In this case CPU event refers to monitoring events per CPU rather than per task. For perf tools this restricts the usage of
-C, --cpu=
Count only on the list of CPUs provided. Multiple CPUs can be provided as a comma-separated list with no space: 0,1.
Ranges of CPUs are specified with -: 0-2. In per-thread mode, this option is ignored. The -a option is still necessary
to activate system-wide monitoring. Default is to count on all CPUs.
-a, --all-cpus
system-wide collection from all CPUs (default if no target is specified)
For perf_event_open this considers the following case:
pid == -1 and cpu >= 0
This measures all processes/threads on the specified CPU. This requires CAP_SYS_ADMIN capability or a /proc/sys/ker‐
nel/perf_event_paranoid value of less than 1.
This may be version specific, the cited documentation is from 4.17. This is another related question.