I am looking for adding QoS for GRE Tunnel and found this info
You can apply a service policy to either the tunnel interface or to the underlying physical interface. The decision of where to apply the policy depends on the QoS objectives. It also depends on which header you need to use for classification.
Apply the policy to a physical interface and enable qos-preclassify on a tunnel interface when you want to classify packets based on the pre-tunnel header.
In our environment, I am using service policy under serial interface, the source interface of Tunnel is F0/0, so from above info, which interface is "physical interface" for my case, serial or F0/0 ?