aha/Documentation/accounting/taskstats.txt

Per-task statistics interface
-----------------------------


Taskstats is a netlink-based interface for sending per-task and
per-process statistics from the kernel to userspace.

Taskstats was designed for the following benefits:

- efficiently provide statistics during lifetime of a task and on its exit
- unified interface for multiple accounting subsystems
- extensibility for use by future accounting patches

Terminology
-----------

"pid", "tid" and "task" are used interchangeably and refer to the standard
Linux task defined by struct task_struct.  per-pid stats are the same as
per-task stats.

"tgid", "process" and "thread group" are used interchangeably and refer to the
tasks that share an mm_struct i.e. the traditional Unix process. Despite the
use of tgid, there is no special treatment for the task that is thread group
leader - a process is deemed alive as long as it has any task belonging to it.

Usage
-----

To get statistics during task's lifetime, userspace opens a unicast netlink
socket (NETLINK_GENERIC family) and sends commands specifying a pid or a tgid.
The response contains statistics for a task (if pid is specified) or the sum of
statistics for all tasks of the process (if tgid is specified).

To obtain statistics for tasks which are exiting, userspace opens a multicast
netlink socket. Each time a task exits, two records are sent by the kernel to
each listener on the multicast socket. The first the per-pid task's statistics
and the second is the sum for all tasks of the process to which the task
belongs (the task does not need to be the thread group leader). The need for
per-tgid stats to be sent for each exiting task is explained in the per-tgid
stats section below.

getdelays.c is a simple utility demonstrating usage of the taskstats interface
for reporting delay accounting statistics.

Interface
---------

The user-kernel interface is encapsulated in include/linux/taskstats.h

To avoid this documentation becoming obsolete as the interface evolves, only
an outline of the current version is given. taskstats.h always overrides the
description here.

struct taskstats is the common accounting structure for both per-pid and
per-tgid data. It is versioned and can be extended by each accounting subsystem
that is added to the kernel. The fields and their semantics are defined in the
taskstats.h file.

The data exchanged between user and kernel space is a netlink message belonging
to the NETLINK_GENERIC family and using the netlink attributes interface.
The messages are in the format

    +----------+- - -+-------------+-------------------+
    | nlmsghdr | Pad |  genlmsghdr | taskstats payload |
    +----------+- - -+-------------+-------------------+


The taskstats payload is one of the following three kinds:

1. Commands: Sent from user to kernel. The payload is one attribute, of type
TASKSTATS_CMD_ATTR_PID/TGID, containing a u32 pid or tgid in the attribute
payload. The pid/tgid denotes the task/process for which userspace wants
statistics.

2. Response for a command: sent from the kernel in response to a userspace
command. The payload is a series of three attributes of type:

a) TASKSTATS_TYPE_AGGR_PID/TGID : attribute containing no payload but indicates
a pid/tgid will be followed by some stats.

b) TASKSTATS_TYPE_PID/TGID: attribute whose payload is the pid/tgid whose stats
is being returned.

c) TASKSTATS_TYPE_STATS: attribute with a struct taskstsats as payload. The
same structure is used for both per-pid and per-tgid stats.

3. New message sent by kernel whenever a task exits. The payload consists of a
   series of attributes of the following type:

a) TASKSTATS_TYPE_AGGR_PID: indicates next two attributes will be pid+stats
b) TASKSTATS_TYPE_PID: contains exiting task's pid
c) TASKSTATS_TYPE_STATS: contains the exiting task's per-pid stats
d) TASKSTATS_TYPE_AGGR_TGID: indicates next two attributes will be tgid+stats
e) TASKSTATS_TYPE_TGID: contains tgid of process to which task belongs
f) TASKSTATS_TYPE_STATS: contains the per-tgid stats for exiting task's process


per-tgid stats
--------------

Taskstats provides per-process stats, in addition to per-task stats, since
resource management is often done at a process granularity and aggregating task
stats in userspace alone is inefficient and potentially inaccurate (due to lack
of atomicity).

However, maintaining per-process, in addition to per-task stats, within the
kernel has space and time overheads. Hence the taskstats implementation
dynamically sums up the per-task stats for each task belonging to a process
whenever per-process stats are needed.

Not maintaining per-tgid stats creates a problem when userspace is interested
in getting these stats when the process dies i.e. the last thread of
a process exits. It isn't possible to simply return some aggregated per-process
statistic from the kernel.

The approach taken by taskstats is to return the per-tgid stats *each* time
a task exits, in addition to the per-pid stats for that task. Userspace can
maintain task<->process mappings and use them to maintain the per-process stats
in userspace, updating the aggregate appropriately as the tasks of a process
exit.

Extending taskstats
-------------------

There are two ways to extend the taskstats interface to export more
per-task/process stats as patches to collect them get added to the kernel
in future:

1. Adding more fields to the end of the existing struct taskstats. Backward
   compatibility is ensured by the version number within the
   structure. Userspace will use only the fields of the struct that correspond
   to the version its using.

2. Defining separate statistic structs and using the netlink attributes
   interface to return them. Since userspace processes each netlink attribute
   independently, it can always ignore attributes whose type it does not
   understand (because it is using an older version of the interface).


Choosing between 1. and 2. is a matter of trading off flexibility and
overhead. If only a few fields need to be added, then 1. is the preferable
path since the kernel and userspace don't need to incur the overhead of
processing new netlink attributes. But if the new fields expand the existing
struct too much, requiring disparate userspace accounting utilities to
unnecessarily receive large structures whose fields are of no interest, then
extending the attributes structure would be worthwhile.

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