cgroups.7: Prefer "controller" over "subsystem"

Replace various uses of "subsystem" with "controller". The
former too was originally used in describing cgroups, but it
is vague to the point of ambiguity. The latter term is a
little less ambiguous.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/cgroups.7

@@ -55,7 +55,7 @@ Subsystems are sometimes also known as
 .IR "resource controllers"
 (or simply, controllers).
 
-The cgroups for a subsystem are arranged in a
+The cgroups for a controller are arranged in a
 .IR hierarchy .
 This hierarchy is defined by creating, removing, and
 renaming subdirectories within the cgroup filesystem.
@@ -69,10 +69,10 @@ by descendant cgroups.
 .\"
 .SS Cgroups version 1 and version 2
 The initial release of the cgroups implementation was in Linux 2.6.24.
-Over time, various cgroup subsystems have been added
+Over time, various cgroup controllers have been added
 to allow the management of various types of resources.
-However, the development of these subsystems was largely uncoordinated,
-with the result that many inconsistencies arose between subsystems
+However, the development of these controllers was largely uncoordinated,
+with the result that many inconsistencies arose between controllers
 and management of the cgroup hierarchies became rather complex.
 (A longer description of these problems can be found in
 the kernel source file
@@ -127,7 +127,7 @@ written to, reflecting resource limits and a few general cgroup
 properties.
 
 In addition, in cgroups v1,
-cgroups can be mounted with no bound subsystem, in which case
+cgroups can be mounted with no bound controller, in which case
 they serve only to track processes.
 An example of this is the
 .I name=systemd
@@ -154,23 +154,23 @@ Cgroups v2 allows manipulation of cgroup membership only for processes
 all threads in the process).
 .\"
 .SS Mounting
-To be available, a given cgroup subsystem must be compiled into the
+To be available, a given cgroup controller must be compiled into the
 kernel.
-Since they are exposed through a virtual filesystem, subsystems
+Since they are exposed through a virtual filesystem, controllers
 must be mounted before they can be controlled.
 The usual place for this is under
 .IR /sys/fs/cgroup .
-If all the desired subsystems can be comounted,
+If all the desired controllers can be comounted,
 then one can do so with the following command:
 
     mount \-t cgroup \-o all cgroup /sys/fs/cgroup
 
 (One can achieve the same result by omitting
 .IR "\-o all" ,
-since it is the default if subsystems are explicitly specified.)
+since it is the default if controllers are explicitly specified.)
 
-If multiple, separately mounted subsystems are desired, then this is
-usually done in per-subsystem subdirectories.
+If multiple, separately mounted controllers are desired, then this is
+usually done in per-controller subdirectories.
 This requires first mounting a tmpfs under
 .I /sys/fs/cgroup
 so that subdirectories can be created.
@@ -191,12 +191,12 @@ done
 .in
 .fi
 
-Comounting subsystems has the effect that a process is in the same cgroup for
-all comounted subsystems.
-Separately mounting subsystems allows a process to
+Comounting controllers has the effect that a process is in the same cgroup for
+all comounted controllers.
+Separately mounting controllers allows a process to
 be in cgroup
 .I /foo1
-for one subsystem while being in
+for one controller while being in
 .I /foo2/foo3
 for another.
 .\"
@@ -334,8 +334,8 @@ For more changes, please see the
 .I Documentation/cgroup-v2.txt
 file in the kernel source.
 .\"
-.SS Cgroups version 1 subsystems
-Each of the cgroups version 1 subsystems is governed
+.SS Cgroups version 1 controllers
+Each of the cgroups version 1 controllers is governed
 by a kernel configuration option (listed below).
 Additionally, the availability of the cgroups feature is governed by the
 .BR CONFIG_CGROUPS
@@ -510,7 +510,7 @@ For each cgroup hierarchy of which the process is a member,
 there is one entry containing three
 colon-separated fields of the form:
 
-     hierarchy-ID:subsystem-list:cgroup-path
+     hierarchy-ID:controller-list:cgroup-path
 
 For example:
 .nf
@@ -530,7 +530,7 @@ that can be matched to a hierarchy ID in
 For the cgroups version 2 hierarchy, this field contains the value 0.
 .IP 2.
 For cgroups version 1 hierarchies,
-this field contains a comma-separated list of the subsystems
+this field contains a comma-separated list of the controllers
 bound to the hierarchy.
 For the cgroups version 2 hierarchy, this field is empty.
 .IP 3.