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    SubjectBroke nice range for RLIMIT NICE
    Hello Ingo,

    I'm guessing that it was you that added the RLIMIT_NICE resource
    limit in 2.6.12. (A passing note to all kernel developers: when
    making changes that affect userland-kernel interfaces, please
    send me a man-pages patch, or at least a notification of the
    change, so that some information makes its way into the manual

    I started documenting RLIMIT_NICE and then noticed an
    inconsistency between the use of this limit and the nice
    value as manipulated by [sg]etpriority().

    This is the documentation I've drafted for RLIMIT_NICE
    in getrlimit.2:

    RLIMIT_NICE(since kernel 2.6.12)
    Specifies a ceiling to which the process nice
    value can be raised using setpriority(2) or
    nice(2). The actual ceiling for the nice value is
    calculated as 19 - rlim_cur.

    And recently I've redrafted the discussion of the nice value
    in getpriority.2 and it now reads:

    Since kernel 1.3.43 Linux has the range -20..19.
    Within the kernel, nice values are actually repre-
    sented using the corresponding range 40..1 (since
    negative numbers are error codes) and these are the
    values employed by the setpriority and getpriority
    system calls. The glibc wrapper functions for
    these system calls handle the translations between
    the user-land and kernel representations of the
    nice value according to the formula
    user_nice = 20 - kernel_nice.

    In other words, there is an off-by-one mismatch between
    these two interfaces: RLIMIT_NICE is expecting to deal
    with values in the range 39..0, while [gs]etpriority()
    works with the range 40..1.

    I suppose that glibc could paper over the cracks here in
    a wrapper for getrlimit(), but it seems more sensible
    to make RLIMIT_NICE consistent with [gs]etpriority() --
    i.e., change the RLIMIT_NICE interface in 2.6.13 before it
    sees wide use in userland. What do you think?



    Michael Kerrisk
    maintainer of Linux man pages Sections 2, 3, 4, 5, and 7

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