kernel: kthread_cpus argument parser overutilizes the stack

[M. Vefa Bicakci]

Brief Description
-----------------

Since commit https://review.opendev.org/plugins/gitiles/starlingx/kernel/+/7b55e47e1d9fe9524b6b6b56cb6cb6efa11938ec the kernel's kthread_cpus= argument parser utilizes a lot of stack space, which may or may not be harmful. This was discovered when building the kernel with the following options:

      CONFIG_MAXSMP=y
      CONFIG_NR_CPUS=8192
      CONFIG_CPUMASK_OFFSTACK=y
      CONFIG_FRAME_WARN=1024

which results in the following compilation-time warning:

        CC kernel/cpu.o
      kernel/cpu.c: In function 'kthread_setup':
      kernel/cpu.c:2475:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]

Please note that the configuration quoted above is *not* used by StarlingX, so the bug being fixed (large stack allocation) will become applicable only if CONFIG_MAXSMP is set to 'y' in the future (or when a large CONFIG_NR_CPUS value is used with CONFIG_CPUMASK_OFFSTACK=y).

Severity
--------
Minor: Given that this issue does not affect StarlingX due to its current kernel configuration, this is a minor issue, but I would like to fix it regardless.

Steps to Reproduce
------------------
Please see above for the build configuration that reproduces the warning.

Expected Behavior
------------------
No kernel build warnings

Actual Behavior
----------------
Kernel build warning about kthread_setup function's frame size exceeding 1KiB.

Reproducibility
---------------
100%

System Configuration
--------------------
Not applicable.

Branch/Pull Time/Commit
-----------------------
The issue is reproduced since the merge of the commit at https://review.opendev.org/plugins/gitiles/starlingx/kernel/+/7b55e47e1d9fe9524b6b6b56cb6cb6efa11938ec

Last Pass
---------
Not applicable.

Timestamp/Logs
--------------
Please see the issue description.

Test Activity
-------------
Exploratory developer testing

Workaround
----------
None.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/starlingx/kernel/+/824460

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/starlingx/kernel/+/824460
Committed: https://opendev.org/starlingx/kernel/commit/8fde1a80702c4fbfb3b4c9fd944a505578fa9ade
Submitter: "Zuul (22348)"
Branch: master

commit 8fde1a80702c4fbfb3b4c9fd944a505578fa9ade
Author: M. Vefa Bicakci <email address hidden>
Date: Tue Jan 11 17:00:27 2022 -0500

    kthread_cpus: Avoid large stack allocation

    Since commit 7b55e47e1d9f ("Restore CPU affinity patches'
    functionality"), the function kthread_setup() has allocated a cpumask
    structure on the stack, which uses 1024 bytes on the stack when
    CONFIG_MAXSMP is enabled. This was detected after setting
    CONFIG_FRAME_WARN to 1024 for testing purposes.

    To clarify, with the following configuration:

      $ grep -e MAXSMP= -e NR_CPUS= -e CPUMASK_OFFSTACK= \
        -e FRAME_WARN= .config
      CONFIG_MAXSMP=y
      CONFIG_NR_CPUS=8192
      CONFIG_CPUMASK_OFFSTACK=y
      CONFIG_FRAME_WARN=1024

    the following build warning is encountered:

        CC kernel/cpu.o
      kernel/cpu.c: In function 'kthread_setup':
      kernel/cpu.c:2475:1: warning: the frame size of 1032 bytes is \
          larger than 1024 bytes [-Wframe-larger-than=]

    This commit resolves this issue by allocating the cpumask structure
    using the boot memory allocator via alloc_bootmem_cpumask_var().
    A similar approach is taken for the housekeeping-related cpumask
    structures in kernel/sched/isolation.c.

    Please note that the issue fixed by this commit (large stack allocation)
    only exists when CONFIG_CPUMASK_OFFSTACK is enabled. StarlingX currently
    disables CONFIG_CPUMASK_OFFSTACK. Disabling CONFIG_CPUMASK_OFFSTACK sets
    CONFIG_NR_CPUS' maximum value to 512 CPUs, which in turn corresponds to
    64 bytes on the stack. In contrast, enabling CONFIG_CPUMASK_OFFSTACK
    (for example, by setting CONFIG_MAXSMP=y) raises the upper limit for
    CONFIG_NR_CPUS to 8192 CPUs, which translates to a 1024-byte bitmap.

    No change in behaviour is intended.

    While carrying out the code changes, space characters in the affected
    function were converted to tab characters as a minor clean-up.

    Testing:
    * Monolithic ISO image build was successful.
    * Installation and bootstrap of the built ISO was successful with a
      virtual machine in All-in-One simplex mode, using the low-latency
      profile.
    * kthread_cpus argument's behaviour was confirmed to not have been
      negatively affected by checking that the CPU affinity of kthreadd
      (i.e., pid 2) matches the kthread_cpus= argument's value.
    * kthread_cpus argument's error handling was confirmed to work as
      expected by booting up the installed system with an invalid argument
      ('kthread_cpus=abc'). In this scenario, the CPU affinity of kthreadd
      was observed to be correctly set to 0-7 (with an 8-CPU VM).

    Change-Id: I31d2175d6084142e63d4b38d7b0c5677046fce4f
    Closes-Bug: 1957188
    Fixes: 7b55e47e1d9f ("Restore CPU affinity patches' functionality")
    Signed-off-by: M. Vefa Bicakci <email address hidden>