Comment 15 for bug 1744370

Just re-discovered my own report here whilst trying to solve another occurrence of the same issue 18 months later.

Now with 18.04 and 4.15.0-56-lowlatency (amd64).

With assistance of grawity in IRC #systemd I found some more information which might help narrow this down.
$ uname -a

Linux T300CHI 4.15.0-56-lowlatency #62-Ubuntu SMP PREEMPT Wed Jul 24 21:03:13 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

$ systemctl list-jobs

   JOB UNIT TYPE STATE
740792 systemd-suspend.service start running
740791 suspend.target start waiting

2 jobs listed.

$ systemctl status systemd-suspend.service

● systemd-suspend.service - Suspend
   Loaded: loaded (/lib/systemd/system/systemd-suspend.service; static; vendor preset: enabled)
   Active: activating (start) since Fri 2019-08-02 08:26:54 BST; 53min ago
     Docs: man:systemd-suspend.service(8)
 Main PID: 16968 (systemd-sleep)
    Tasks: 1 (limit: 4915)
   CGroup: /system.slice/systemd-suspend.service
           â””─16968 /lib/systemd/systemd-sleep suspend

Aug 02 08:26:54 T300CHI systemd[1]: Starting Suspend...
Aug 02 08:26:55 T300CHI systemd-sleep[16968]: Suspending system...

$ sudo cat /proc/$(pidof systemd-sleep)/stack

[<0>] __vt_event_wait.isra.2.part.3+0x40/0x90
[<0>] vt_waitactive+0x7b/0xd0
[<0>] vt_move_to_console+0x46/0xc0
[<0>] pm_prepare_console+0x23/0x40
[<0>] pm_suspend+0x19c/0x3b0
[<0>] state_store+0x85/0xf0
[<0>] kobj_attr_store+0x12/0x20
[<0>] sysfs_kf_write+0x3c/0x50
[<0>] kernfs_fop_write+0x125/0x1a0
[<0>] __vfs_write+0x1b/0x40
[<0>] vfs_write+0xb1/0x1a0
[<0>] SyS_write+0x55/0xc0
[<0>] do_syscall_64+0x73/0x130
[<0>] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[<0>] 0xffffffffffffffff