Add README

Add a README briefly explaining what this example does.

Signed-off-by: Simon Sundberg <[email protected]>

netstacklat/README.md

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+# Netstacklat - Monitor latency within the network stack
+Netstacklat is a simple tool for monitoring latency within the Linux
+network stack for ingress traffic. The tool relies on the kernel time
+stamping received packets (`SOF_TIMESTAMPING_RX_SOFTWARE`),
+specifically setting `sk_buff->tstamp`, and then reports when packets
+arrive at various hooks relative to this timestamp, i.e. the time
+between the latency between the packet being timestamped and reaching
+the hook.
+
+The tool is based on the following bpftrace script from Jesper
+Dangaard Brouer:
+'''console
+sudo bpftrace -e '
+	kfunc:tcp_v4_do_rcv,
+	kfunc:tcp_data_queue,
+	kfunc:udp_queue_rcv_one_skb
+	{
+		$tai_offset=37000000000;
+		$now=nsecs(tai)-$tai_offset; @cnt[probe]=count(); @total[probe]=count();
+		$ts=args->skb->tstamp; $delta=$now-(uint64)$ts;
+		@hist_ns[probe]=hist($delta);
+		@stats[probe]=stats($delta);
+		//printf("now:%llu - ts:%llu = delta:%llu\n", $now, $ts, $delta);
+	}
+	interval:s:10 {time("\n%H:%M:%S\n");
+		print(@cnt); clear(@cnt);
+		print(@total);
+		print(@stats);
+		print(@hist_ns);
+	}'
+'''
+
+The eBPF part of the tool (`netstacklat.bpf.c`) is designed to be
+compatible with
+[ebpf_exporter](https://github.com/cloudflare/ebpf_exporter), so that
+the data can easily be exported to Prometheus.