Re: [PATCH v11 7/8] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

[Michal Koutný]

On Tue, Jun 28, 2022 at 04:10:29AM +0900, Tejun Heo <tj@xxxxxxxxxx> wrote:
> What I'm trying to say is that cpuset.cpus of child_1 and child_2 are
> owned by the parent,

Cf

On Mon, Jun 13, 2022 at 08:00:56AM -1000, Tejun Heo <tj@xxxxxxxxxx> wrote:
> On Mon, Jun 13, 2022 at 07:55:49PM +0200, Michal Koutný wrote:
> > I don't think child_*/cpuset.cpus must be owned by root.
> 
> I meant the parent.

I'm slightly confused.

> so a feature which blocks siblings from intersecting each other
> doesn't make whole lot of sense because all those files are under the
> control of the parent who would have the power to enable or disable
> the restrition anyway.

file				owner
parent/				user (mkdir)
`- cpuset.cpus			root
`- cpuset.cpus.partition	root	(P)
`- child_1/			user
  ` cpuset.cpus			user	(*)
`- child_2/			user
  ` cpuset.cpus			user	(*)

The writes to child cpuset.cpus may/may not invalidate parent's (P)
partition validity (whether a cpu is left to it to host possible tasks).
child_1 vs child_2 overlap affects only whether the children cgroups are
a valid partition.

I think you mean: writes to children cpuset.cpus should be allowed,
possible exclusivity violation should be reported in
parent/cpuset.cpus.partition.

What I thought was OK: prevent (fail) writes to children cpuset.cpus
that'd violate the exclusivity (or would take the last cpu from parent
if it's necessary to host a task).
IMO, it's similar to failed writes to parent/cgroup.subtree_control in a
delegated subtree if the parent still has some tasks (that'd violate
internal node constraint).

What I think might still be OK: allow writes to children cpuset.cpus
that violate exclusivity and report that in children's
cpuset.cpus.partition. Writes that'd take last cpu from parent should
still fail (similar to the failing subtree_control writes above).

Hope that clarifies,
Michal