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3fbd56f0e7
[ a.k.a. Revert "Revert "ARM64: Dynamically allocate cpumasks and
increase supported CPUs to 512""; originally reverted because of a
bug in the cpufreq-dt code not using zalloc_cpumask_var() ]
Currently defconfig selects NR_CPUS=256, but some vendors (e.g. Ampere
Computing) are planning to ship systems with 512 CPUs. So that all CPUs on
these systems can be used with defconfig, we'd like to bump NR_CPUS to 512.
Therefore this patch increases the default NR_CPUS from 256 to 512.
As increasing NR_CPUS will increase the size of cpumasks, there's a fear that
this might have a significant impact on stack usage due to code which places
cpumasks on the stack. To mitigate that concern, we can select
CPUMASK_OFFSTACK. As that doesn't seem to be a problem today with
NR_CPUS=256, we only select this when NR_CPUS > 256.
CPUMASK_OFFSTACK configures the cpumasks in the kernel to be
dynamically allocated. This was used in the X86 architecture in the
past to enable support for larger CPU configurations up to 8k cpus.
With that is becomes possible to dynamically size the allocation of
the cpu bitmaps depending on the quantity of processors detected on
bootup. Memory used for cpumasks will increase if the kernel is
run on a machine with more cores.
Further increases may be needed if ARM processor vendors start
supporting more processors. Given the current inflationary trends
in core counts from multiple processor manufacturers this may occur.
There are minor regressions for hackbench. The kernel data size
for 512 cpus is smaller with offstack than with onstack.
Benchmark results using hackbench average over 10 runs of
hackbench -s 512 -l 2000 -g 15 -f 25 -P
on Altra 80 Core
Support for 256 CPUs on stack. Baseline
7.8564 sec
Support for 512 CUs on stack.
7.8713 sec + 0.18%
512 CPUS offstack
7.8916 sec + 0.44%
Kernel size comparison:
text data filename Difference to onstack256 baseline
25755648
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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.