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SubjectRe: Attaching a cgroup subsystem to multiple hierarchies
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Hi Michal

On 6 Feb 2014, at 18:59, Michal Hocko <mhocko@suse.cz> wrote:

> On Wed 05-02-14 14:39:52, Glyn Normington wrote:
>> Reading cgroups.txt and casting around the net leads me to believe
>> that it is possible to attach a cgroup subsystem (e.g. memory) to
>> multiple hierarchies, but this seems to result in “mirrored”
>> hierarchies which are automatically kept in step with each other -
>> essentially it looks like the same hierarchy at multiple file system
>> paths.
>>
>> Take the following interaction for example:
>>
>> \begin{verbatim}
>> $ pwd
>> /home/vagrant
>> $ mkdir mem1
>> $ mkdir mem2
>> $ sudo su
>> # mount -t cgroup -o memory none /home/vagrant/mem1
>> # mount -t cgroup -o memory none /home/vagrant/mem2
>> # cd mem1
>> # mkdir inst1
>> # ls inst1
>> cgroup.clone_children memory.failcnt ...
>> # ls ../mem2
>> cgroup.clone_children inst1 memory.limit_in_bytes ...
>> # cd inst1
>> # echo 1000000 > memory.limit_in_bytes
>> # cat memory.limit_in_bytes
>> 1003520
>> # cat ../../mem2/inst1/memory.limit_in_bytes
>> 1003520
>> # echo $$ > tasks
>> # cat tasks
>> 1365
>> 1409
>> # cat ../../mem2/inst1/tasks
>> 1365
>> 1411
>>
>> Is this working as intended?
>
> Yes, it doesn't make any sense to have two different views on the same
> controllers.

Then wouldn’t it be better for the second mount to fail?

>
>> Is there some other way to attach a subsystem to *distinct*
>> hierarchies?
>
> No.
>
>> Distinct hierarchies would allow distinct cgroups, distinct settings
>> (e.g. memory.limit_in_bytes) and distinct partitions of the tasks in
>> the system.
>
> Which one should be applied then?

Good question. All of them, I would say: the constraints due to distinct settings would be ANDed together.

The implementation would be more complex and less efficient as a subsystem's resources consumed by a process would need charging against each hierarchy to which the subsystem was attached.

I very much doubt this would be worth implementing and I’m not at all suggesting it.

>
>>
>> Note: I don’t have a good use for this function - I’m simply
>> trying to reverse engineer the semantics of cgroups to get a precise
>> understanding.
>
> I think there is no need to reverse engineer ;)
> Documentation/cgroups/cgroups.txt in the kernel tree does give a decent
> description IMO.

I disagree. For example, cgroups.txt does not clearly state whether or not a single subsystem may be attached to distinct hierarchies.

This seems to have caused confusion elsewhere. For example, Red Hat write “… a single subsystem can be attached to two hierarchies if both of those hierarchies have only that subsystem attached.” ([1]).

>
> --
> Michal Hocko
> SUSE Labs

Regards,
Glyn

[1] See “Rule 2” on p. 10 of https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/pdf/Resource_Management_Guide/Red_Hat_Enterprise_Linux-6-Resource_Management_Guide-en-US.pdf--
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