The code setting the cgroup path vor cgroups v1 reads like this:

/*
 * Set directory to subsystem specific files based
 * on the contents of the mountinfo and cgroup files.
 */
void CgroupV1Controller::set_subsystem_path(char *cgroup_path) {
  char buf[MAXPATHLEN+1];
  if (_root != NULL && cgroup_path != NULL) {
    if (strcmp(_root, "/") == 0) {
      int buflen;
      strncpy(buf, _mount_point, MAXPATHLEN);
      buf[MAXPATHLEN-1] = '\0';
      if (strcmp(cgroup_path,"/") != 0) {
        buflen = strlen(buf);
        if ((buflen + strlen(cgroup_path)) > (MAXPATHLEN-1)) {
          return;
        }
        strncat(buf, cgroup_path, MAXPATHLEN-buflen);
        buf[MAXPATHLEN-1] = '\0';
      }
      _path = os::strdup(buf);
    } else {
      if (strcmp(_root, cgroup_path) == 0) {
        strncpy(buf, _mount_point, MAXPATHLEN);
        buf[MAXPATHLEN-1] = '\0';
        _path = os::strdup(buf);
      } else {
        char *p = strstr(cgroup_path, _root);
        if (p != NULL && p == _root) {
          if (strlen(cgroup_path) > strlen(_root)) {
            int buflen;
            strncpy(buf, _mount_point, MAXPATHLEN);
            buf[MAXPATHLEN-1] = '\0';
            buflen = strlen(buf);
            if ((buflen + strlen(cgroup_path) - strlen(_root)) > (MAXPATHLEN-1)) {
              return;
            }
            strncat(buf, cgroup_path + strlen(_root), MAXPATHLEN-buflen);
            buf[MAXPATHLEN-1] = '\0';
            _path = os::strdup(buf);
          }
        }
      }
    }
  }
}

That is for the substring case, we see a 'p = strstr(cgroup_path, _root)' check that is followed by 'if (p != NULL && p == _root)'. We enter this branch only iff _root != '/' AND cgroup_path != _root. So the only way that the pointer points to '_root' (p == _root) is when cgroup_path is the same pointer than _root, but in that case we should have entered the branch where it is checked that cgroup_path == _root. Thus, this is dead code.

I believe the intention of the original author of this code is to check whether there is a substring match of cgroup_path and _root. If cgroup_path starts with _root, the remainder after _root is being appended to the _mount_path. At least that's what the corresponding Java code does.