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). But sure, I'm not opposed to you carrying this line of work forward. I can always lend a hand if you need help later or if you need to hand it off to me. > >I think you probably just got caught out by the recursive nature of the bio > >submission path -- makes creating a graph of submitted bios and their > >associated per-bio-data and generated cookies a mess to track (again, > >like herding cats). > > > >Could also be you didn't quite understand the DM code's various > >structures. > > > >In any case, the block core changes needed to make bio-based IO polling > >work is the main limiting factor right now. > > Yes the logic is kind of subtle and maybe what I'm concerned here is > really should be concerned at the coding phase. Definitely, lots of little details and associations. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
