It's an informal expression but it takes on different meanings with different emotions as the others said. You can use it with a voice of exasperation, e.g. when you have told somebody how to do something correctly but they still haven't listened to you and make the same mistake."What ARE you doing?" "What do you think you are doing?" You can use it with a voice of slight annoyance. There's the sarcastic use. You can use it in a an easy going sort of manner to your friend - "Hey, what are you doing?""what are you up to?" It's not the sort of thing to say with guests at a formal event.

This grammar is totally used in a non-formal situation, especially speaking to your friends or families or someone who is close to you. In the conversation you posted, I guess this person B doesn't normally get up so early, so person A said this sentence"干嘛啊你" instead of "你干嘛啊" to express some kind of surprise. In some kind of short sentence, in order to express some feeling like surprise or sarcasm(in a joke way), we normally put subject at the end of sentence.

“干嘛啊，你” is an informal way of expression, and it's very common in oral Chinese . the formal way is "你干嘛啊“

I remember a sentence"我吃饭在"

两者说法是不一样的。 “干嘛啊你？”，具有很强的感情色彩，同时，也强调了“干嘛”，但事实上可能根本不是在询问，而是在劝阻。比如，“大过年的，干嘛你？“意思是说，大过年的，大家都开开心心的，你不要这样做，大家会因为你而不开心，这里并不是在询问。而说成“你干嘛”往往只是一种简单的询问，如果表达上面的用法，比如”大过年的，你干嘛？“，却给人一种挑衅的感觉，好象要打架的样子。 在句子在后面加上，”你、我“之类，会缓和语气，表达某种情感。这是汉语的规律，与地域无关。比如：我容易么我！最后一我，就表达我的情绪，我觉得自己很不容易，自己不被理解很委屈等等。”你以为你是谁啊你？“，最后一个就表达了”你有什么了不起的，你别自己当成一回事“等等类似的言外之意。 不过，这种表达方式应该属于口语表达。 简而言之，最后加是”你、我“，和放在句首，是不同的结构，不同表达方式，感情色彩、含义等等都是不同的。当然有时也可表达相同的含义，但那不主要的，不同才是主要的。