[PATCHSET cgroup/for-3.15] cgroup: implement unified hierarchy

[Tejun Heo]

Hello,

(this is too late for the upcoming merge window and is targeting the
next devel cycle)

cgroup currently allows creating arbitrary number of hierarchies and
any number of controllers may be associated with a given tree.  This
allows for huge amount of variance how tasks are associated with
various cgroups and controllers; unfortunately, the variance is
extreme to the extent that it unnecessarily complicates capabilities
which can otherwise be straight-forward and hinders implementation of
features which can benefit from coordination among different
controllers.

Here are some of the issues which we're facing with the current
multiple hierarchies.

* cgroup membership of a task can't be described in finite number of
  paths.  As there can be arbitrary number of hierarchies, the key
  describing a task's cgroup membership can be arbitrarily long.  This
  is painful when userland or other parts of the kernel needs to take
  cgroup membership into account and leads to proliferation of
  controllers which are just there to identify membership rather than
  actually control resources, which in turn exacerbates the problem.

* Different controllers may or may not reside on the same hierarchy.
  Features or optimizations which can benefit from sharing the
  hierarchical organization either can't be implemented or becomes
  overly complicated.

* Tasks of a process may belong to different cgroups, which doesn't
  make any sense for some controllers.  Those controllers end up
  ignoring such configurations in their own ways leading to
  inconsistent behavior.  In addition, in-process resource control
  fundamentally isn't something which belongs to cgroup.  As it has to
  be visible to the binary for the process, it must be part of the
  stable programming interface which is easily accessible to the
  process proper in an easy race-free way.

* The current cgroup allows cgroups which have child cgroups to have
  tasks in it.  This means that the child cgroups end up competing
  against the internal tasks.  This introduces inherent ambiguity as
  the two are separate types of entities and the latter doesn't have
  the same control knobs assigned to them.

  Different controllers are dealing with the issue in different ways.
  cpu treats internal tasks and child cgroups as equivalents, which
  makes giving a child cgroup a given ratio of the parent's cpu time
  difficult as the number of competing entities may fluctuate without
  any indication.  blkio, in my misguided attempt to deal with the
  issue, introduced a whole duplicate set of knobs for internal tasks
  and deal with them as if they belong to a separate child cgroup
  making the interface and implementation a mess.  memcg seems
  somewhat ambiguous on the issue but there are attempts to introduce
  ad-hoc modifications to tilt the way it's handled to suit specific
  use cases.

  This is an inherent problem.  All of the solutions that different
  controllers came up with are unsatisfactory, the different behaviors
  greatly increases the level of inconsistency and complicates the
  controller implementations.

This patchset finally implements the default unified hierarchy.  The
goal is providing enough flexibility while enforcing stricter common
structure where appropriate to address the above listed issues.

Controllers which aren't bound to other hierarchies are
automatically attached to the unified hierarchy, which is different in
that controllers are enabled explicitly for each subtree.
"cgroup.subtree_control" controls which controllers are enabled on the
child cgroups.  Let's assume a hierarchy like the following.

  root - A - B - C
               \ D

root's "cgroup.subtree_control" determines which controllers are
enabled on A.  A's on B.  B's on C and D.  This coincides with the
fact that controllers on the immediate sub-level are used to
distribute the resources of the parent.  In fact, it's natural to
assume that resource control knobs of a child belong to its parent.
Enabling a controller in "cgroup.subtree_control" declares that
distribution of the respective resources of the cgroup will be
controlled.  Note that this means that controller enable states are
shared among siblings.

The default hierarchy has an extra restriction - only cgroups which
don't contain any task may have controllers enabled in
"cgroup.subtree_control".  Combined with the other properties of the
default hierarchy, this guarantees that, from the view point of
controllers, tasks are only on the leaf cgroups.  In other words, only
leaf csses may contain tasks.  This rules out situations where child
cgroups compete against internal tasks of the parent.

This patchset contains the following twelve patches.

 0001-cgroup-update-cgroup-subsys_mask-to-child_subsys_mas.patch
 0002-cgroup-introduce-effective-cgroup_subsys_state.patch
 0003-cgroup-implement-cgroup-e_csets.patch
 0004-cgroup-make-css_next_child-skip-missing-csses.patch
 0005-cgroup-reorganize-css_task_iter.patch
 0006-cgroup-teach-css_task_iter-about-effective-csses.patch
 0007-cgroup-cgroup-subsys-should-be-cleared-after-the-css.patch
 0008-cgroup-allow-cgroup-creation-and-suppress-automatic-.patch
 0009-cgroup-add-css_set-dfl_cgrp.patch
 0010-cgroup-update-subsystem-rebind-restrictions.patch
 0011-cgroup-prepare-migration-path-for-unified-hierarchy.patch
 0012-cgroup-implement-dynamic-subtree-controller-enable-d.patch

0001 updates subsys_mask handling again to morph cgrp->subsys_mask to
cgrp->child_subsys_mask.

0002-0003 introduce effective cgroup.  The cgroup on the unified
hierarchy a task belongs to when viewed from a controller.

0004-0007 update iterators to handle effective cgroup correctly.

0008-0011 prepare various paths for explicit controller
enable/disable.

0012 implements explicit controller enable/disable.

The patchset is on top of cgroup/for-3.15 01a971406177 ("cgroup: Use
RCU_INIT_POINTER(x, NULL) in cgroup.c") and also available in the
following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git review-unified

diffstat follows.

 include/linux/cgroup.h |   44 ++-
 kernel/cgroup.c        |  651 +++++++++++++++++++++++++++++++++++++++++--------
 2 files changed, 585 insertions(+), 110 deletions(-)

Thanks.

--
tejun

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