[PATCH v2 0/4] blk-flush: optimize non-postflush requests

[chengming.zhou@linux.dev]

From: Chengming Zhou <zhouchengming@bytedance.com>

Hello,

This series optimize flush handling for non-postflush requests. Now we
unconditionally replace rq->end_io to make rq return twice back to the
flush state machine for post-flush.

Obviously, non-postflush requests don't need it, they don't need to
end request twice, so they don't need to replace rq->end_io callback.
And the same for requests with the FUA bit on hardware with FUA support.

The previous approach [1] we take is to move blk_rq_init_flush() to
REQ_FSEQ_DATA stage and only replace rq->end_io if it needs post-flush.

But this way add more magic to the already way too magic flush sequence.
Christoph suggested that we can kill the flush sequence entirely, and
just split the flush_queue into a preflush and a postflush queue.

So this series implement the suggested approach that use two queues:
preflush and postflush requests have separate pending list and running
list, so we know what to do for each request in flush_end_io(), and
we don't need the flush sequence entirely.

Thanks for comments!

[1] https://lore.kernel.org/lkml/20230710133308.GB23157@lst.de/

Chengming Zhou (4):
  blk-flush: flush_rq should inherit first_rq's cmd_flags
  blk-flush: split queues for preflush and postflush requests
  blk-flush: kill the flush state machine
  blk-flush: don't need to end rq twice for non postflush

 block/blk-flush.c      | 181 +++++++++++++++++++++--------------------
 block/blk.h            |   3 +-
 include/linux/blk-mq.h |   1 -
 3 files changed, 96 insertions(+), 89 deletions(-)

-- 
2.41.0