homework and exercises - Solving a problem using Newtonian mechanics and D'Alembert principle

yI have to solve that problem with two methods (applying Newtonian mechanics and the D'Alembert principle.

The problem consists in two balls inside a spherical cylinder, it consists in determine the minimum value of $M$ making the tube not to knock down (where $M$ is the mass of the cylinder and $m$ the masses of the two spheres).

I have issues with both methods. With Newtonian method, I don't know what influence has $M$ on the problem, because I can choose a reference point in the center of the cylinder and there will be no torque. With D'Alembert principle, the problem is I have no idea what virtual displacement I have to choose.

The Newtonian process brings me to this meaningless expression if the normal force acts on the lower right corner.