[PATCHv3 8/8] cgroup: Add documentation for cgroup namespaces

Eric W. Biederman ebiederm at xmission.com
Wed Jan 7 14:45:05 UTC 2015


Richard Weinberger <richard at nod.at> writes:

> Am 07.01.2015 um 00:20 schrieb Aditya Kali:
>> I understand your point. But it will add some complexity to the code.
>> 
>> Before trying to make it work for non-unified hierarchy cases, I would
>> like to get a clearer idea.
>> What do you expect to be mounted when you run:
>>   container:/ # mount -t cgroup none /sys/fs/cgroup/
>> from inside the container?
>> 
>> Note that cgroup-namespace wont be able to change the way cgroups are
>> mounted .. i.e., if say cpu and cpuacct subsystems are mounted
>> together at a single mount-point, then we cannot mount them any other
>> way (inside a container or outside). This restriction exists today and
>> cgroup-namespaces won't change that.
>
> I wondered why cgroup namespaces won't change that and looked at your patches
> in more detail.
> What you propose as cgroup namespace is much more a cgroup chroot() than
> a namespace.
> As you pass relative paths into the namespace you depend on the mount structure
> of the host side.
> Hence, the abstraction between namespaces happens on the mount paths of the initial
> cgroupfs. But we really want a new cgroupfs instance within a container and not just
> a cut out of the initial cgroupfs mount.
>
> I fear you approach is over simplified and won't work for all cases. It may work
> for your specific use case at Google but we really want something generic.
> Eric, what do you think?

I think I probably need to go back upthread and read the patches.

I think it is a reasonable practical requirement that a widely used long
term supported distribution like RHEL 7 needs to be able to run in a linux
container bizarre init system and all.  And that we the abstractions
should be that that we should be able to migrate such a beast.

There are a couple of issues in play and I think we need actual testing
rather than reports that something shouldn't work before we reject a set
of patches.    Aditya in one of his replies to me has reported a
configuration that he expects will work.  So I think that configuration
needs to be tested.

cgroups is a weird beast and the problems tend not to lie where a person
would first expect.

I suspect no one strongly cares if the cgroup hierarchy is unified or
not.  By unified hierarchy I mean that  every mount of cgroupfs has the
same directories with the same processes in each directory.

I do think people will care which controllers will show up in differ
mounts of cgroupfs, and I think that is relevant to process migration.




I am going to segway into scope of what is achievable with a cgroup namespace.

- If there are files in cgroupfs that are not safe to delegate we can
  not support those files in a container. 

  Last I looked there were such files and systemd used them.

- Which controllers share hierarchies of processes to track resources is
  a core cgroup issue and not a cgroup namespace issue.

  If we find problems with using a unified hierarchy support we need to
  go fix cgroups in general not cgroupfs.

Eric


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