request-energy-ebpf

Tiny eBPF experiment that attaches to a cgroup, tracks HTTP requests, and injects an X-Energy-nJ header on responses.

Prerequisites

• clang (for both userland and BPF)
• libbpf headers/libs (libbpf-devel on Fedora) and pkg-config
• A cgroup you can write to (examples below use /sys/fs/cgroup/httpdemo)

Build

make

This produces:

• http_energy.bpf.o – the BPF program
• http_energy – the user-space loader/attacher

Clean up:

make clean

Run (example)

1. Start a server in the target cgroup (adjust the path to your cgroup):

   sudo bash -c 'echo $$ > /sys/fs/cgroup/httpdemo/cgroup.procs; python3 -m http.server 8080'

2. In another shell, load the programs (optional third arg sets energy-per-nanosecond; default 5):

   sudo ./http_energy /sys/fs/cgroup/httpdemo ./http_energy.bpf.o 5

3. Curl the server and check for the X-Energy-nJ header:

   curl -v http://127.0.0.1:8080/

How energy is computed

• On connection establish (sockops), the program records opened_ns = ktime_get_ns().
• When the first response chunk (sk_msg) is seen, it computes energy_nj = (now - opened_ns) * energy_per_ns, capping the interval at 10s and the printed value at 12 digits. The header is injected only when the message payload is small (≤256 bytes) and linear; otherwise it is left untouched.

If you hit verifier or permission issues, ensure the cgroup path is correct and that your kernel supports SK_MSG programs. Use make clean && make after code changes.