I'm trying to do something that I hope is relatively simple, namely to
run a simple kexec -l ... command on system shutdown before the /boot
file system is unmounted. I'm trying this on two different machines,
both of which are running current versions of arch linux with systemd
204. The machines are configured very similarly, except that one has
a serial console and some NFS mounts, while the other does not.
Unfortunately, I cannot find a single script that works on both
machines.
On the machine with a serial console, the following unit file seems to
work (in /etc/systemd/system/kexec-load@.service, enabled with
systemctl enable kexec-load@linux). This is a slightly modified
version of a suggestion on the arch linux wiki:
[Unit]
Description=load %i kernel into the current kernel
Documentation=man:kexec(8)
DefaultDependencies=no
Before=shutdown.target umount.target final.target
After=sysinit.target
[Service]
Type=oneshot
ExecStart=/usr/bin/kexec -l /boot/vmlinuz-%i
--initrd=/boot/initramfs-%i.img --reuse-cmdline
[Install]
WantedBy=kexec.target
However, when I try this script on the machine without a serial
console, the script gets run at the very end of the shutdown process,
after all the file systems are unmounted (and hence at a point where
the new kernel image is unavailable). I can boot with
systemd.log_level=debug, but since there's no serial console and I
can't get my job to fire before the journal system is shutdown, it's
hard for me to capture all the output on shutdown.
Conversely, I devised a kind of hacky way to make it work on the
non-serial-console machine what doesn't work on the serial console
one. Here my idea is to have a job whose ExecStop loads the new
kernel. I'm not sure why that job fires only on startup and not
shutdown of the serial console machine, but if I solve one of these
problems, I'd rather get the WantedBy=kexec.target approach working,
if that's possible, so I'll omit the details for now.
For what it's worth, here's a relevant subset of the output of the
systemd --test command on the machine that does not work (i.e., the
non-serial console) with the above unit file:
/usr/lib/systemd/systemd --system --test --unit=kexec.target
-> By units:
-> Unit shutdown.target:
After: kexec-load@linux.service
ReferencedBy: kexec-load@linux.service
-> Unit systemd-journald.socket:
Before: kexec-load@linux.service
ReferencedBy: kexec-load@linux.service
-> Unit sysinit.target:
Before: kexec-load@linux.service
ReferencedBy: kexec-load@linux.service
-> Unit final.target:
After: kexec-load@linux.service
ReferencedBy: kexec-load@linux.service
-> Unit umount.target:
After: kexec-load@linux.service
ReferencedBy: kexec-load@linux.service
-> Unit kexec.target:
Wants: kexec-load@linux.service
References: kexec-load@linux.service
-> Unit kexec-load@linux.service:
Name: kexec-load@linux.service
Fragment Path: /etc/systemd/system/kexec-load@.service
-> ExecStart:
Command Line: /usr/bin/kexec -l
/boot/vmlinuz-%i --initrd=/boot/initramfs-%i.img --reuse-cmdline
So here the dependencies seem fine. Yet when I look at the "by jobs"
category:
-> By jobs:
-> Job 3:
Action: kexec.target -> start
State: waiting
Forced: no
Irreversible: no
-> Job 4:
Action: systemd-kexec.service -> start
State: waiting
Forced: no
Irreversible: no
-> Job 5:
Action: shutdown.target -> start
State: waiting
Forced: no
Irreversible: no
-> Job 83:
Action: umount.target -> start
State: waiting
Forced: no
Irreversible: no
-> Job 88:
Action: boot.mount -> stop
State: waiting
Forced: no
Irreversible: no
-> Job 90:
Action: final.target -> start
State: waiting
Forced: no
Irreversible: no
-> Job 91:
Action: kexec-load@linux.service -> start
State: waiting
Forced: no
Irreversible: no
[end of file]
Job 91 is the very last job. So while I'm not 100% sure how to parse
this output, it sure looks as though kexec-load is getting started
*after* boot.mount is stopped, which is indeed the behavior I see when
rebooting my machine with "systemctl kexec". On the other hand, maybe
"waiting" means these jobs are waiting for dependencies?
At any rate, I'd be grateful for any advice on what the problem is, or
even better for any meta-advice on how to debug these things in
general. (In a sense, debugging my dependencies and solving the
problem are the same thing, as getting my shutdown script to run while
the journal service is active would solve the problem.)
Also, in order to short-circuit some potentially redundant discussion,
I should say that I've also tried adding various permutations of the
following to my unit file, all to no avail:
RequiresMountsFor=/boot
RequiresMountsFor=/boot/vmlinuz-linux
Requires=boot.mount
After=boot.mount
Thanks.
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