Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.

Cover sweet potatoes with water in a large pot; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and transfer potatoes to a large bowl.

Mash potatoes with white sugar, milk, eggs, salt, vanilla extract, and cinnamon. Spread mixture in prepared baking dish.

Mix brown sugar, pecans, and flour in another bowl. Pour margarine over pecan mixture; toss until mixture resembles coarse crumbs. Sprinkle over potato mixture.

Bake in the oven until pecan topping is hard, about 1 hour.