It depends, really.
Milan was the centre of reformist socialist leader Filippo Turati, while in the South clientelarism ran rampant and the Socialist Party itself was seen as too detached from the rural masses (Salvemini was a great critic of this) It's highly unlikely that reformist Trade Unions and the Parrty will mount something revolutionary, bringing the great mass of industrial workers with them, if nothing really bad happens at State-level. And it should also happen before the Bolshevik Revolution, otherwise ruling classes will get too much scared and be ready for everything.
I suggest either a very botched government response to the Red Week in Ancona in 1914, which leads to heavy police repression, deads and injureds, eventually martial law as soon as Italy joins the war and then revolution; or something in the middle of the conflict, such as anarcho-syndicalists leaders survive their first years in the trenches and, as Caporetto hits the Italian Army, they lead an armed uprising against the abysmal Cadorna leadership, allying with mainstream socialists after the government cracks down on all the oppositions.
In both cases, Mussolini would be at the forefront of revolutionary efforts, and could well become the new Italy's Minister for Propaganda, with a serious claim on future leadership, especially in the first case.