|When used as the subject with a linking verb, all is usually singular. (Theodore Bernstein notes that when all means the only thing or everything, it is singular.) Note in the last two examples that the singular regulates both the subordinate and main verbs:
All I want for Christmas is my two front teeth.
All we found was candy wrappers and soda cans.
All that’s left of the structure is oak footings.
All that remains of the Hejaz Railway is scraps of rusty iron.
All may take a plural verb when the context is plural, often dictated by a prepositional phrase:
All of us are happy to be here.
All the books were saved from the fire.
All were happy to serve on the jury.
Generally omit of when all is followed by a noun:
He ate all the cake, but he ate all of it.