Re: [PATCH v4 0/2] block/dm: support bio polling

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Mar 04 2022 at  8:43P -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> On Fri, Mar 04, 2022 at 04:26:21PM -0500, Mike Snitzer wrote:
> > Hi,
> > 
> > I've rebased Ming's latest [1] ontop of dm-5.18 [2] (which is based on
> > for-5.18/block). End result available in dm-5.18-biopoll branch [3]
> > 
> > These changes add bio polling support to DM.  Tested with linear and
> > striped DM targets.
> > 
> > IOPS improvement was ~5% on my baremetal system with a single Intel
> > Optane NVMe device (555K hipri=1 vs 525K hipri=0).
> > 
> > Ming has seen better improvement while testing within a VM:
> >  dm-linear: hipri=1 vs hipri=0 15~20% iops improvement
> >  dm-stripe: hipri=1 vs hipri=0 ~30% iops improvement
> > 
> > I'd like to merge these changes via the DM tree when the 5.18 merge
> > window opens.  The first block patch that adds ->poll_bio to
> > block_device_operations will need review so that I can take it
> > through the DM tree.  Reason for going through the DM tree is there
> > have been some fairly extensive changes queued in dm-5.18 that build
> > on for-5.18/block.  So I think it easiest to just add the block
> > depenency via DM tree since DM is first consumer of ->poll_bio
> > 
> > FYI, Ming does have another DM patch [4] that looks to avoid using
> > hlist but I only just saw it.  bio_split() _is_ involved (see
> > dm_split_and_process_bio) so I'm not exactly sure where he is going
> > with that change. 
> 
> io_uring(polling) workloads often cares latency, so big IO request
> isn't involved usually, I guess. Then bio_split() is seldom called in
> dm_split_and_process_bio(), such as if 4k random IO is run on dm-linear
> or dm-stripe via io_uring, bio_split() won't be run into.
> 
> Single list is enough here, and efficient than hlist, just need
> a little care to delete element from the list since linux kernel doesn't
> have generic single list implementation.

OK, makes sense, thanks for clarifying. But yeah its a bit fiddley for sure.

> > But that is DM-implementation detail that we'll
> > sort out.
> 
> Yeah, that patch also needs more test.

Yeap, sounds good.