reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf.git/commit/?id=20d564bba6b3806c26498061299a88330561efa5
        git remote add netfilter-nf https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf.git
        git fetch --no-tags netfilter-nf testing
        git checkout 20d564bba6b3806c26498061299a88330561efa5

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash net/sched/