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    Subject[RFC][PATCH 00/10] taskstats: Enhancements for precise accounting
    Currently tools like "top" gather the task information by reading procfs
    files. This has several disadvantages:

    * It is very CPU intensive, because a lot of system calls (readdir, open,
    read, close) are necessary.
    * No real task snapshot can be provided, because while the procfs files are
    read the system continues running.
    * The procfs times granularity is restricted to jiffies.

    In parallel to procfs there exists the taskstats binary interface that uses
    netlink sockets as transport mechanism to deliver task information to
    user space. There exists a taskstats command "TASKSTATS_CMD_ATTR_PID"
    to get task information for a given PID. This command can already be used for
    tools like top, but has also several disadvantages:

    * You first have to find out which PIDs are available in the system. Currently
    we have to use procfs again to do this.
    * For each task two system calls have to be issued (First send the command and
    then receive the reply).
    * No snapshot mechanism is available.

    The intention of this patch set is to provide better support for tools like
    top. The goal is to:

    * provide a task snapshot mechanism where we can get a consistent view of
    all running tasks.
    * provide a transport mechanism that does not require a lot of system calls
    and that allows implementing low CPU overhead task monitoring.
    * provide microsecond CPU time granularity.

    Together with this kernel patch set also user space code for a new top
    utility (ptop) is provided that exploits the new kernel infrastructure. See
    patch 10 for more details.

    TEST1: System with many sleeping tasks

    for ((i=0; i < 1000; i++))
    sleep 1000000 &

    # ptop_new_proc

    pid user sys ste total Name
    (#) (%) (%) (%) (%) (str)
    541 0.37 2.39 0.10 2.87 top
    3743 0.03 0.05 0.00 0.07 ptop_new_proc

    Compared to the old top command that has to scan more than 1000 proc
    directories the new ptop consumes much less CPU time (0.05% system time
    on my s390 system).

    TEST2: Show snapshot consistency with system that is 100% busy

    System with 3 CPUs:

    for ((i=0; i < $(cat /proc/cpuinfo | grep "^processor" | wc -l); i++))
    ./loop &

    # ptop_snap_proc

    pid user sys ste cuser csys cste delay total Elap+ Name
    (#) (%) (%) (%) (%) (%) (%) (%) (%) (hm) (str)
    23891 99.84 0.06 0.09 0.00 0.00 0.00 0.01 99.99 0:00 loop
    23881 99.66 0.06 0.09 0.00 0.00 0.00 0.20 99.81 0:00 loop
    23886 99.65 0.06 0.09 0.00 0.00 0.00 0.20 99.80 0:00 loop
    2413 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 4:17 sshd
    V:V:S 299.36 0.36 0.27 0.00 0.00 0.00 0.40 300.00 4:22

    With the snapshot mechanism the sum of all tasks CPU times (user + system +
    steal) will be exactly 300.00% CPU time with this testcase. Using
    ptop_snap_proc (see patch 10) this works fine on s390.

    The code is not final and still has a few TODOs. But it is good enough for a
    first round of review. The following kernel patches are provided:

    [01] Prepare-0: Use real microsecond granularity for taskstats CPU times.
    [02] Prepare-1: Restructure taskstats.c in order to be able to add new commands
    more easily.
    [03] Prepare-2: Separate the finding of a task_struct by PID or TGID from
    filling the taskstats.
    [04] Add new command "TASKSTATS_CMD_ATTR_PIDS" to get a snapshot of multiple
    [05] Add procfs interface for taskstats commands. This allows to get a complete
    and consistent snapshot with all tasks using two system calls (ioctl and
    read). Transferring a snapshot of all running tasks is not possible using
    the existing netlink interface, because there we have the socket buffer
    size as restricting factor.
    [06] Add TGID to taskstats.
    [07] Add steal time per task accounting.
    [08] Add cumulative CPU time (user, system and steal) to taskstats.
    [09] Fix exit CPU time accounting.

    [10] Besides of the kernel patches also user space code is provided that
    exploits the new kernel infrastructure. The user space code provides the
    1. A proposal for a taskstats user space library:
    1.1 Based on netlink (requires libnl-devel-1.1-5)
    2.1 Based on the new /proc/taskstats interface (see [05])
    2. A proposal for a task snapshot library based on taskstats library (1.1)
    3. A new tool "ptop" (precise top) that uses the libraries

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