Add FCOS variant to use rt-kernel now that PREEMPT_RT is merged into 6.12

[tkarls]

Describe the enhancement

I would like to be able to use FCOS with kernel compiled with the RT patches (PREEMPT_RT) now that this is merged into the mainline kernel (but still require to be compiled with the proper config) to better support latency critical applications.

System details

This would only be feasible once FCOS starts to ship with kernel 6.12 - I don't expect to have any external patches applied.

Additional information

No response

[jlebon]

I think this probably makes most sense to discuss in the context of #1726. Having multiple FCOS variants adds a lot of complexity unfortunately. One approach is to provide the definitions for it like centos-bootc does and let users build it themselves with podman build. Another approach, which while not great definitely has its benefits, is to swap out the kernel in a layered build (see e.g. https://github.com/coreos/layering-examples/blob/main/kernel-rt/Containerfile, but a lot of the bits there can be made cleaner soon).

That said, for now, it should also work to use rpm-ostree override replace to do this client-side. See e.g. the RHCOS unofficial docs for this: https://github.com/openshift/os/blob/master/docs/faq.md#q-how-do-i-replace-the-current-kernel-with-kernel-rt-or-a-new-kernel-version-in-rhcos.

[Nemric]

I think that could be related : https://bugzilla.redhat.com/show_bug.cgi?id=2329409

[travier]

My understanding is that now that it's merged in the upstream kernel, once https://bugzilla.redhat.com/show_bug.cgi?id=2329409 is enabled, there will be a single kernel build and the real time part would be enabled with a kernel argument likely, so there would be nothing for us to do here to have it.

[travier]

Reading https://www.kernelconfig.io/CONFIG_PREEMPT_RT?q=PREEMPT_RT&kernelversion=6.12.1&arch=x86, maybe it's not the case yet and Fedora will have to build two kernels to get it.

[tkarls]

Yes, it is merged into the kernel. But my understanding is that it is a compile time switch and not a boot time switch. So it would require two separate built kernels.

[LorbusChris]

This might require a Fedora Change proposal to get the kernel-rt RPM built then