[RFC PATCH] dm: fix excessive dm-mq context switching

[Sagi Grimberg]

>> Hi Mike,
>>
>> So I gave your patches a go (dm-4.6) but I still don't see the
>> improvement you reported (while I do see a minor improvement).
>>
>> null_blk queue_mode=2 submit_queues=24
>> dm_mod blk_mq_nr_hw_queues=24 blk_mq_queue_depth=4096 use_blk_mq=Y
>>
>> I see 620K IOPs on dm_mq vs. 1750K IOPs on raw nullb0.
>
> blk_mq_nr_hw_queues=24 isn't likely to help you (but with these patches,
> the first being the most important, it shouldn't hurt either provided
> you have 24 cpus).

I tried with less but as you said, it didn't have an impact...

> Could be you have multiple NUMA nodes and are seeing problems from that?

I am running on a dual socket server, this can most likely be the
culprit...

> I have 12 cpus (in the same physical cpu) and only a single NUMA node.
> I get the same results as blk_mq_nr_hw_queues=12 with
> blk_mq_nr_hw_queues=4 (same goes for null_blk submit_queues).
> I've seen my IOPs go from ~950K to ~1400K.  The peak null_blk can get on
> my setup is ~1950K.  So I'm still seeing a ~25% drop with dm-mq (but
> that is much better than the over 50% drop I saw seeing).

That's what I was planning on :(

>> Is there something I'm missing?
>
> Not sure, I just emailed out all my patches (and cc'd you).  Please
> verify you're using the latest here (same as 'dm-4.6' branch):
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/log/?h=for-next
>
> I rebased a couple times... so please diff what you have tested against
> this latest 'dm-4.6' branch.

I am. I'll try to instrument what's going on...