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    SubjectBug in recent kernel timer implementation
    Date
    Hi,

    Seems like kernel timer implementation is buggy since some kernel
    version (between 3.16.0 and 4.9.0).

    Comments for add_timer in timer.c:

    * The kernel will do a ->function(@timer) callback from the
     * timer interrupt at the ->expires point in the future. The
     * current time is 'jiffies'.

    Linux x86, Linux x64. HZ=250.

    init_timer(&my_timer);
    my_timer.function = testjiffy_timer_function;
    my_timer.expires = jiffies + 1;
    add_timer(&my_timer);

    So, according to comments in add_timer, testjiffy_timer_function should
    be called next timer tick, 4ms later.

    If you will run this code in Linux 3.16.0 - it will call
    testjiffy_timer_function in the next timer tick (4ms later), as expected.

    If you will run this code in Linux 4.9.0 - it will call
    testjiffy_timer_function in the on second timer tick (8ms later)...

    If you will modify code for Linux > 4.15 (setup_timer/mod_timer) - you
    will get the same strange behaviour...

    You can get code from here

    https://stackoverflow.com/questions/16920238/reliability-of-linux-kernel-add-timer-at-resolution-of-one-jiffy

    3.16.0

    [52728.313029] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4
    [52728.313720]  testjiffy_timer_function: runcount 1, jif=13110976,
    oldjif=13110975, delta=1
    [52728.318743]  testjiffy_timer_function: runcount 2, jif=13110977,
    oldjif=13110976, delta=1
    [52728.323304]  testjiffy_timer_function: runcount 3, jif=13110978,
    oldjif=13110977, delta=1
    [52728.325824]  testjiffy_timer_function: runcount 4, jif=13110979,
    oldjif=13110978, delta=1
    [52728.330811]  testjiffy_timer_function: runcount 5, jif=13110980,
    oldjif=13110979, delta=1
    [52728.335802]  testjiffy_timer_function: runcount 6, jif=13110981,
    oldjif=13110980, delta=1
    [52728.338305]  testjiffy_timer_function: runcount 7, jif=13110982,
    oldjif=13110981, delta=1
    [52728.343298]  testjiffy_timer_function: runcount 8, jif=13110983,
    oldjif=13110982, delta=1
    [52728.345806]  testjiffy_timer_function: runcount 9, jif=13110984,
    oldjif=13110983, delta=1
    [52728.350885]  testjiffy_timer_function: runcount 10, jif=13110985,
    oldjif=13110984, delta=1
    [52729.317263] Exit testjiffy

    4.9.0

    [52073.085341] Init testjiffy: 0 ; HZ: 250 ; 1/HZ (ms): 4
    [52073.092322]  testjiffy_timer_function: runcount 1, jif=4307910976,
    oldjif=4307910974, delta=2
    [52073.099818]  testjiffy_timer_function: runcount 2, jif=4307910978,
    oldjif=4307910976, delta=2
    [52073.109830]  testjiffy_timer_function: runcount 3, jif=4307910980,
    oldjif=4307910978, delta=2
    [52073.117362]  testjiffy_timer_function: runcount 4, jif=4307910982,
    oldjif=4307910980, delta=2
    [52073.124845]  testjiffy_timer_function: runcount 5, jif=4307910984,
    oldjif=4307910982, delta=2
    [52073.132315]  testjiffy_timer_function: runcount 6, jif=4307910986,
    oldjif=4307910984, delta=2
    [52073.139816]  testjiffy_timer_function: runcount 7, jif=4307910988,
    oldjif=4307910986, delta=2
    [52073.149824]  testjiffy_timer_function: runcount 8, jif=4307910990,
    oldjif=4307910988, delta=2
    [52073.157303]  testjiffy_timer_function: runcount 9, jif=4307910992,
    oldjif=4307910990, delta=2
    [52073.164824]  testjiffy_timer_function: runcount 10, jif=4307910994,
    oldjif=4307910992, delta=2
    [52074.089574] Exit testjiffy


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