How to trace credit_flow block/unblock ?

[kamyuentse]

Hello, I want to trace internal credit flow block/unblock. But I got nothing after adding the following to credit_flow.erl.

block(From) ->
    ?TRACE_BLOCKED(self(), From),
    rabbit_log:info("call credit_flow:block/1 ~p~n", [From]),
    case blocked() of
        false -> put(credit_blocked_at, erlang:monotonic_time());
        true  -> ok
    end,
    ?UPDATE(credit_blocked, [], Blocks, [From | Blocks]).

What's wrong for it? Or any other way to trace the internal credit flow status?