---
layout: post
title: Bird CPU usage
date: 2025-01-07 19:06 +0100
categories: tech
lang: en
---

**Preface**: This is kind of a "bug hunt log". I extensively posted on Mastodon
about it, but wanted to keep the progress in writing as well. This might lead
like random rambling in some parts, because I edited and rearranged the
paragraphs of this post a few times. Sorry for that.

**Preface 2**: Starting point: In September 2024 I upgraded to bird2
`2.15.1-1~bpo12+1` - probably a self-built backport of mine.  This, I "had to
to", since I wanted the Babel metrics measurement, which is not present in
Debian stable (`2.0.12-7`). Yeah, I won't complain. For 99 % of the stuff I do,
Debian stable is *perfect*. I want a server without regularly breaking things,
after all.  Only recently (2015-01-07), I added the bird / network.cz repos and
installed bird2 from there.

Several times already, I noticed this in my Munin monitoring:

{% image
img="https://pics.uvokchee.de/upload/2025/01/07/20250107180512-c1453895.png"
alt="RRD tool graphic showing a high CPU usage" %}

I found it strange, but had no time to inspect further.

Recently, I tried to investigate what happens on the server there.  htop showed
high CPU usage for both bird and dnsmasq (always together) in these times.

Fuming (angry) a bit, I went with a brute-force approach:

```
#!/bin/bash

# Configuration
THRESHOLD=1.0
# 3 hours
DURATION=$((3*80*80))
MAIL_TO="lolnope"
SUBJECT="High Load Average Alert"
BODY="The load average has been above ${THRESHOLD} for more than 3 hours."
REBOOT_CMD="/sbin/reboot"

# Function to check the load average
check_load() {
    # 15 min loadavg
    loadavg=$(awk '{print $3}' /proc/loadavg)
    echo "$(date): Current Load Average: $loadavg"

    if (( $(echo "$loadavg > $THRESHOLD" | bc -l) )); then
        echo "$(date): Load average is above threshold."
        return 0
    else
        echo "$(date): Load average is below threshold."
        return 1
    fi
}

# Monitor the load average
start_time=$(date +%s)
while true; do
    if check_load; then
        current_time=$(date +%s)
        elapsed_time=$((current_time - start_time))

        if [ "$elapsed_time" -gt "$DURATION" ]; then
            echo "$(date): Load average has been above threshold for more than 3 hours."

            # Send alert email
            (echo "$BODY"; ps -e -o %cpu,%mem,cmd --sort pcpu | tail) | mail -s "$SUBJECT" "$MAIL_TO"

            # Reboot the server - room for improvement
#            systemctl stop bird
#            systemctl start bird
            $REBOOT_CMD
            break
        fi
    else
        start_time=$(date +%s)
    fi
    sleep 300 # Check every 5 minutes
done

```

Specifically, the output of ps

```
22.7  2.7 /usr/sbin/bird -f -u bird -g bird
33.3  0.1 ps -e -o %cpu,%mem,cmd --sort pcpu
37.4  0.0 /usr/sbin/dnsmasq -x /run/dnsmasq/dnsmasq.pid -u dnsmasq -7 /etc/dnsmasq.d,.dpkg-dist,.dpkg-old,.dpkg-new,.bak --local-service --trust-anchor=.,20326,8,2,e06d44b80b8f1d39a95c0b0d7c65d08458e880409bbc683457104237c7f8ec8d
```

confirmed the quick look at htop - although the "percentage" is a bit weird.
From the manpage:

> Currently, it is the CPU time used divided by the time the process has been
> running (cputime/realtime ratio), expressed as a percentage.

(So if the process runs "long enough" and only starts misbehaving after a year,
it won't show up?).

I asked an LLM what to do, in addition to strace, and it suggested perf.
Unfortunately, this requires debug symbols [1]. [And while Debian does provide
debug symbols](https://wiki.debian.org/HowToGetABacktrace) - it doesn't for
dnsmasq (yet) in bookworm.  Luckily, the nice people at labs.nic.cz provide a
dbgsym package in their Bird(2) Debian repository.

Aside, the script above could benefit from some improvements, instead of
rebooting, I could actually record and strace and perf when this happens, and
only then reboot / restart the service.

Now, stracing dnsmasq (when "idle") reveals some recvmsg of type
`RTM_NEWROUTE`.  I have *no idea* why dnsmasq would need that. But I already
*assume* the high CPU usage occurs when Bird exports lots of routes to the
kernel. *I have no hecking clue why dnsmasq would need to use the CPU at the
same time. If someone can enlighten me, **please** message me.*

Also, in journalctl, I see lots of the infamous `Kernel dropped some netlink
messages, will resync on next scan.` messages at times - the message apparently
nobody has a solution to, and even though there are mailing list posts telling
to sysctl `net.core.rmem_default`, I doesn't seem to yield a solution.

I actually did a `perf record` of bird when the CPU usage was high, and I saw
that `rt_event` was running 33% of the time. I don't know what to make of that.

So… step-by-step? I have both DN42 and clearnet bird running on this VPS, in
parallel. So maybe start by disabling one of these? Or, I have a better idea,
keep it enabled, and disable all protocols! (`birdc disable \"*\"`).  That
helped, until midnight. When suddenly the CPU went up again. WTF?  Let's have a
look at `birdc s p`. All protocols up. Huh!?!?

Let's investigate further:

* Oooo, log rotation happens at midnight
* Fuck, I specified the same log files for both bird's

Well, log rotation. Which I added myself. This does a `birdc configure`
afterward. Which means the protocols go up again, because I disabled them on the
command line, not in the config.

Ungh. Okay, this is getting really ugly. `systemctl disable --now bird-clear`.
Now let's run this for a few days...

That seem to have helped. I now decided to edit the clearnet config and disable
all "external" protocols (to IXPs), keeping Babel, RPKI, and IBGP enabled.
Immediately after starting bird-clearnet, the CPU usage went up again. To be
expected, for the initial sync. But it kept being high. So, I disabled the RPKI
protocols as well and... suddenly, the CPU usage is down???

(2025-01-09) So, now, the RPKI stuff... I saw that the connection to one of my
RPKI servers wouldn't work (Fort Validator), and Fort complained about `Bad RTR
version: 2`.  The [Bird
docs](https://bird.network.cz/?get_doc&v=20&f=bird-6.html) mention a `max
version` option, and with that, the connection worked. And the CPU usage was
down. *However*, this was not "the root issue". This option is only present
since bird 2.16, which I only installed on 2025-01-07 [2]. And I had the CPU
problems back in November last year! So this is "something completely
different", it appears. I currently have the BGP sessions to virtual IXPs
disabled, and will run it like this for a few days. The average load is 0.4.
Still too high, compared to my other VPS.

[1] At least when I want to see the binaries function names.  Kernel symbols
seem to show up fine. \
[2] Which is pretty annoying, when you chase one bug, you change something in
your config, and then something else breaks. That's on me.