linux-kernel - Understanding cgroups

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Message-ID: <4CC99A65.9050706@sssup.it>
Date:	Thu, 28 Oct 2010 17:44:37 +0200
From:	Tommaso Cucinotta <tommaso.cucinotta@...up.it>
To:	linux-kernel@...r.kernel.org
CC:	Peter Zijlstra <peterz@...radead.org>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Paul Menage <menage@...gle.com>
Subject: Understanding cgroups

Hi,

I'm trying to get some understanding of the current cgroups in-kernel 
implementation
(after having read Documentation/cgroup* and having browsed a bit the code).
To this purpose, I tried to draw the relationships among the involved 
data structures
(I'm limited to its relationship with [real-time] scheduling), and 
obtained this:

   http://retis.sssup.it/~tommaso/cgroups.odg
   http://retis.sssup.it/~tommaso/cgroups.eps

(You can see in the bottom left part of the diagram a little 
"key/legend/pattern" for representing lists).

I might have done mistakes, however the greatest doubts that I have now 
concern the relative
cardinalities of the various associated items. Namely:
a) why doesn't a cgroup object directly point to a css_set one, but to a 
list of them (via cg_cgroup_list elements) ?
     it seems that a cgroup object may be associated to multiple css_set 
objects, which in turn contain vectors of
     cgroup_subsys_state;
b) however, cgroup.subsys[] would point to a single cgroup_subsys_state 
object per subsys_id, so, what is the
     difference between cgroup.subsys[] and css_set.subsys[] ? (or, are 
these all redundant pointers and point
     to the same cgroup_subsys_state objects ?)
c) is css_set.cg_links used to point to (the head of) a list of 
cg_cgroup_link objects, or is it used to link multiple css_set objects 
into a list ? In the latter case, where is the head of the list pointed 
to from ?

Apologies for the newbie questions that I might have posted; FYI, I'm 
trying to set-up RT scheduling groups without using the VFS-based 
cgroups interface.

Thanks in advance, regards (please, reply in cc to my e-mail address).

     Tommaso

-- 
Tommaso Cucinotta, Computer Engineering PhD, Researcher
ReTiS Lab, Scuola Superiore Sant'Anna, Pisa, Italy
Tel +39 050 882 024, Fax +39 050 882 003
http://retis.sssup.it/people/tommaso

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