Re: [PATCH v7 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

[Michal Koutný]

Hello.

On Fri, Aug 27, 2021 at 06:50:10PM -0400, Waiman Long <llong@xxxxxxxxxx> wrote:
> So the new rules will be:

When I followed the thread, it seemed to me you're talking past each
other a bit. I'd suggest the following terminology:

- config space: what's written by the user and saved,

- reality space: what's currently available (primarily subject to
  on-/offlinng but I think it'd be helpful to consider here also what's
  given by the parent),

- effect space: what's actually possible and happening.

Not all elements of config_space x reality_space (Cartesian product) can
be represented in the effect_space (e.g. root partition with no
(effective) cpus).

IIUC, Waiman's "high bar" is supposed to be defined over transitions in
the config_space. However, there can be independent changes in the
reality_space so the rules should be actually formulated in the
effect_space:

The conditions for being a valid partition root rewritten into the effect
space:

> 1) The "cpuset.cpus" is not empty and the list of CPUs are exclusive.
- effective CPUs are non-empty and exclusive wrt siblings
- (E.g. setting empty cpuset.cpus might be possible but it invalidates
  the partition root, same as offlining or removal by an ancestor.)

> 2) The parent cgroup is a partition root (can be an invalid one).
- parent cgroup is a (valid) partition
- (Being valid partition means owning "stolen" cpus from the parent, if
  the parent is not valid partition itself, you can't steal what is not
  owned.)
- (And I think it's OK that: "the child partitions will stay invalid
  forever unless the parent become a valid partition again" [1].)

> 3) The "cpuset.cpus" is a subset of the parent's cpuset.cpus.allowed.
- I'm not sure what is the use of this condition (together with the
  rewrite of the 1st condition which covers effective cpus). I think it
  would make sense if being a valid parition root guaranteed that all
  configured cpuset.cpus will be available, however, that's not the case
  IIUC (e.g. due to offlining).

> 4) No child cgroup with cpuset enabled.
- A child cgroup with cpuset enabled is OK in the effect space
  (achievable by switching first and creating children later).
- For technical reasons this may be a condition on the transitions in
  the config_space.

Generally, most config changes should succeed and user should check (or
watch) how they landed in combination with the reality_space.

Regards,
Michal

[1] This follows the general model where ancestors can "preempt"
resources from their subtree.