ebpf trace doesn't work during cpu hotplug

[Ming Lei]

Hi,

Looks ebpf trace doesn't work during cpu hotplug, see the following trace:

1) trace two functions called during CPU unplug via bcc/trace

/usr/share/bcc/tools/trace -T 'takedown_cpu "%d", arg1'  'take_cpu_down'

2) put cpu7 offline via:

echo 0 > /sys/devices/system/cpu/cpu7/online

3) only trace on 'takedown_cpu' is dumped via bcc/trace:

TIME     PID     TID     COMM            FUNC             -
03:23:17 733     733     bash            takedown_cpu     7

The lost trace on 'take_cpu_down' can never be shown, even though
CPU7 is switched ON again.

take_cpu_down is called via stop_machine_cpuslocked.

Thanks,
Ming Lei