Interface for enabling context tracking

[Junxuan Liao]

Hi all,

From what I can tell, tracepoints context_tracking:user_enter and
user_exit might be useful for performance analysis. e.g. Figuring out how
much time is spent handling page faults or in system calls. However
context tracking is by default inactive and the only way to enable it is
to turn on nohz_full for some CPUs.

Is it a good idea to support turning on and off context tracking through
some interface accessible from the userspace?

Thanks,
Junxuan