Date:	Wed, 16 Apr 2014 10:35:39 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCHSET cgroup/for-3.16] cgroup: implement unified hierarchy,
 v2

On 2014/4/15 5:36, Tejun Heo wrote:
> Hello,
> 
> This is v2 of the unified hierarchy patchset.  Changes from v1[1] are,
> 
> * Rebased on top of v3.15-rc1
> 
> * Interface file "cgroup.controllers" which was only available in the
>   root is now available in all cgroups.  This allows, e.g., a
>   sub-manager in charge of a subtree to tell which controllers are
>   available to it.
> 
> 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
> 

Acked-by: Li Zefan <lizefan@...wei.com>

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