Real-time preemption not being applied to the kernel

[concerned12345]

Building the latest 5.4.24 kernel with Yocto Zeus for the Embedian iMX8MM SMARC SoM. I've added a .bbappend file with correct SRC_URI and checksum, then build with bitbake.

I then go into `bitbake -c menuconfig linux-smarcimx8mm` and select `Embedded System` which then allows me to select a fully preemptive real-time kernel. I select the fully preemptive option and build again. When I go back to `...menuconfig...` I see that full preemption is not selected and instead low latency desktop is selected. The LOCALVERSION variable in config doesn't get updated either.

I've tried several approaches but I cannot get the fully preemptive kernel to be built.

Also, after selecting full preemption Yocto complains the kernel has been tainted when doing a build.

[Zhiming_Liu]

Hi,

After you selected the full preemption in menuconfig, you can find what has been added about full preemption,like CONFIG_XXXXX. Then you can copy the config to imx8_xxx.defconfig.Then you can make the kernel.It's a effective way changing the kernel config that  i have found.

Best Regards,

Zhiming

[concerned12345]

I believe I found the config file that menuconfig saves to, problem is menuconfig doesn't always save my configuration. Example: I turned off full preemption and saved. When I went back to menuconfig it still had full preemption selected.

I've noticed from the patch file that CONFIG_PREEMPT_RT_FULL is no longer used, is this correct? Is this replaced with CONFIG_PREEMPT_RT along with other configuration options?

I was able to get `bitbake -c diffconfig virtual/kernel` to output the differences which I will use as a fragment file (*.cfg) which will be applied from my bbappend file. Sometimes I cannot get a difference because menuconfig won't always save my changes, don't know what's going on there.

Lastly, is there a place within the output kernel code (or build artifacts) where I can see what `uname -v` will output? I'd like to check if `PREEMPT RT` is being applied to the version string without having to build an SD card and fire up the hardware with the new image.

[Zhiming_Liu]

Hi,

When you exit menuconfig, the config file is saved in .config in kernel main folder,then you can find /imx-yocto-bsp/build-xwayland_8qmm/tmp/work-shared/imx8mmevk/kernel-source/arch/arm64/configs/imx_v8_defconfig. Open the defconfig,the add the CONFIG__XXX from .config to defconfig.You can try CONFIG_PREEMPT_RT.

[concerned12345 · Accepted solution]

tl;dr - I had to run menuconfig and build several times before real time preemption was compiled into the kernel.

I set the settings through menuconfig (embedded system and fully preemptive) and did a `bitbake -c diffconfig virtual/kernel` and viewed the fragments.cfg file output. Turns out CONFIG_PREEMPT_RT_FULL is no longer used, instead multiple other configurations are set.

I'm still having a problem of `bitbake -c menuconfig virtual/kernel` not always saving my settings. I even added a do_configure() function to my bbappend kernel recipe file to load in my fragment file (*.cfg). I still don't get an RT kernel. The only thing that seems to work is run menuconfig and build several times before the settings take. I know you suggested to change the main imx..._defconfig file but this should not be the correct solution. Fragment files should work and saving settings from menuconfig should also work the first time.

I verify real time preemption is set by going to the folder:

~/smarc-imx8mm-zeus-release/core-image-minimal/tmp/work/smarcimx8mm2g-poky-linux/linux-smarcimx8mm/5.4-r0/build/include/generated

I then open file compile.h and look at the UTS_VERSION string to see if "PREEMPT" and "RT" are present since this is where `uname -v` gets its information.