proc.5: Document /proc/[pid]/cgroup

Upstreamed from Red Hat / Fedora

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

man5/proc.5

@@ -94,11 +94,32 @@ to the process at exec time.
 The format is one \fIunsigned long\fP ID
 plus one \fIunsigned long\fP value for each entry.
 The last entry contains two zeros.
-.\" FIXME Describe /proc/[pid]/cgroup and
-.\"       /proc/[pid]/task/[tid]/cgroup
-.\" 	  Info in Documentation/cgroups.txt
-.\"       Added in 2.6.24
-.\"	  CONFIG_CGROUPS
+.TP
+.I /proc/[pid]/cgroup " (since Linux 2.6.24)"
+.\" 	  Info in Documentation/cgroups/cgroups.txt
+This file describes control groups to which the process/task belongs.
+For each cgroup hierarchy there is one entry containing
+colon-separated fields of the form:
+.nf
+.ft CW
+
+    5:cpuacct,cpu,cpuset:/daemons
+.ft
+.fi
+.IP
+The colon-separated fields are, from left to right:
+.RS 11
+.IP 1. 3
+hierarchy ID number
+.IP 2.
+set of subsystems bound to the hierarchy
+.IP 3.
+control group in the hierarchy to which the process belongs
+.RE
+.IP
+This file is only present if the
+.B CONFIG_CGROUPS
+kernel configuration option is enabled.
 .\"
 .\" FIXME Describe /proc/[pid]/clear_refs
 .\"       Added in 2.6.22