Re: [PATCH RFC cgroup/for-3.7] cgroup: mark subsystems with brokenhierarchy support and whine if cgroups are nested for them

From: Tejun Heo
Date: Tue Sep 11 2012 - 14:22:08 EST

Hello, Vivek.

On Tue, Sep 11, 2012 at 02:16:00PM -0400, Vivek Goyal wrote:
> Ok, so whole point of warning seems to be so that we can change the
> behavior in future and say to user space they few kernel releases back we
> had started printing a warning that creating hierarchy is wrong and
> move to a flat setup. So don't complain to us now.?

Yes, pretty much. At the moment, it's simply broken.

> Are you planning to get rid of .user_hierarchy file from memory cgroup
> too? If you are planning not to put such a file in blkio controller,
> then it will make sense to remove it from mem_cgorup too.

Yes, or at least make it RO 1 eventually.

> The point I am trying to make is that deep hierarchies (5-6 levels) are
> /going to be a reality and if accounting overhead is not manageable then
> enabling hierarchy by default might not be a practical solution even
> if you implement hierarchy support (like memory cgroup), and in that
> case retaining .use_hierarchy will make sense.

That doesn't make any sense to me. If you don't want feature and
overhead of hierarchy, you just need to not create a hierarchy. If
hierarchical behavior isn't needed, why create hierarchy at all?

> IIUC, are you saying that now none of the controller will have flat
> hiearchy support because there is no way to be able to create flat
> hierarchy. (Any new group is child of root group). So are we moving
> towards a model where every controller is hierarhical and there is
> no concept of flat hierarchy.



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