Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy

Jan Kara
2023-02-08T13:45:05Z

On Fri 03-02-23 11:45:32, Bart Van Assche wrote:
> On 2/2/23 17:48, Hou Tao wrote:
> > I don't get it on how to remove IOPRIO_POL_PROMOTION when calculating the final
> > ioprio for bio. IOPRIO_POL_PROMOTION is not used for IOPRIO_CLASS values but
> > used to determinate on how to calculate the final ioprio for bio: choosing the
> > maximum or minimum between blkcg ioprio and original bio bi_ioprio.
>
> Do the block layer code changes shown below implement the functionality
> that you need?

Just one question guys: So with my a78418e6a04c ("block: Always initialize
bio IO priority on submit") none-to-rt policy became effectively a noop as
Hou properly noticed. Are we aware of any users that were broken by this?
Shouldn't we rather fix the code so that none-to-rt starts to operate
correctly again? Or maybe change the none-to-rt meaning to be actually
promote-to-rt?

I have to admit I'm wondering a bit what was the intended usecase behind
the introduction of none-to-rt policy. Can someone elaborate? promote-to-rt
makes some sense to me - we have a priviledged cgroup we want to provide
low latency access to IO but none-to-rt just does not make much sense to
me...

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR