blk-mq: support batching dispatch from scheduler

Hi Guys,

More and more drivers want to get batching requests queued from
block layer, such as mmc[1], and tcp based storage drivers[2]. Also
current in-tree users have virtio-scsi, virtio-blk and nvme.

For none, we already support batching dispatch.

But for io scheduler, every time we just take one request from scheduler
and pass the single request to blk_mq_dispatch_rq_list(). This way makes
batching dispatch not possible when io scheduler is applied. One reason
is that we don't want to hurt sequential IO performance, becasue IO
merge chance is reduced if more requests are dequeued from scheduler
queue.

Tries to start the support by dequeuing more requests from scheduler
if budget is enough and device isn't busy.

Simple fio test over virtio-scsi shows IO can get improved by 5~10%.

Patches can be found from the following tree too:

	https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-bat...

Patch 1 ~ 7 are improvement and cleanup, which can't applied without
supporting batching dispatch.

Patch 8 ~ 9 starts to support batching dispatch from scheduler.


Please review and comment!


[1] https://lore.kernel.org/linux-block/20200512075501.GF1531...
[2] https://lore.kernel.org/linux-block/fe6bd8b9-6ed9-b225-f8...


Ming Lei (9):
  blk-mq: pass request queue into get/put budget callback
  blk-mq: pass hctx to blk_mq_dispatch_rq_list
  blk-mq: don't predicate last flag in blk_mq_dispatch_rq_list
  blk-mq: move getting driver tag and bugget into one helper
  blk-mq: move .queue_rq code into one helper
  blk-mq: move code for handling partial dispatch into one helper
  blk-mq: remove dead check from blk_mq_dispatch_rq_list
  blk-mq: pass obtained budget count to blk_mq_dispatch_rq_list
  blk-mq: support batching dispatch in case of io scheduler

 block/blk-mq-sched.c    |  96 ++++++++++++++--
 block/blk-mq.c          | 248 +++++++++++++++++++++-------------------
 block/blk-mq.h          |  15 +--
 drivers/scsi/scsi_lib.c |   8 +-
 include/linux/blk-mq.h  |   4 +-
 5 files changed, 226 insertions(+), 145 deletions(-)

Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Baolin Wang <baolin.wang7@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>

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2.25.2