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+CPU Accounting Controller
+The CPU accounting controller is used to group tasks using cgroups and
+account the CPU usage of these groups of tasks.
+The CPU accounting controller supports multi-hierarchy groups. An accounting
+group accumulates the CPU usage of all of its child groups and the tasks
+directly present in its group.
+Accounting groups can be created by first mounting the cgroup filesystem.
+# mount -t cgroup -ocpuacct none /sys/fs/cgroup
+With the above step, the initial or the parent accounting group becomes
+visible at /sys/fs/cgroup. At bootup, this group includes all the tasks in
+the system. /sys/fs/cgroup/tasks lists the tasks in this cgroup.
+/sys/fs/cgroup/cpuacct.usage gives the CPU time (in nanoseconds) obtained
+by this group which is essentially the CPU time obtained by all the tasks
+in the system.
+New accounting groups can be created under the parent group /sys/fs/cgroup.
+# cd /sys/fs/cgroup
+# mkdir g1
+# echo $$ > g1/tasks
+The above steps create a new group g1 and move the current shell
+process (bash) into it. CPU time consumed by this bash and its children
+can be obtained from g1/cpuacct.usage and the same is accumulated in
+/sys/fs/cgroup/cpuacct.usage also.
+cpuacct.stat file lists a few statistics which further divide the
+CPU time obtained by the cgroup into user and system times. Currently
+the following statistics are supported:
+user: Time spent by tasks of the cgroup in user mode.
+system: Time spent by tasks of the cgroup in kernel mode.
+user and system are in USER_HZ unit.
+cpuacct controller uses percpu_counter interface to collect user and
+system times. This has two side effects:
+- It is theoretically possible to see wrong values for user and system times.
+ This is because percpu_counter_read() on 32bit systems isn't safe
+ against concurrent writes.
+- It is possible to see slightly outdated values for user and system times
+ due to the batch processing nature of percpu_counter.