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Add warning about cgroup v1 force flag #2749

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Oct 24, 2024
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Add warning about cgroup v1 force flag #2749

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3 changes: 2 additions & 1 deletion doc/manual/install-judgehost.rst
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Expand Up @@ -172,7 +172,8 @@ any other tasks on the same CPU core the judgedaemon is using:

On modern distros (e.g. Debian bullseye and Ubuntu Jammy Jellyfish) which have
cgroup v2 enabled by default, you need to add ``systemd.unified_cgroup_hierarchy=0``
as well. Then run ``update-grub`` and reboot.
as well. If you are running systemd v257, you also need to add `SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1`.
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v257 or higher I assume?

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They say they will drop it fully in 248, so no

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@mpsijm mpsijm Oct 24, 2024 •
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I'm currently running v256 and I can't run with cgroup v1 at all.

If I remember correctly, only v255 requires this extra command-line parameter, and in v256, support was dropped entirely, so the flag is no longer needed there.

It may also be nice to warn users about this, something like: "Support for cgroup v1 is dropped entirely in systemd v256 or higher. DOMjudge will support cgroup v2 starting from version 8.4."

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https://github.com/systemd/systemd/blob/main/NEWS seems to suggest that for 257:

    * Support for cgroup v1 ('legacy' and 'hybrid' hierarchies) is now
      considered obsolete and systemd by default will refuse to boot under
      it. To forcibly reenable cgroup v1 support,
      SYSTEMD_CGROUP_ENABLE_LEGACY_FORCE=1 must be set on kernel command
      line. The complete removal of cgroup v1 is scheduled for v258.

You are saying this is wrong?

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@mpsijm mpsijm Oct 24, 2024 •
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Oh, never mind, I misremembered 🙂 Looking back at #1072 (comment), enabling cgroup v1 in v256 makes it harder to boot the host machine (i.e. they've added a 30 second timeout) 😅

On the other hand, @RagnarGrootKoerkamp couldn't boot his Arch at all anymore (#1072 (comment)). I assume he also ran v256 back then, though (because v257 is not released yet) 🤔

In any case, the kernel parameter appears to have been necessary since v256, which is confirmed by #1072 (comment). The command-line parameter is indeed not mentioned in the v255 changelog.

Then run ``update-grub`` and reboot.
After rebooting check that ``/proc/cmdline`` actually contains the
added kernel options. On VM hosting providers such as Google Cloud or
DigitalOcean, ``GRUB_CMDLINE_LINUX_DEFAULT`` may be overwritten
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