181 lines
8.0 KiB
ReStructuredText
181 lines
8.0 KiB
ReStructuredText
=============================
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Per-task statistics interface
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=============================
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Taskstats is a netlink-based interface for sending per-task and
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per-process statistics from the kernel to userspace.
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Taskstats was designed for the following benefits:
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- efficiently provide statistics during lifetime of a task and on its exit
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- unified interface for multiple accounting subsystems
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- extensibility for use by future accounting patches
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Terminology
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-----------
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"pid", "tid" and "task" are used interchangeably and refer to the standard
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Linux task defined by struct task_struct. per-pid stats are the same as
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per-task stats.
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"tgid", "process" and "thread group" are used interchangeably and refer to the
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tasks that share an mm_struct i.e. the traditional Unix process. Despite the
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use of tgid, there is no special treatment for the task that is thread group
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leader - a process is deemed alive as long as it has any task belonging to it.
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Usage
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-----
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To get statistics during a task's lifetime, userspace opens a unicast netlink
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socket (NETLINK_GENERIC family) and sends commands specifying a pid or a tgid.
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The response contains statistics for a task (if pid is specified) or the sum of
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statistics for all tasks of the process (if tgid is specified).
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To obtain statistics for tasks which are exiting, the userspace listener
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sends a register command and specifies a cpumask. Whenever a task exits on
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one of the cpus in the cpumask, its per-pid statistics are sent to the
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registered listener. Using cpumasks allows the data received by one listener
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to be limited and assists in flow control over the netlink interface and is
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explained in more detail below.
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If the exiting task is the last thread exiting its thread group,
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an additional record containing the per-tgid stats is also sent to userspace.
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The latter contains the sum of per-pid stats for all threads in the thread
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group, both past and present.
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getdelays.c is a simple utility demonstrating usage of the taskstats interface
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for reporting delay accounting statistics. Users can register cpumasks,
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send commands and process responses, listen for per-tid/tgid exit data,
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write the data received to a file and do basic flow control by increasing
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receive buffer sizes.
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Interface
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---------
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The user-kernel interface is encapsulated in include/linux/taskstats.h
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To avoid this documentation becoming obsolete as the interface evolves, only
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an outline of the current version is given. taskstats.h always overrides the
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description here.
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struct taskstats is the common accounting structure for both per-pid and
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per-tgid data. It is versioned and can be extended by each accounting subsystem
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that is added to the kernel. The fields and their semantics are defined in the
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taskstats.h file.
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The data exchanged between user and kernel space is a netlink message belonging
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to the NETLINK_GENERIC family and using the netlink attributes interface.
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The messages are in the format::
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+----------+- - -+-------------+-------------------+
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| nlmsghdr | Pad | genlmsghdr | taskstats payload |
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+----------+- - -+-------------+-------------------+
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The taskstats payload is one of the following three kinds:
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1. Commands: Sent from user to kernel. Commands to get data on
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a pid/tgid consist of one attribute, of type TASKSTATS_CMD_ATTR_PID/TGID,
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containing a u32 pid or tgid in the attribute payload. The pid/tgid denotes
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the task/process for which userspace wants statistics.
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Commands to register/deregister interest in exit data from a set of cpus
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consist of one attribute, of type
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TASKSTATS_CMD_ATTR_REGISTER/DEREGISTER_CPUMASK and contain a cpumask in the
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attribute payload. The cpumask is specified as an ascii string of
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comma-separated cpu ranges e.g. to listen to exit data from cpus 1,2,3,5,7,8
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the cpumask would be "1-3,5,7-8". If userspace forgets to deregister interest
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in cpus before closing the listening socket, the kernel cleans up its interest
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set over time. However, for the sake of efficiency, an explicit deregistration
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is advisable.
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2. Response for a command: sent from the kernel in response to a userspace
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command. The payload is a series of three attributes of type:
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a) TASKSTATS_TYPE_AGGR_PID/TGID : attribute containing no payload but indicates
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a pid/tgid will be followed by some stats.
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b) TASKSTATS_TYPE_PID/TGID: attribute whose payload is the pid/tgid whose stats
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are being returned.
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c) TASKSTATS_TYPE_STATS: attribute with a struct taskstats as payload. The
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same structure is used for both per-pid and per-tgid stats.
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3. New message sent by kernel whenever a task exits. The payload consists of a
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series of attributes of the following type:
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a) TASKSTATS_TYPE_AGGR_PID: indicates next two attributes will be pid+stats
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b) TASKSTATS_TYPE_PID: contains exiting task's pid
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c) TASKSTATS_TYPE_STATS: contains the exiting task's per-pid stats
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d) TASKSTATS_TYPE_AGGR_TGID: indicates next two attributes will be tgid+stats
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e) TASKSTATS_TYPE_TGID: contains tgid of process to which task belongs
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f) TASKSTATS_TYPE_STATS: contains the per-tgid stats for exiting task's process
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per-tgid stats
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--------------
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Taskstats provides per-process stats, in addition to per-task stats, since
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resource management is often done at a process granularity and aggregating task
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stats in userspace alone is inefficient and potentially inaccurate (due to lack
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of atomicity).
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However, maintaining per-process, in addition to per-task stats, within the
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kernel has space and time overheads. To address this, the taskstats code
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accumulates each exiting task's statistics into a process-wide data structure.
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When the last task of a process exits, the process level data accumulated also
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gets sent to userspace (along with the per-task data).
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When a user queries to get per-tgid data, the sum of all other live threads in
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the group is added up and added to the accumulated total for previously exited
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threads of the same thread group.
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Extending taskstats
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-------------------
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There are two ways to extend the taskstats interface to export more
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per-task/process stats as patches to collect them get added to the kernel
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in future:
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1. Adding more fields to the end of the existing struct taskstats. Backward
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compatibility is ensured by the version number within the
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structure. Userspace will use only the fields of the struct that correspond
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to the version its using.
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2. Defining separate statistic structs and using the netlink attributes
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interface to return them. Since userspace processes each netlink attribute
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independently, it can always ignore attributes whose type it does not
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understand (because it is using an older version of the interface).
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Choosing between 1. and 2. is a matter of trading off flexibility and
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overhead. If only a few fields need to be added, then 1. is the preferable
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path since the kernel and userspace don't need to incur the overhead of
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processing new netlink attributes. But if the new fields expand the existing
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struct too much, requiring disparate userspace accounting utilities to
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unnecessarily receive large structures whose fields are of no interest, then
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extending the attributes structure would be worthwhile.
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Flow control for taskstats
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--------------------------
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When the rate of task exits becomes large, a listener may not be able to keep
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up with the kernel's rate of sending per-tid/tgid exit data leading to data
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loss. This possibility gets compounded when the taskstats structure gets
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extended and the number of cpus grows large.
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To avoid losing statistics, userspace should do one or more of the following:
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- increase the receive buffer sizes for the netlink sockets opened by
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listeners to receive exit data.
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- create more listeners and reduce the number of cpus being listened to by
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each listener. In the extreme case, there could be one listener for each cpu.
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Users may also consider setting the cpu affinity of the listener to the subset
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of cpus to which it listens, especially if they are listening to just one cpu.
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Despite these measures, if the userspace receives ENOBUFS error messages
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indicated overflow of receive buffers, it should take measures to handle the
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loss of data.
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