  blk-mq: add a helper to initialize shared_tag_info
  blk-mq: support to track active queues from blk_mq_tags
  blk-mq: precalculate available tags for hctx_may_queue()
  blk-mq: add new helpers blk_mq_driver_tag_busy/idle()
  blk-mq-tag: delay tag sharing until fail to get driver tag
  blk-mq-tag: allow shared queue/hctx to get more driver tags

 block/blk-core.c       |   2 -
 block/blk-mq-debugfs.c |  30 +++++-
 block/blk-mq-tag.c     | 226 +++++++++++++++++++++++++++++++++++++++--
 block/blk-mq.c         |  12 ++-
 block/blk-mq.h         |  64 +++++++-----
 include/linux/blk-mq.h |  36 +++++--
 include/linux/blkdev.h |  11 +-
 7 files changed, 328 insertions(+), 53 deletions(-)

-- 
2.39.2