I am currently trying to self learn general relativity. I understand you need knowledge of an action principle, but what I am not so sure of is how deep of an understanding I need. I currently have a book on classical field theory and I am at the point of basic Lagrangian's and holonomic systems. I can compute lagrangians and I am very familiar with the E-L equation, but I have yet to come to non-linear dynamics and hamiltonian mechanics. My question boils down to how much do I need for an introduction to G-rel?