group_add_at
err = group_add_at(&pgd->cmt_pgs, parent, idx);
err = group_add_at(&pgd->cmt_pgs, pg, idx - 1);
err = group_add_at(cmt_pgs, pg, levels - level - 1);
ret = group_add_at(pg_hw, g, (uint_t)hw);
int group_add_at(group_t *, void *, uint_t);