The ListUnitFiles operations in systemd causing high CPU consumption and causing delays in other system operations and makes system unusable

Issue

• ListUnitFiles impacts systemd a lot, causing high CPU consumption and delay in operations.

• The "systemctl list-unit-files" command when run often, kills the system usability and makes system unusable, as CPU usage by systemd spikes up.

• With RHEL8.7 / systemd-239-68.el8, we can see that the ListUnitFiles DBus operation is very costly.

• When executing systemd under strace, we can observe that on a system having 145 units ("minimal installation" of RHEL8.7), the readlinkat/openat/getdents64 syscalls are called hundreds or even thousands of times on same files, causing high CPU consumption.

• Overall CPU usage by systemd becomes high and impact other operations on the system and makes system unusable.

• Consistent high CPU for PID 1
• systemd consumes a lot of kernel space
• Systemctl commands are hanging and timing out