iotdk: cpu_stats function doesn't work as expected

[maksimmasalski]

Describe the bug
cpu_stats profiling function doesn’t work as expected, it always report cpu usage 100 even if cpu in idle thread for a long time. It looks like the cpu_stats hooks does not have a chance to count the switch in of idle thread (stats_hw_tick.idle always 0).

Hello World! iotdk
CPU usage: 100
CPU usage: 100
CPU usage: 100
CPU usage: 100
CPU usage: 100

It always report cpu usage 100%, and global variable stats_hw_tick.idle is 0, but I confirmed cpu can enter idle

To Reproduce

1. Use latest master c64e0ce
2. SDK 0.11.4
3. Go to samples/hello_world
4. Add to prj.conf configuration for turning on cpu_stats:

CONFIG_TRACING=y
CONFIG_TRACING_CPU_STATS=y
CONFIG_TRACING_CPU_STATS_LOG=y

5. west build -p -b iotdk samples/hello_world/
6. west flash
7. View the output

Expected behavior
When system idle, the cpu usage reported should not be always 100%
My expectation

Hello World! iotdk
CPU usage: 0
CPU usage: 0
CPU usage: 0
CPU usage: 0
CPU usage: 0

Impact
Wrong cpu_stats function execution on the arc board.

Environment (please complete the following information):

• OS: Linux Ubuntu 18.04.4 LTS
• Toolchain Zephyr SDK 0.11.4
• Commit SHA c64e0ce

[nashif]

Here we are not recording switched_in/switched_out correctly for all threads and cpu stats depends on such information. switched_in might need to move to the arch context switch code and not be in the c code.

[jocelyn-li]

@evgeniy-paltsev I am interested with this fix. Do you have timeline for the fix?