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).
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.
Thanks. I will continue working on this.
Jeffle
