reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git/commit/?id=03a12b026323247a320495fed3719d39cffdbe9b
        git remote add tj-cgroup https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git
        git fetch --no-tags tj-cgroup for-next
        git checkout 03a12b026323247a320495fed3719d39cffdbe9b
        # 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 kernel/cgroup/