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Cpumask code is written in assumption that when CONFIG_CPUMASK_OFFSTACK is enabled, all cpumasks have boot-time defined size, otherwise the size is always NR_CPUS. The latter is wrong because the number of possible cpus is always calculated on boot, and it may be less than NR_CPUS. On my 4-cpu arm64 VM the nr_cpu_ids is 4, as expected, and nr_cpumask_bits is 256, which corresponds to NR_CPUS. This not only leads to useless traversing of cpumask bits greater than 4, this also makes some cpumask routines fail. For example, cpumask_full(0b1111000..000) would erroneously return false in the example above because tail bits in the mask are all unset. This patch deprecates nr_cpumask_bits and wires it to nr_cpu_ids unconditionally, so that cpumask routines will not waste time traversing unused part of cpu masks. It also fixes cpumask_full() and similar routines. As a side effect, because now a length of cpumasks is defined at run-time even if CPUMASK_OFFSTACK is disabled, compiler can't optimize corresponding functions. It increases kernel size by ~2.5KB if OFFSTACK is off. This is addressed in the following patch. Signed-off-by: Yury Norov <yury.norov@gmail.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.