On Wed, Jul 24, 2024 at 10:52:21AM +0200, Peter Zijlstra wrote:

[...]

> > > Also, why does that thing hard depend on DEBUG_BTF? (at least having
> > > that option enabled no longer explodes build times like it used to)
> > 
> > That's necessary for building the schedulers, at least, I think. We didn't
> > have that earlier and people were getting confused.
> 
> It's what made it difficult for me to even build this stuff, simple
> things like:
> 
>   cd foo-build; ../scipts/config --enable CONFIG_SCHED_CLASS_EXT; cd -
> 
> don't work (nor error out on the lack of dependencies), and my build
> scripts were force disabling the BTF crud -- and thus silently also the
> scx thing.
> 
> It looks like I can change my builds scripts, because the reason I did
> that was that BTF made the build times explode and that is no longer the
> case.

Yeah so to clarify the situation here -- BTF is a hard requirement for
sched_ext because BPF requires it in order to call kfuncs [0] (note that
the APIs have changed significantly since that article was written). The
reason it's required, among other things, is so that the verifier can
check that a BPF prog is safely invoking a kernel function with the
proper types.

[0]: https://lwn.net/Articles/856005/