Wandering Thoughts archives

2024-03-07

Some notes about the Cloudflare eBPF Prometheus exporter for Linux

I've been a fan of the Cloudflare eBPF Prometheus exporter for some time, ever since I saw their example of per-disk IO latency histograms. And the general idea is extremely appealing; you can gather a lot of information with eBPF (usually from the kernel), and the ability to turn it into metrics is potentially quite powerful. However, actually using it has always been a bit arcane, especially if you were stepping outside the bounds of Cloudflare's canned examples. So here's some notes on the current version (which is more or less v2.4.0 as I write this), written in part for me in the future when I want to fiddle with eBPF-created metrics again.

If you build the ebpf_exporter yourself, you want to use their provided Makefile rather than try to do it directly. This Makefile will give you the choice to build a 'static' binary or a dynamic one (with 'make build-dynamic'); the static is the default. I put 'static' into quotes because of the glibc NSS problem; if you're on a glibc-using Linux, your static binary will still depend on your version of glibc. However, it will contain a statically linked libbpf, which will make your life easier. Unfortunately, building a static version is impossible on some Linux distributions, such as Fedora, because Fedora just doesn't provide static versions of some required libraries (as far as I can tell, libelf.a). If you have to build a dynamic executable, a normal ebpf_exporter build will depend on the libbpf shared library you can find in libbpf/dest/usr/lib. You'll need to set a LD_LIBRARY_PATH to find this copy of libbpf.so at runtime.

(You can try building with the system libbpf, but it may not be recent enough for ebpf_exporter.)

To get metrics from eBPF with ebpf_exporter, you need an eBPF program that collects the metrics and then a YAML configuration that tells ebpf_exporter how to handle what the eBPF program provides. The original version of ebpf_exporter had you specify eBPF programs in text in your (YAML) configuration file and then compiled them when it started. This approach has fallen out of favour, so now eBPF programs must be pre-compiled to special .o files that are loaded at runtime. I believe these .o files are relatively portable across systems; I've used ones built on Fedora 39 on Ubuntu 22.04. The simplest way to build either a provided example or your own one is to put it in the examples directory and then do 'make <name>.bpf.o'. Running 'make' in the examples directory will build all of the standard examples.

To run an eBPF program or programs, you copy their <name>.bpf.o and <name>.yaml to a configuration directory of your choice, specify this directory in theebpf_exporter '--config.dir' argument, and then use '--config.names=<name>,<name2>,...' to say what programs to run. The suffix of the YAML configuration file and the eBPF object file are always fixed.

The repository has some documentation on the YAML (and eBPF) that you have to write to get metrics. However, it is probably not sufficient to explain how to modify the examples or especially to write new ones. If you're doing this (for example, to revive an old example that was removed when the exporter moved to the current pre-compiled approach), you really want to read over existing examples and then copy their general structure more or less exactly. This is especially important because the main ebpf_exporter contains some special handling for at least histograms that assumes things are being done as in their examples. When reading examples, it helps to know that Cloudflare has a bunch of helpers that are in various header files in the examples directory. You want to use these helpers, not the normal, standard bpf helpers.

(However, although not documented in bpf-helpers(7), '__sync_fetch_and_add()' is a standard eBPF thing. It is not so much documented as mentioned in some kernel BPF documentation on arrays and maps and in bpf(2).)

One source of (e)BPF code to copy from that is generally similar to what you'll write for ebpf_exporter is bcc/libbpf-tools (in the <name>.bpf.c files). An eBPF program like runqlat.bpf.c will need restructuring to be used as an ebpf_exporter program, but it will show you what you can hook into with eBPF and how. Often these examples will be more elaborate than you need for ebpf_exporter, with more options and the ability to narrowly select things; you can take all of that out.

(When setting up things like the number of histogram slots, be careful to copy exactly what the examples do in both your .bpf.c and in your YAML, mysterious '+ 1's and all.)

linux/EbpfExporterNotes written at 23:01:56; Add Comment


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