Goliath

Today I read the passage about the battle between David and Goliath. David had taken a break from serving Saul to feed his father's sheep.

David went back and forth from Saul to tend his father’s sheep at Bethlehem. (1 Samuel 17:15)

Then his father sends him to bring provisions to the battle line and he hears Goliath boasting and he stands up to it, Saul sends him off and and David kills Goliath. After this there is a puzzling discussion. Still in 1 Samuel:

⁵⁶ The king said, “Find out whose son this young man is.”
⁵⁷ As soon as David returned from killing the Philistine, Abner took him and brought him before Saul, with David still holding the Philistine’s head.
⁵⁸ “Whose son are you, young man?” Saul asked him.
David said, “I am the son of your servant Jesse of Bethlehem.”

It is puzzling that Saul doesn't know who David is. David works for him on a regular basis playing the harp to make the evil spirit go off Saul. Saul was the one who tried to suit up David to face Goliath. How can he not know who David is?