|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
|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|- It is possible to see slightly outdated values for user and system times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.