clocksource 解释
================

.. versionchanged:: @2018-01-14
   创建

.. index :: Linux
   single: Linux; clocksource

内核日志： ::

    clocksource: timekeeping watchdog on CPU7: Marking clocksource 'tsc' as unstable because the skew is too large:
    clocksource:                       'hpet' wd_now: ec7fd07a wd_last: f6240e67 mask: ffffffff
    clocksource:                       'tsc' cs_now: db522085db96 cs_last: 9d30c4ea512c mask: ffffffffffffffff
    clocksource: Switched to clocksource hpet

If system clcok source is hpet, then gettimeofday will do syscall gettimeofday,
not for clocksource tsc. So we should add gettimeofday to seccomp too,
otherwise: ::

    [WARN/seccomp] invalid syscall attempted: gettimeofday(96)
    [CRIT/seccomp] invalid syscall not allowed: stop here

如下测试代码演示差异：

.. code-block:: c
    :linenos:

    #include <stdio.h>
    #include <sys/time.h>

    int main(int argc, char **argv)
    {
            struct timeval tv;

            gettimeofday(&tv, NULL);
            printf("%ld\n", tv.tv_sec);
            return 0;
    }

下面是不同 clocksource 时 strace 的差异： ::

    $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
    hpet
    $ strace -f ./a.out
    execve("./a.out", ["./a.out"], [/* 39 vars */]) = 0
    brk(NULL)                               = 0x559844c55000
    …
    gettimeofday({tv_sec=1512111369, tv_usec=671919}, NULL) = 0
    fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
    brk(NULL)                               = 0x559844c55000
    brk(0x559844c76000)                     = 0x559844c76000
    write(1, "1512111369\n", 111512111369
    )            = 11
    exit_group(0)                           = ?
    +++ exited with 0 +++

    $ cat /sys/devices/system/clocksource/clocksource0/current_clocksource
    tsc
    $ strace -f ./a.out
    execve("./a.out", ["./a.out"], [/* 47 vars */]) = 0
    brk(NULL)                               = 0x7f5b0b28e000
    …
    fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 96), ...}) = 0
    brk(NULL)                               = 0x7f5b0b28e000
    brk(0x7f5b0b2af000)                     = 0x7f5b0b2af000
    write(1, "1512111377\n", 111512111377
    )            = 11
    exit_group(0)                           = ?
    +++ exited with 0 +++