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

Waiman Long
2022-06-13T03:02:53Z

On 6/12/22 13:49, Tejun Heo wrote:
> Hello,
>
> On Tue, May 10, 2022 at 11:34:12AM -0400, Waiman Long wrote:
>> + External events like hotplug or changes to "cpuset.cpus" can
>> + cause a valid partition root to become invalid and vice versa.
>> + Note that a task cannot be moved to a cgroup with empty
>> + "cpuset.cpus.effective".
>> +
>> + For a valid partition root or an invalid partition root with
>> + the exclusivity rule enabled, changes made to "cpuset.cpus"
>> + that violate the exclusivity rule will not be allowed.
> My memory is failing but this is the same thing that we were discussing
> before, right? The point was that the different behaviors re. system events
> and config actions seemed unncessary and IIRC Michal was of the same opinion
> (please correct me if I'm misremembering).

That is the behavior enforced by setting the CPU_EXCLUSIVE bit in cgroup
v1. I haven't explicitly change it to make it different in cgroup v2.
The major reason is that I don't want change to one cpuset to affect a
sibling partition as it may make the code more complicate to validate if
a partition is valid.

>
>> + A valid non-root parent partition may distribute out all its CPUs
>> + to its child partitions when there is no task associated with it.
> I'm probably forgetting something. Was this necessary because of threaded
> cgroup support because otherwise the above condition is superflous?

The top cpuset cannot have empty cpus.effective whereas the non-root
partition roots can. Maybe I should reword it to make it more clear.

Thanks,
Longman