Re: [RFC 0/3] Add support of iopoll for dm device

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




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.

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

--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel




[Index of Archives]     [DM Crypt]     [Fedora Desktop]     [ATA RAID]     [Fedora Marketing]     [Fedora Packaging]     [Fedora SELinux]     [Yosemite Discussion]     [KDE Users]     [Fedora Docs]

  Powered by Linux