Re: can we drop the bio based path in null_blk

[Jens Axboe <axboe@kernel.dk>]

On 1/24/24 1:31 PM, Pankaj Raghav (Samsung) wrote:
> On Tue, Jan 23, 2024 at 09:49:42AM +0100, Christoph Hellwig wrote:
>> As we found out recently null_blk never splits bios in bio mode, thus
>> ignoring a lot of it's paramters and having buggy zoned device
>> handling.
>>
>> Is there any good reason to keep this mode around given that all relevant
>> hardware drivers use blk-mq, and the non-so-relevant ones not using
>> blk-mq probably should?
> 
> The subject says removing the bio mode in null_blk but here you are
> asking an open question about the non-so-relevant ones should move to
> blk-mq. My input is for the latter part, FWIW.
> 
> I tried to convert brd from bio to using blk-mq last year. One of the
> conclusion we reached was that we will see a performance hit when we use
> blk-mq for RAM backed devices because having tag management, etc was
> adding overhead to these drivers[1](You were also part of this
> discussion, so you probably remember it!).
> 
> Unless there is a mode in blk-mq that can provide a real fast path for
> these drivers, moving to blk-mq might not be possible?

Yeah, by default, blk-mq will do a bunch of things that you don't need,
like the tag generation. We may be able to have a fops->submit_bio()
hook that just puts a request on the stack with the bio stuffed in
there, and have per-cpu hardware queues to avoid any issues there. I
feel like if we did that, then it should be the same performance. That
won't necessarily solve things like queue quisce etc, however... But
it'd be a start, maybe.

-- 
Jens Axboe