Re: [PATCH 5/8] dm: always setup ->orig_bio in alloc_io

Ming Lei
2022-04-13T12:26:55Z

On Wed, Apr 13, 2022 at 02:12:47AM -0400, Mike Snitzer wrote:
> On Tue, Apr 12 2022 at 9:56P -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
>
> > On Tue, Apr 12, 2022 at 04:52:40PM -0400, Mike Snitzer wrote:
> > > On Tue, Apr 12 2022 at 4:56P -0400,
> > > Ming Lei <ming.lei@redhat.com> wrote:
> > >
> > > > The current DM codes setup ->orig_bio after __map_bio() returns,
> > > > and not only cause kernel panic for dm zone, but also a bit ugly
> > > > and tricky, especially the waiting until ->orig_bio is set in
> > > > dm_submit_bio_remap().
> > > >
> > > > The reason is that one new bio is cloned from original FS bio to
> > > > represent the mapped part, which just serves io accounting.
> > > >
> > > > Now we have switched to bdev based io accounting interface, and we
> > > > can retrieve sectors/bio_op from both the real original bio and the
> > > > added fields of .sector_offset & .sectors easily, so the new cloned
> > > > bio isn't necessary any more.
> > > >
> > > > Not only fixes dm-zone's kernel panic, but also cleans up dm io
> > > > accounting & split a bit.
> > >
> > > You're conflating quite a few things here. DM zone really has no
> > > business accessing io->orig_bio (dm-zone.c can just as easily inspect
> > > the tio->clone, because it hasn't been remapped yet it reflects the
> > > io->origin_bio, so there is no need to look at io->orig_bio) -- but
> > > yes I clearly broke things during the 5.18 merge and it needs fixing
> > > ASAP.
> >
> > You can just consider the cleanup part of this patches, :-)
>
> I will. But your following list doesn't reflect any "cleanup" that I
> saw in your patchset. Pretty fundamental changes that are similar,
> but different, to the dm-5.19 changes I've staged.
>
> > 1) no late assignment of ->orig_bio, and always set it in alloc_io()
> >
> > 2) no waiting on on ->origi_bio, especially the waiting is done in
> > fast path of dm_submit_bio_remap().
>
> For 5.18 waiting on io->orig_bio just enables a signal that the IO was
> split and can be accounted.
>
> For 5.19 I also plan on using late io->orig_bio assignment as an
> alternative to the full-blown refcounting currently done with
> io->io_count. I've yet to quantify the gains with focused testing but
> in theory this approach should scale better on large systems with many
> concurrent IO threads to the same device (RCU is primary constraint
> now).
>
> I'll try to write a bpfrace script to measure how frequently "waiting on
> io->orig_bio" occurs for dm_submit_bio_remap() heavy usage (like
> dm-crypt). But I think we'll find it is very rarely, if ever, waited
> on in the fast path.

The waiting depends on CPU and device's speed, if device is quicker than
CPU, the wait should be longer. Testing in one environment is usually
not enough.

>
> > 3) no split for io accounting
>
> DM's more recent approach to splitting has never been done for benefit
> or use of IO accounting, see this commit for its origin:
> 18a25da84354c6b ("dm: ensure bio submission follows a depth-first tree walk")
>
> Not sure why you keep poking fun at DM only doing a single split when:
> that is the actual design. DM splits off orig_bio then recurses to
> handle the remainder of the bio that wasn't issued. Storing it in
> io->orig_bio (previously io->bio) was always a means of reflecting
> things properly. And yes IO accounting is one use, the other is IO
> completion. But unfortunately DM's IO accounting has always been a
> mess ever since the above commit. Changes in 5.18 fixed that.
>
> But again, DM's splitting has _nothing_ to do with IO accounting.
> Splitting only happens when needed for IO submission given constraints
> of DM target(s) or underlying layers.

What I meant is that the bio returned from bio_split() is only for
io accounting. Yeah, the comment said it can be for io completion too,
but that is easily done without the splitted bio.

>
> All said, I will look closer at your entire set and see if it better
> to go with your approach. This patch in particular is interesting
> (avoids cloning and other complexity of bio_split + bio_chain):
> https://patchwork.kernel.org/project/dm-devel/patch/20220412085616.1409626-6-ming.lei@redhat.com/

That patch shows we can avoid the extra split, also shows that the
splitted bio from bio_split() is for io accounting only.

thanks,
Ming