Re: [PATCH block-5.14] Revert "block/mq-deadline: Add cgroup support"

[Tejun Heo <tj@kernel.org>]

Hello, Damien.

On Fri, Aug 13, 2021 at 02:18:04AM +0000, Damien Le Moal wrote:
> Command duration limits (CDL) and Sequestered commands features are being
> drafted in SPC/SBC and ACS to give the device better hints than just a on/off
> high priority bit. I am currently prototyping these features and I am reusing
> the ioprio interface for that. Here is how this works:

And I think ioprio would be the right interface for it, combined with
some mechanism which can limit which class can take up how many of the
available command slots. Without that, it'd be really easy for anyone
to saturate the command queues and expressing priorities inside the
device won't do much.

> CDL can completely replace the existing binary on/off NCQ priority in a more
> flexible manner as the user can set different duration limits for high vs low
> priority. E.g. high priority is equivalent to a very short limit while low
> priority is equivalent to longer or no limits.

The problem with complex optional hardware features is often the
accompanying variability in terms of availability, reliability and
behavior. The track record has been pretty sad. That isn't to say this
won't be useful for anybody but it'd need careful coordination in
terms of picking hardware vendor and model and ensuring vendor
support, which kinda severely limits the usefulness.

Thanks.

-- 
tejun