webOS OSE kernel doesn't work with ftrace feature

[honggyukim]

The Linux kernel inside webOS OSE doesn't work with ftrace feature as follows:

root@raspberrypi3:/sys/kernel/debug/tracing# cat tracing_on
1

root@raspberrypi3:/sys/kernel/debug/tracing# cat trace
# tracer: nop
#
# entries-in-buffer/entries-written: 0/0   #P:4
#
#                              _-----=> irqs-off
#                             / _----=> need-resched
#                            | / _---=> hardirq/softirq
#                            || / _--=> preempt-depth
#                            ||| /     delay
#           TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
#              | |       |   ||||       |         |

root@raspberrypi3:/sys/kernel/debug/tracing# cat current_tracer
nop

root@raspberrypi3:/sys/kernel/debug/tracing# cat available_tracers
blk function_graph wakeup_dl wakeup_rt wakeup irqsoff function nop

root@raspberrypi3:/sys/kernel/debug/tracing# echo "function_graph" > current_tracer

Right after writing "function_graph" to "current_tracer", the system freezes and cannot do anything.
The same problem happens when I tested to enable "function" tracer as follows:

root@raspberrypi3:/sys/kernel/debug/tracing# echo function > current_tracer

Except for those, it seems that other tracers work fine.

It may not be an important issue for webOS OSE, but I'm just leaving a note for later reference.

[honggyukim]

Here is the kernel and system info.

root@raspberrypi3:/sys/kernel/debug/tracing# uname -a
Linux raspberrypi3 4.4.50 #1 SMP Wed Jan 9 12:17:48 UTC 2019 armv7l GNU/Linux