linux-kernel - Re: [PATCH 0/7] cgroups: Allow to bind/unbind subsystems to/from non-trival hierarchy

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Date:	Thu, 28 Oct 2010 16:33:41 -0700
From:	Paul Menage <menage@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"akpm >> Andrew Morton" <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	containers@...ts.linux-foundation.org
Subject: Re: [PATCH 0/7] cgroups: Allow to bind/unbind subsystems to/from
 non-trival hierarchy

On Sun, Oct 24, 2010 at 6:07 PM, Li Zefan <lizf@...fujitsu.com> wrote:
>
> Paul had a patch to allow some subsystems to be added to multi-hierarchies,
> which may help.

I more or less dropped it since I couldn't see a big use for it - all
the examples I had were more easily implemented as basic cgroup
control files (and hence available in all hierarchies automatically)

> But it forbids accessing t->cgroups, which makes this
> feature of limited use.

Not exactly - it meant that you couldn't do a fast dereference via
t->cgroups to get to your cgroup_subsys_state, since there was no
longer a unique css for each task for that subsystem - there would be
one per mounted instance of the subsystem. Instead it became necessary
to follow a chain of pointers to find the appropriate css for the
hierarchy in question.

In general this also means that it's harder for the multi-bound
subsystem have any real-world meaning - it's mostly useful for
stateless subsystems, or ones where the cgroup_subsys_state contents
have no direct connection with machine-wide data.

Paul
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