Dublin timezone cause systemd cpu high

[altairchan]

Systemd version(249) in Jammy Jellyfish is too old, When using the Dublin time zone for daylight saving time switching，the CPU usage will suddenly increase to 100% starting from 2024-03-31 00:00:00 and last for 1 hour, but return to normal at 2024-03-31 02:00:00. plz backport the bugfix to v249 or update 255 version in Jammy Jellyfish to fix it. thank you.

[altairchan (altairchan)]

how to reduce:
1. Stop clock synchronization
systemctl stop chrony

2. Set the timezone to Dublin
timedatectl set-timezone Europe/Dublin

3. Change the time to waitting for Daylight saving time switch
timedatectl set-time "2024-03-30 23:59:50"

At 0 o'clock, the system is executing atop logrotation.

# cat /etc/cron.d/atop
PATH=/bin:/usr/bin:/sbin:/usr/sbin

# daily restart of atop at midnight
0 0 * * * root [ -d "/run/systemd/system" ] || /usr/share/atop/atop.daily&
0 0 * * * root [ -d "/run/systemd/system" ] && systemctl restart atop

[Ivan Vučica (ivucica)]

Encountered last night (2025-03-30) on Debian 12 on systemd package 252.36-1~deb12u1. 100% CPU usage by /sbin/init stopped either by stopping .timer units manually or by waiting until clock changed 00:59:59Z -> 02:00:00+0100.

This is evidenced by one of the multiple affected machines logs showing .timer-covered .service units stopped restarting exactly at that time.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Skia (hyask)]

I'm not sure about the proposed fix, but:
* Dublin is probably not the only timezone impacted.
* We should probably try to reproduce that and confirm the bug on Jammy, but also check Noble, just to be sure.

[Skia (hyask)]

I took a bit of time to try to reproduce, without success. Here were my steps:

$ lxc launch ubuntu:jammy --vm jammy
Launching jammy
$ lxc exec jammy bash
root@jammy:~# apt update && apt install -y atop
...
root@jammy:~# systemctl stop systemd-timesyncd
root@jammy:~# timedatectl set-timezone Europe/Dublin
root@jammy:~# timedatectl set-time "2024-03-30 23:59:50"

In another terminal before messing with the time:
$ lxc exec jammy htop

I did not see init eating 100% CPU, the machine remained mostly idle.

Could you please provide more details on how to reproduce the bug?