|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
|"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
|"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.
|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.
|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
|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
|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
|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
|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
|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.
|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
|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.