[dm-devel] [PATCH 1/2] block: add resubmit_bio_noacct()

[Ming Lei]

On Mon, Jan 10, 2022 at 09:35:22AM -0800, Christoph Hellwig wrote:
> On Mon, Jan 10, 2022 at 03:51:40PM +0800, Ming Lei wrote:
> > Add block layer API of resubmit_bio_noacct() for handling blk-throttle
> > iops limit correctly. Typical use case is that bio split, and it isn't
> > good to export blk_throtl_charge_bio_split() for drivers, so add new API
> > for serving such purpose.
> 
> Umm, submit_bio_noacct is meant exactly for this case of resubmitting
> a bio.  We should not need another API for that.

Follows the background of the issue first:

1) IOPS limit throttle needs to cover split bio since iostat accounts
split bio actually, and user space setup iops limit throttle by the
feedback from iostat/diskstat;

2) block throttle is block layer internal stuff, and we shouldn't expose
blk_throtl_charge_bio_split() to driver.

Maybe rename the new API as submit_split_bio_noacct(), but we can't
reuse submit_bio_noacct() simply, otherwise blk_throtl_charge_bio_split()
needs to be exported.

Another ides is to clearing BIO_THROTTLED before calling submit_bio_noacct(),
meantime blk-throttle code needs to change to avoid double accounting of bio
bytes, so the caller of submit_bio_noacct() still needs some change.
This way can get smooth IOPS throttle, but needs to call __blk_throtl_bio
for split bio one more time.

Or other idea for this bio split vs. iops limit issue?


Thanks,
Ming