forked from luck/tmp_suning_uos_patched
a3baf649ca
Some documentation for delay accounting. Signed-off-by: Shailabh Nagar <nagar@watson.ibm.com> Signed-off-by: Balbir Singh <balbir@in.ibm.com> Cc: Jes Sorensen <jes@sgi.com> Cc: Peter Chubb <peterc@gelato.unsw.edu.au> Cc: Erich Focht <efocht@ess.nec.de> Cc: Levent Serinol <lserinol@gmail.com> Cc: Jay Lan <jlan@engr.sgi.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
149 lines
6.3 KiB
Plaintext
149 lines
6.3 KiB
Plaintext
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 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, userspace opens a multicast
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netlink socket. Each time a task exits, two records are sent by the kernel to
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each listener on the multicast socket. The first the per-pid task's statistics
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and the second is the sum for all tasks of the process to which the task
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belongs (the task does not need to be the thread group leader). The need for
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per-tgid stats to be sent for each exiting task is explained in the per-tgid
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stats section below.
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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.
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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. The payload is one attribute, of type
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TASKSTATS_CMD_ATTR_PID/TGID, containing a u32 pid or tgid in the attribute
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payload. The pid/tgid denotes the task/process for which userspace wants
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statistics.
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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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is being returned.
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c) TASKSTATS_TYPE_STATS: attribute with a struct taskstsats 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. Hence the taskstats implementation
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dynamically sums up the per-task stats for each task belonging to a process
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whenever per-process stats are needed.
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Not maintaining per-tgid stats creates a problem when userspace is interested
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in getting these stats when the process dies i.e. the last thread of
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a process exits. It isn't possible to simply return some aggregated per-process
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statistic from the kernel.
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The approach taken by taskstats is to return the per-tgid stats *each* time
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a task exits, in addition to the per-pid stats for that task. Userspace can
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maintain task<->process mappings and use them to maintain the per-process stats
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in userspace, updating the aggregate appropriately as the tasks of a process
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exit.
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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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----
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