Failed to trace Round Trip Time (RTT) in kernel functions

The kernel version is 4.14.62-65.117.amzn1.x86_64. I used attach_kprobe() to inject code into three kernel functions about RTT. But it failed in two of them.
tcp_ack_update_rtt(): cannot attach kprobe, probe entry may not exist.
tcp_update_rtt_min(): ERROR: tcp_update_rtt_min() kernel function not found or traceable.
tcp_synack_rtt_meas(): it works.
Could anybody explain why attach_kprobe() fails for the first two functions? Thanks very much for the help.