[PATCH v2 00/11] blk-mq: Reduce static requests memory footprint for shared sbitmap

John Garry
2021-08-09T14:38:04Z

Currently a full set of static requests are allocated per hw queue per
tagset when shared sbitmap is used.

However, only tagset->queue_depth number of requests may be active at
any given time. As such, only tagset->queue_depth number of static
requests are required.

The same goes for using an IO scheduler, which allocates a full set of
static requests per hw queue per request queue.

This series changes shared sbitmap support by using a shared tags per
tagset and request queue. Ming suggested something along those lines in
v1. But we'll keep name "shared sbitmap" name as it is fimilar. In using
a shared tags, the static rqs also become shared, reducing the number of
sets of static rqs, reducing memory usage.

Patch "blk-mq: Use shared tags for shared sbitmap support" is a bit big,
and could be broken down. But then maintaining ability to bisect
becomes harder and each sub-patch would get more convoluted.

For megaraid sas driver on my 128-CPU arm64 system with 1x SATA disk, we
save approx. 300MB(!) [370MB -> 60MB]

Baseline is 2112f5c1330a (for-5.15/block) loop: Select I/O scheduler ...

Changes since v1:
- Switch to use blk_mq_tags for shared sbitmap
- Add new blk_mq_ops init request callback
- Add some RB tags (thanks!)

John Garry (11):
blk-mq: Change rqs check in blk_mq_free_rqs()
block: Rename BLKDEV_MAX_RQ -> BLKDEV_DEFAULT_RQ
blk-mq: Relocate shared sbitmap resize in blk_mq_update_nr_requests()
blk-mq: Invert check in blk_mq_update_nr_requests()
blk-mq-sched: Rename blk_mq_sched_alloc_{tags -> map_and_request}()
blk-mq: Pass driver tags to blk_mq_clear_rq_mapping()
blk-mq: Add blk_mq_tag_update_sched_shared_sbitmap()
blk-mq: Add blk_mq_ops.init_request_no_hctx()
scsi: Set blk_mq_ops.init_request_no_hctx
blk-mq: Use shared tags for shared sbitmap support
blk-mq: Stop using pointers for blk_mq_tags bitmap tags

block/bfq-iosched.c | 4 +-
block/blk-core.c | 2 +-
block/blk-mq-debugfs.c | 8 +--
block/blk-mq-sched.c | 92 +++++++++++++------------
block/blk-mq-sched.h | 2 +-
block/blk-mq-tag.c | 111 +++++++++++++-----------------
block/blk-mq-tag.h | 12 ++--
block/blk-mq.c | 144 +++++++++++++++++++++++----------------
block/blk-mq.h | 8 +--
block/kyber-iosched.c | 4 +-
block/mq-deadline-main.c | 2 +-
drivers/block/rbd.c | 2 +-
drivers/scsi/scsi_lib.c | 6 +-
include/linux/blk-mq.h | 20 +++---
include/linux/blkdev.h | 5 +-
15 files changed, 218 insertions(+), 204 deletions(-)

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2.26.2