$pm->start(
        cb => sub {
            my($pm, $exit_code) = @_;
            local $SIG{USR1} = sub { $started_all_process = 1; };
            isnt $$, $pm->manager_pid, 'called by child';       # <<== HERE
            until ($started_all_process) {}; # wait
            note "exit_code: $exit_code";
            $pm->finish($exit_code);
            fail 'finish failed';
        },
        args => [$exit_code]
    );