1=========================
2CPU Accounting Controller
3=========================
4
5The CPU accounting controller is used to group tasks using cgroups and
6account the CPU usage of these groups of tasks.
7
8The CPU accounting controller supports multi-hierarchy groups. An accounting
9group accumulates the CPU usage of all of its child groups and the tasks
10directly present in its group.
11
12Accounting groups can be created by first mounting the cgroup filesystem::
13
14  # mount -t cgroup -ocpuacct none /sys/fs/cgroup
15
16With the above step, the initial or the parent accounting group becomes
17visible at /sys/fs/cgroup. At bootup, this group includes all the tasks in
18the system. /sys/fs/cgroup/tasks lists the tasks in this cgroup.
19/sys/fs/cgroup/cpuacct.usage gives the CPU time (in nanoseconds) obtained
20by this group which is essentially the CPU time obtained by all the tasks
21in the system.
22
23New accounting groups can be created under the parent group /sys/fs/cgroup::
24
25  # cd /sys/fs/cgroup
26  # mkdir g1
27  # echo $$ > g1/tasks
28
29The above steps create a new group g1 and move the current shell
30process (bash) into it. CPU time consumed by this bash and its children
31can be obtained from g1/cpuacct.usage and the same is accumulated in
32/sys/fs/cgroup/cpuacct.usage also.
33
34cpuacct.stat file lists a few statistics which further divide the
35CPU time obtained by the cgroup into user and system times. Currently
36the following statistics are supported:
37
38user: Time spent by tasks of the cgroup in user mode.
39system: Time spent by tasks of the cgroup in kernel mode.
40
41user and system are in USER_HZ unit.
42
43cpuacct controller uses percpu_counter interface to collect user and
44system times. This has two side effects:
45
46- It is theoretically possible to see wrong values for user and system times.
47  This is because percpu_counter_read() on 32bit systems isn't safe
48  against concurrent writes.
49- It is possible to see slightly outdated values for user and system times
50  due to the batch processing nature of percpu_counter.
51