Hi Jens,

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%.

Baolin has tested previous versions and found performance on MMC can be improved.

Patch 1 ~ 4 are improvement and cleanup, which can't applied without
supporting batching dispatch.

Patch 5 ~ 6 starts to support batching dispatch from scheduler.



[1] https://lore.kernel.org/linux-block/20200512075501.GF1531898@T590/#r (local)
[2] https://lore.kernel.org/linux-block/fe6bd8b9-6ed9-b225-f80c-314746133722@grimberg.me/ (local)

V5:
	- code style changes suggested by Damien

V4:
	- fix releasing budgets and avoids IO hang(5/6)
	- dispatch more batches if the device can accept more(6/6)
	- verified by running more tests

V3:
	- add reviewed-by tag
	- fix one typo
	- fix one budget leak issue in case that .queue_rq returned *_RESOURCE in 5/6

V2:
	- remove 'got_budget' from blk_mq_dispatch_rq_list
	- drop patch for getting driver tag & handling partial dispatch

Hey Ming,

What ever happened to this one?