I think just WARN_ON(cpu_online(fcpu)) would be satisfactory. In my
experience, the downstream effects of violating this condition are
varied and quite difficult to debug. Seems only appropriate to emit a
warning and stack trace before the OS inevitably becomes unstable.

I still have to try but wouldn't this be a problem for the boot-cpu?
I mean boot-cpu would be marked online while it tries to do numa_setup_cpu.
No?

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Thanks and Regards
Srikar Dronamraju