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-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 a 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, the userspace listener
-sends a register command and specifies a cpumask. Whenever a task exits on
-one of the cpus in the cpumask, its per-pid statistics are sent to the
-registered listener. Using cpumasks allows the data received by one listener
-to be limited and assists in flow control over the netlink interface and is
-explained in more detail below.
-
-If the exiting task is the last thread exiting its thread group,
-an additional record containing the per-tgid stats is also sent to userspace.
-The latter contains the sum of per-pid stats for all threads in the thread
-group, both past and present.
-
-getdelays.c is a simple utility demonstrating usage of the taskstats interface
-for reporting delay accounting statistics. Users can register cpumasks,
-send commands and process responses, listen for per-tid/tgid exit data,
-write the data received to a file and do basic flow control by increasing
-receive buffer sizes.
-
-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. Commands to get data on
-a pid/tgid consist of 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.
-
-Commands to register/deregister interest in exit data from a set of cpus
-consist of one attribute, of type
-TASKSTATS_CMD_ATTR_REGISTER/DEREGISTER_CPUMASK and contain a cpumask in the
-attribute payload. The cpumask is specified as an ascii string of
-comma-separated cpu ranges e.g. to listen to exit data from cpus 1,2,3,5,7,8
-the cpumask would be "1-3,5,7-8". If userspace forgets to deregister interest
-in cpus before closing the listening socket, the kernel cleans up its interest
-set over time. However, for the sake of efficiency, an explicit deregistration
-is advisable.
-
-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
-are being returned.
-
-c) TASKSTATS_TYPE_STATS: attribute with a struct taskstats 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. To address this, the taskstats code
-accumulates each exiting task's statistics into a process-wide data structure.
-When the last task of a process exits, the process level data accumulated also
-gets sent to userspace (along with the per-task data).
-
-When a user queries to get per-tgid data, the sum of all other live threads in
-the group is added up and added to the accumulated total for previously exited
-threads of the same thread group.
-
-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.
-
-Flow control for taskstats
---------------------------
-
-When the rate of task exits becomes large, a listener may not be able to keep
-up with the kernel's rate of sending per-tid/tgid exit data leading to data
-loss. This possibility gets compounded when the taskstats structure gets
-extended and the number of cpus grows large.
-
-To avoid losing statistics, userspace should do one or more of the following:
-
-- increase the receive buffer sizes for the netlink sockets opened by
-listeners to receive exit data.
-
-- create more listeners and reduce the number of cpus being listened to by
-each listener. In the extreme case, there could be one listener for each cpu.
-Users may also consider setting the cpu affinity of the listener to the subset
-of cpus to which it listens, especially if they are listening to just one cpu.
-
-Despite these measures, if the userspace receives ENOBUFS error messages
-indicated overflow of receive buffers, it should take measures to handle the
-loss of data.
-
-----