[dm-devel] [RFC 0/3] Add support of iopoll for dm device

[JeffleXu]

On 11/2/20 11:28 PM, Mike Snitzer wrote:
> On Sun, Nov 01 2020 at 10:14pm -0500,
> JeffleXu <jefflexu at linux.alibaba.com> 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.

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.


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.


Now we have to maintain one data structure for every original bio, 
something like

```

struct dm_poll_instance {

     ...

     struct list_head cookies;

};

```


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;

     }

     ...

}

```



-- 
Jeffle
Thanks