reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=cd91b502f1b4bb81e82fbae38678c698ee5ac026
        git remote add tip https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
        git fetch --no-tags tip locking/futex
        git checkout cd91b502f1b4bb81e82fbae38678c698ee5ac026
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=x86_64 olddefconfig
        make W=1 ARCH=x86_64 
        make W=1 headers
        make W=1 -C tools/testing/selftests