Hello, Michal.

On Wed, Jun 27, 2012 at 02:51:19PM +0200, Michal Hocko wrote:

quoted

Yeah, this is something I'm seriously considering doing from cgroup
core.  ie. generating a warning message if the user nests cgroups w/
controllers which don't support full hierarchy.

This is a good idea.

And I want each controller either to do proper hierarchy or not at all
and disallow switching the behavior while mounted - at least disallow
switching off hierarchy support dynamically.

quoted

Just disallow clearing .use_hierarchy if it was mounted with the
option? 

Dunno, mount option just doesn't feel right. We do not offer other
attributes to be set by them so it would be just confusing. Besides that
it would require an integration into existing tools like cgconfig which
is yet another pain just because of something that we never promissed to
keep a certain way. There are many people who don't work with mount&fs
cgroups directly but rather use libcgroup for that...

If the default behavior has to be switched without extra input from
userland, that should be noisy like hell and slow.  e.g. generate
warning messages whenever userland does something which is to be
deprecated - nesting when .use_hierarchy == 0, mixing .use_hierarchy
== 0 / 1, and maybe later on, using .use_hierarchy == 0 at all.

Hmm.... we need to switch other controllers over to hierarchical
behavior too.  We may as well just do it from cgroup core.  Once we
rule out all users of pseudo hierarchy - nesting with controllers
which don't support hierarchy - switching on hierarchy support
per-controller shouldn't cause much problem.

How about the following then?

* I'll add a way for controllers to tell cgroup core that full
  hierarchy support is supported and a cgroup mount option to enable
  hierarchy (cgroup core itself already uses a number of mount options
  and libgroup or whatever should be able to deal with it).

  cgroup will refuse to mount if the hierarchy option is specified
  with a controller which doesn't support hierarchy and it will also
  whine like crazy if the userland tries to nest without the mount
  option specified.

  Each controller must enforce hierarchy once so directed by cgroup
  mount option.

* While doing that, all applicable controllers will be updated to
  support hierarchy.

* After sufficient time has passed, nesting without the mount option
  specified will be failed by cgroup core.

As for memcg's .use_hierarchy, make it RO 1 if the cgroup indicates
that hierarchy should be used.  Otherwise, I don't know but make sure
it gets phased out out use somehow.

Thanks.

-- 
tejun

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