tick/nohz: Expose housekeeping CPUs in sysfs

tick/nohz: Expose housekeeping CPUs in sysfs

Expose the current system-defined list of housekeeping CPUs in a new
sysfs file: /sys/devices/system/cpu/housekeeping.

This provides userspace performance tuning tools and resource managers
with a canonical, reliable method to accurately identify the cores
responsible for essential kernel maintenance workloads (RCU, timer
callbacks, and unbound workqueues). Currently, tooling must manually
calculate the housekeeping set by parsing complex kernel boot parameters
(like isolcpus= and nohz_full=) and system topology, which is prone to
error. This dedicated file simplifies the configuration of low-latency
workloads.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Link: https://patch.msgid.link/20251011012853.7539-2-atomlin@atomlin.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>