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

[Bart Van Assche <bvanassche@acm.org>]

On 8/11/21 10:41 AM, Tejun Heo wrote:
>  From e150c6478e453fe27b5cf83ed5d03b7582b6d35e Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Wed, 11 Aug 2021 07:29:20 -1000
> 
> This reverts commit 08a9ad8bf607 ("block/mq-deadline: Add cgroup support")
> and a follow-up commit c06bc5a3fb42 ("block/mq-deadline: Remove a
> WARN_ON_ONCE() call"). The added cgroup support has the following issues:
> 
> * It breaks cgroup interface file format rule by adding custom elements to a
>    nested key-value file.
> 
> * It registers mq-deadline as a cgroup-aware policy even though all it's
>    doing is collecting per-cgroup stats. Even if we need these stats, this
>    isn't the right way to add them.
> 
> * It hasn't been reviewed from cgroup side.

Agreed that I should have Cc-ed you on the cgroup patches. But where 
were you while my mq-deadline patch series was out for review? The first 
version of that patch series was published on May 27 and the patch 
series was merged on June 21 so there was almost one month time to post 
review feedback.

Additionally, the above description is not very helpful. If it is not 
allowed to add custom elements by adding more pd_stat_fn callbacks, why 
does that callback even exist? Why does the cgroup core not complain if 
a new policy is registered that defines a pd_stat_fn callback?

You write that this isn't the right way to collect per cgroup stats. 
What is the "right way"? Has this been documented somewhere?

Bart.