Bug#1051514: grub-common: Please remove grub-common.service from boot hot path

[Julian Andres Klode]

On Sun, Sep 10, 2023 at 03:45:52PM +0200, Paul Menzel wrote:
> Dear Andres,
> 
> 
> Thank you for your reply.
> 
> Am 10.09.23 um 13:54 schrieb Julian Andres Klode:
> > On Sat, Sep 09, 2023 at 12:21:29AM +0200, Paul Menzel wrote:
> > > Package: grub-common
> > > Version: 2.12~rc1-9
> > > Severity: normal
> 
> > > The unit `grub-common.service` is installed to the multi-user.target and
> > > therefore run during boot-up, slowing down the boot, especially as shell
> > > commands are used.
> > > 
> > > ```
> > > $ systemctl cat grub-common.service
> > > # /lib/systemd/system/grub-common.service
> > > [Unit]
> > > Description=Record successful boot for GRUB
> > > After=suspend.target hibernate.target hybrid-sleep.target
> > > suspend-then-hibernate.target
> > > ConditionPathExists=/boot/grub/grub.cfg
> > > 
> > > [Service]
> > > Type=oneshot
> > > Restart=no
> > > ExecStartPre=/bin/sh -c '[ -s /boot/grub/grubenv ] || rm -f
> > > /boot/grub/grubenv; mkdir -p /boot/grub'
> > > ExecStart=grub-editenv /boot/grub/grubenv unset recordfail
> > > ExecStartPost=/bin/sh -c 'if grub-editenv /boot/grub/grubenv list | grep -q
> > > initrdless_boot_fallback_triggered=1; then echo "grub: GRUB_FORCE_PARTUUID
> > > set, initrdless boot paniced, fallback triggered."; fi'
> > > StandardOutput=kmsg
> > > 
> > > [Install]
> > > WantedBy=multi-user.target suspend.target hibernate.target
> > > hybrid-sleep.target suspend-then-hibernate.target
> > > ```
> > 
> > As you can see, there are no Before relations in the service,
> > so grub-common is not in the hot path, nothing depends on it
> > having finished startup.
> > 
> > I'm sure this was well intended and not a troll attempt, but
> > systemd doesn't start services in a sequence, so services can
> > run in parallel and there is in general very little ordering
> > requirements.
> 
> That is exactly, what I was saying. Due to the missing ordering, systemd
> starts this service as early as possible causing pressure on the possibly
> scarce system resources at the beginning. So care has to be taken
> introducing these things. Looking into decreasing the start time of an
> Ubuntu system once, grub-common showed up there, so it will make the startup
> time of quite a lot of Debian systems longer now too. Please keep in mind,
> that Debian is also run on older systems.

I think bug reports like this are ill-advised, it is not our decision
to make how to start services and abusing timers for this is wrong.

If you want units that do not contribute to the target to run only
when idle, then my suggestion would be to add such a feature to
systemd itself rather than go around trying to hack each such instance
with timers.

That said, I strongly disagree with the assessment. Boot is not
interactive so it is less of a priority to have it fast vs having
a performant interactive session.

On the extreme, if you boot into your desktop and then it eats up
all your cores and locks up the UI on your resource constrained
device that's a worse experience than waiting longer at boot and
then having things work correctly.
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