[dm-devel] blk-mq DM changes for 3.20 [was: Re: blk-mq request allocation stalls]

[Jens Axboe]

On 01/28/2015 10:44 AM, Mike Snitzer wrote:
> On Wed, Jan 28 2015 at 11:42am -0500,
> Jens Axboe <axboe at kernel.dk> wrote:
>
>> On 01/27/2015 11:42 AM, Mike Snitzer wrote:
>>> Hey Jens,
>>>
>>> I _think_ we've resolved the issues Bart raised for request-based DM's
>>> support for blk-mq devices (anything remaining seems specific to iSER's
>>> blk-mq support which is in development).  Though Keith did have that one
>>> additional patch for that block scatter gather attribute that we still
>>> need to review closer.
>>>
>>> Anyway, I think what we have is a solid start and see no reason to hold
>>> these changes back further.  So I've rebased the 'dm-for-3.20' branch of
>>> linux-dm.git ontop of 3.19-rc6 and reordered the required block changes
>>> to be at the front of the series, see:
>>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=dm-for-3.20
>>>
>>> (these changes have been in Linux next for a month, via linux-dm.git
>>> 'for-next')
>>>
>>> With your OK, I'd be happy to carry the required block changes and
>>> ultimately request Linus pull them for 3.20 (I can backfill your Acks if
>>> you approve).  BUT I also have no problem with you picking up the block
>>> changes to submit via your block tree (I'd just have to rebase ontop of
>>> your 3.20 branch once you pull them in).
>>
>> I'd prefer to take these prep patches through the block tree.
>
> Great, should I send the patches or can you cherry-pick?

I already cherry picked them, they are in the for-3.20/core branch.

>> Only one I don't really like is this one:
>>
>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-for-3.20&id=23556c2461407495099d1eb20b0de43432dc727d
>>
>> I prefer keeping the alloc path as lean as possible, normal allocs
>> always initialize ->bio since they need to associate a bio with it.
>
> Would be very surprised if this initialization were measurable but..

That's what people always say, and then keep piling more crap in...

> I could push this initialization into the DM-mpath driver (just after
> blk_get_request, like Keith opted for) but that seemed really gross.

It's already doing blk_rq_init() now, so not a huge change and not that 
nasty.

-- 
Jens Axboe