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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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