How does tcppktlat tool work ?

[mind-fuzzy]

As per the documentation of tcppktlat tool, it seems to trace the latency between TCP received pkt and picked up by userspace thread.

And looking at the implementation it seems to use the raw tracepoint  tcp_probe and tcp_rcv_space_adjust to track the incoming and outgoing time to then calculate the latency.

tcp_probe tracepoint is at tcp_rcv_established which to my understanding is when the packet is received when the connection is in an established state. Which makes sense to me to track the incoming time.

However, I don't quite get how using tcp_rcv_space_adjust would provide us the outgoing time.. Isn't it traced when the receive buffer needs to be adjusted, which isn't always. So wouldn't it be better if tcp_recvmsg is used ?

[yonghong-song]

@mind-fuzzy The current tcppktlat can only track incoming packet latency between tcp receiving packet and the packet assigned to a user space. it does not support tracking latency for outgoing packets.

[mind-fuzzy]

the packet assigned to a user space

ah yes, I was wondering if tcp_rcv_space_adjust provides the right tracepoint to track when the packet is assigned to user space.

[mind-fuzzy]

This function should be called every time data is copied to user space. It calculates the appropriate TCP receive buffer space.

My bad. https://elixir.bootlin.com/linux/v6.1.14/source/net/ipv4/tcp_input.c#L701