cgroups.7: Rework intro text a little

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

man7/cgroups.7

@@ -28,48 +28,14 @@
 cgroups \- Linux control groups
 .SH DESCRIPTION
 Control cgroups, usually referred to as cgroups,
-are a Linux kernel feature which provides for grouping of processes and
-resource tracking and limitations for those groups.
-While several systems have been introduced to help in configuring and
-managing cgroups, the kernel's cgroup interface is provided through
+are a Linux kernel feature which which allow processes to
+be organized into hierarchical groups whose usage of
+various types of resources can then be limited and monitored.
+The kernel's cgroup interface is provided through
 a pseudo-filesystem called cgroupfs.
 Grouping is implemented in the core cgroup kernel code,
 while resource tracking and limits are implemented in
-a set of per-resource-type subsystems (memory, CPU, and so on) which may be
-enabled as separate hierarchies, or joined into comounted hierarchies.
-
-Each hierarchy constitutes a separate mount of the cgroup filesystem,
-with the subsystems enabled in that hierarchy listed in the mount options.
-For each mounted hierarchy,
-the directory tree mirrors the control group hierarchy.
-Each control group is represented by a directory, with each of its child
-control cgroups represented as a child directory.
-For instance,
-.IR /user/joe/1.session
-represents control group
-.IR 1.session ,
-which is a child of cgroup
-.IR joe ,
-which is a child of
-.IR /user .
-Under each cgroup directory is a set of files which can be read or
-written to, reflecting resource limits and a few general cgroup
-properties.
-
-In general, cgroup limits are hierarchical, meaning that the limits placed on
-.IR /user/joe
-cannot be exceeded by
-.IR /usr/joe/1.session .
-There are currently exceptions to this rule,
-but stricter adherence is a goal as cgroups are being largely reworked.
-
-In addition, cgroups can be mounted with no bound subsystem, in which case
-they serve only to track processes.
-An example of this is the
-.I name=systemd
-cgroup which is used by
-.BR systemd (1)
-to track services and user sessions.
+a set of per-resource-type subsystems (memory, CPU, and so on).
 .\"
 .SS Terminology
 A
@@ -96,6 +62,11 @@ renaming subdirectories within the cgroup filesystem.
 At each level of the hierarchy, attributes (e.g., limits) can be defined;
 these attributes may govern or propagate
 to child cgroups and their descendants in the hierarchy.
+
+The limits, control, and accounting provided by cgroups are hierarchical.
+Thus, for example, the limits placed on
+a cgroup at a higher level in the hierarchy cannot be exceeded
+by descendant cgroups.
 .\"
 .SS Cgroups version 1 and version 2
 The initial release of the cgroups implementation was in Linux 2.6.24.
@@ -130,6 +101,39 @@ that are supported under version 2,
 while also using version 1 controllers
 where version 2 does not yet support those controllers.
 .\"
+.SS Cgroups version 1
+Under cgroups v1, each controller may be mounted against a separate
+cgroup filesystem that provides its own hierarchical organization of the
+processes on the system.
+It is also possible comount multiple (or even all) cgroups v1 controllers
+against the same cgroup filesystem, meaning that the comounted controllers
+manage the same hierarchical organization of processes.
+
+For each mounted hierarchy,
+the directory tree mirrors the control group hierarchy.
+Each control group is represented by a directory, with each of its child
+control cgroups represented as a child directory.
+For instance,
+.IR /user/joe/1.session
+represents control group
+.IR 1.session ,
+which is a child of cgroup
+.IR joe ,
+which is a child of
+.IR /user .
+Under each cgroup directory is a set of files which can be read or
+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
+they serve only to track processes.
+An example of this is the
+.I name=systemd
+cgroup which is used by
+.BR systemd (1)
+to track services and user sessions.
+.\"
 .SS Tasks (threads) versus processes
 In cgroups v1, a distinction is drawn between
 .I processes