ftracetest in ubuntu_kselftests_ftrace hang with ARM64k kernel on step 19

Issue found on Focal hwe 5.15 generic ARM64k / lowlatency ARM64k instance (scobee-kernel).

This is not a regression but a report to follow up with bug 1959424, which describes the ftrace hangs on step 16 with ARM64.

Bug 1959424 happens when you try to run the whole ubuntu_kernel_selftests test suite with ftracetest in it.

After we're splitting ftracetest out to ubuntu_kselftests_ftrace, it's failing on step 19 instead.

Test log:
05:14:10 DEBUG| Running 'make run_tests -C ftrace TEST_PROGS=ftracetest TEST_GEN_PROGS='' TEST_CUSTOM_PROGS='''
05:14:10 DEBUG| [stdout] make: Entering directory '/home/ubuntu/autotest/client/tmp/ubuntu_kselftests_ftrace/src/linux/tools/testing/selftests/ftrace'
05:14:10 DEBUG| [stdout] TAP version 13
05:14:10 DEBUG| [stdout] 1..1
05:14:10 DEBUG| [stdout] # selftests: ftrace: ftracetest
05:14:10 DEBUG| [stdout] # === Ftrace unit tests ===
05:14:10 DEBUG| [stdout] # [1] Basic trace file check [PASS]
05:14:15 DEBUG| [stdout] # [2] Basic test for tracers [PASS]
05:14:16 DEBUG| [stdout] # [3] Basic trace clock test [PASS]
05:14:16 DEBUG| [stdout] # [4] Basic event tracing check [PASS]
05:14:16 DEBUG| [stdout] # [5] Change the ringbuffer size [PASS]
05:14:17 DEBUG| [stdout] # [6] Snapshot and tracing setting [PASS]
05:14:17 DEBUG| [stdout] # [7] trace_pipe and trace_marker [PASS]
05:14:17 DEBUG| [stdout] # [8] Test ftrace direct functions against tracers [UNRESOLVED]
05:14:17 DEBUG| [stdout] # [9] Test ftrace direct functions against kprobes [UNRESOLVED]
05:14:18 DEBUG| [stdout] # [10] Generic dynamic event - add/remove eprobe events [PASS]
05:14:18 DEBUG| [stdout] # [11] Generic dynamic event - add/remove kprobe events [PASS]
05:14:18 DEBUG| [stdout] # [12] Generic dynamic event - add/remove synthetic events [PASS]
05:14:18 DEBUG| [stdout] # [13] Generic dynamic event - selective clear (compatibility) [PASS]
05:14:18 DEBUG| [stdout] # [14] Generic dynamic event - generic clear event [PASS]
05:14:19 DEBUG| [stdout] # [15] Generic dynamic event - check if duplicate events are caught [PASS]
05:14:19 DEBUG| [stdout] # [16] event tracing - enable/disable with event level files [PASS]
05:14:19 DEBUG| [stdout] # [17] event tracing - restricts events based on pid notrace filtering [PASS]
05:14:20 DEBUG| [stdout] # [18] event tracing - restricts events based on pid [PASS]
06:29:15 INFO | Timer expired (4500 sec.), nuking pid 18954