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  48- It is possible to see slightly outdated values for user and system times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