On Wed, Nov 04 2020 at 1:47am -0500, JeffleXu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: > > On 11/2/20 11:28 PM, Mike Snitzer wrote: > >On Sun, Nov 01 2020 at 10:14pm -0500, > >JeffleXu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: > > > >>On 10/27/20 2:53 AM, Mike Snitzer wrote: > >>>What you detailed there isn't properly modeling what it needs to. > >>>A given dm_target_io could result in quite a few bios (e.g. for > >>>dm-striped we clone each bio for each of N stripes). So the fan-out, > >>>especially if then stacked on N layers of stacked devices, to all the > >>>various hctx at the lowest layers is like herding cats. > >>> > >>>But the recursion in block core's submit_bio path makes that challenging > >>>to say the least. So much so that any solution related to enabling > >>>proper bio-based IO polling is going to need a pretty significant > >>>investment in fixing block core (storing __submit_bio()'s cookie during > >>>recursion, possibly storing to driver provided memory location, > >>>e.g. DM initialized bio->submit_cookie pointer to a blk_qc_t within a DM > >>>clone bio's per-bio-data). > >>> > >>>SO __submit_bio_noacct would become: > >>> > >>> retp = &ret; > >>> if (bio->submit_cookie) > >>> retp = bio->submit_cookie; > >>> *retp = __submit_bio(bio); > >>Sorry for the late reply. Exactly I missed this point before. IF you > >>have not started working on this, I'd like to try to implement this as > >>an RFC. > >I did start on this line of development but it needs quite a bit more > >work. Even the pseudo code I provided above isn't useful in the context > >of DM clone bios that have their own per-bio-data to assist with this > >implementation. Because the __submit_bio_noacct() recursive call > >drivers/md/dm.c:__split_and_process_bio() makes is supplying the > >original bio (modified to only point to remaining work). > > Yes I noticed this recently. Since the depth-first splitting > introduced in commit 18a25da84354 > > ("dm: ensure bio submission follows a depth-first tree walk"), one > bio to dm device can be > > split into multiple bios to this dm device. > > ``` > > one bio to dm device (dm0) = one dm_io (to nvme0) + one bio to this > same dm device (dm0) > > ``` > > > In this case we need a mechanism to track all split sub-bios of the > very beginning original bio. Yes, splitting causes additional potential for sub-bios. There are other cases that cause a 1-to-many bio generation (e.g. dm-striped) or splitting cases where a DM target makes use of dm_accept_partial_bio (e.g. dm-snapshot, dm-integrity, dm-writecache, etc). > I'm doubted if this should be implemented in block layer like: > > ``` > > struct bio { > > ... > > struct list_head cookies; > > }; > > ``` > > After all it's only used by bio-based queue, or more specifically > only dm device currently. I do think this line of work really should be handled in block core because I cannot see any reason why MD or bcache or whatever bio-based device wouldn't want the ability to better support io_uring (with IO poll). > Another design I can come up with is to maintain a global data > structure for the very beginning > original bio. Currently the blocking point is that now one original > bio to the dm device (@bio of dm_submit()) can correspond to multiple > dm_io and thus we have nowhere to place the @cookies list. Yes, and that will always be the case. We need the design to handle an arbitrary sprawl of splitting from a given bio. The graph of bios resulting from that fan-out needs to be walked at various levels -- be it the top-level original bio's submit_bio() returned cookie or some intermediate point in the chain of bios. The problem is the lifetime of the data structure created for a given split bio versus layering boundaries (that come from block core's simplistic recursion via bio using submit_bio). > Now we have to maintain one data structure for every original bio, > something like > > ``` > > struct dm_poll_instance { > > ... > > struct list_head cookies; > > }; > > ``` I do think we need a hybrid where at the point of recursion we're able to make the associated data structure available across the recursion boundary so that modeling the association in a chain of split bios is possible. (e.g. struct dm_poll_data or dm_poll_instance as you named it, _but_ that struct definition would live in block core, but would be part of per-bio-data; so 'struct blk_poll_data' is more logical name when elevated to block core). It _might_ be worthwhile to see if a new BIO_ flag could be added to allow augmenting the bio_split + bio_chain pattern to also track this additional case of carrying additional data per-bio while creating bio-chains. I may not be clear yet, said differently: augmenting bio_chain to not only chain bios, but to _also_ thread/chain together per-bio-data that lives within those chained bios. SO you have the chain of bios _and_ the chain of potentially opaque void * that happens to point to a list head for a list of 'struct blk_poll_data'. Does that make sense? > We can transfer this dm_poll_instance between split bios by > bio->bi_private, like > > ``` > > dm_submit_bio(...) { > > struct dm_poll_instance *ins; > > if (bio->bi_private) > > ins = bio->bi_private; > > else { > > ins = alloc_poll_instance(); > > bio->bi_private = ins; > > } > > ... > > } > > ``` Sadly, we cannot (ab)use bi_private for this given its (ab)used via the bio_chain() interface. It's almost like we need to add a new pointer in the bio that isn't left for block core to hijack. There is the well-worn pattern of saving off the original bi_private, hooking a new endio method and then when that endio is called restoring bi_private but we really want to avoid excessive indirect function calls for this usecase. The entire point of implementing blk_poll support is for performance after all. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel