Comment 2 for bug 1992201

Reviewed: https://review.opendev.org/c/starlingx/kernel/+/860750
Committed: https://opendev.org/starlingx/kernel/commit/f85a7fc271b95e35a81a8e6c69d49445b9fd54a1
Submitter: "Zuul (22348)"
Branch: master

commit f85a7fc271b95e35a81a8e6c69d49445b9fd54a1
Author: Jim Somerville <email address hidden>
Date: Fri Oct 7 16:30:40 2022 -0400

    rcu: Avoid running boost kthreads on isolated CPUs

    Problem:
    The rcub tasks are allowed to run on isolated CPUs. This
    is a two-fold problem. The first is that these rcub tasks
    steal precious yielded CPU time away from kworker
    tasks that need to run on the isolated cpu. The second
    is that they interfere with nohz_full behavior.

    Solution:
    This is already solved for us upstream, so we simply
    backport the patch here. It affines the rcub tasks
    away from any nohz_full cpus. Note that even though
    this issue only pertains to our rt kernel, we also
    apply the patch to the std kernel for completeness.

    Verification:
    - Boot rt kernel without this patch applied, and check the
    affinity of the rcub tasks. They are set to run on any
    cpu, even the nohz_full ones.
    - Boot rt kernel with this patch included, and now see that
    the nohz_full cpus are excluded from rcub affinity.

    Closes-Bug: 1992201

    Signed-off-by: Jim Somerville <email address hidden>
    Change-Id: I53cbfeddcd7c89b17bd7465134c02311caa7914f