Switch User+DPMS then click Unlock causes a delay with black screen

[Eddie B]

ed@userver:~$ lsb_release -rd
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04
ed@userver:~$ apt-cache policy pkgname lightdm-gtk-greeter
lightdm-gtk-greeter:
  Installed: 2.0.6-0ubuntu1
  Candidate: 2.0.6-0ubuntu1
  Version table:
 *** 2.0.6-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status
N: Unable to locate package pkgname

I'm not sure if the issue is in the LightDM greeter, but it only happens after I use the greeter.

I expect my desktop to get displayed soon after I click "Unlock" on the greeter.

What happens is that there is a delay with black screen of at least 2 seconds (often 10 minutes).

     Details:

When I "Switch User" and then click unlock button, I get a black screen with no mouse cursor for 2 seconds, and then black screen with mouse cursor for approx 0.65 seconds for every minute my PC was in DPMS (after 495 minutes in DPMS the black screen is up for a very long 321 seconds). If I'm on greeter screen and keep moving mouse to avoid DPMS, then I only have the 2 second delay.

This only happens with NVidia drivers (nouveau drivers don't have any delay at all). I tested on fresh installs of both Ubuntu 19.10 with default gdm and UbuntuStudio 19.10 with LightDM and now UbuntuStudio 20.04 nightly build. If I prevent DPMS by moving my mouse every few minutes, then I only have the 2 second delay. I am not able to reproduce this issue on UbuntuStudio 18.04.

Methods that cause delay:
- Click "Switch User" on the Whisker menu.
- Click "Switch User" on the "Action Buttons" on panel 0.
- dm-tool switch-to-greeter (displays greeter)
- dm-tool lock (starts DPMS, then greeter)

Methods that DO NOT cause delay:
- Click "Lock" on the Whisker menu.
- Click "Lock" on the "Action Buttons" on panel 0.
- xdotool key Ctrl+alt+l (runs screensaver, only delay if click "Switch User")
- xflock4 (runs screensaver, only delay if click "Switch User")
- xset dpms force off (runs screensaver, only delay if click "Switch User")

Another interesting symptom is that the first 2 second black screen does have the mouse pointer in 19.10, but not in 20.04. Also, If I Lock instead of Switch User, then my desktop comes up immediately.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: lightdm-gtk-greeter 2.0.6-0ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-14.17-lowlatency 5.4.18
Uname: Linux 5.4.0-14-lowlatency x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu18
Architecture: amd64
Date: Sun Mar 1 00:56:12 2020
InstallationDate: Installed on 2020-02-29 (0 days ago)
InstallationMedia: Ubuntu-Studio 20.04 LTS "Focal Fossa" - Alpha amd64 (20200227)
ProcEnviron:
 LANGUAGE=en_US
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm-gtk-greeter
UpgradeStatus: No upgrade log present (probably fresh install)

[Eddie B (fasteddieb216)]

Also, during the black screen the mouse is sluggish and the keyboard is not responsive until desktop finally shows (not even ctrl-alt-F2), except the num-lock key does turn light on and off. Caps lock has no affect.

Another thing is that after clicking unlock on the greeter the mouse cursor jumps to a location near the top right corner (about 1" down and half inch over) on 19.10 and to center of screen on 20.04 and that cursor shows as the correct cursor type for what will be at that location when the screen finally shows up (so if a terminal is at that location it will show as the I shaped text cursor).

And I do get some log activity during the black screen, so my PC isn't locked up totally, but there is nothing consistent.

[Eddie B (fasteddieb216)]

I modified lightdm-gtk-greeter to put more debug messages in and found that the delay starts sometime after lightdm-gtk-greeter closes. I'm trying to do the same thing with lightdm to see where the black screen delay starts, but I'm having trouble getting my modified lightdm to replace lightdm.

I'm not sure if this is helpful, but I found that if I install light-locker then the black screen delay also happens when I choose to lock the screen. If I uninstall light-locker then only "switch user/unlock" causes delay and "lock/unlock" displays desktop immediately like stated in original post.

[Launchpad Janitor (janitor)]

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

[Aloys (ggaloys)]

I have also the NVidia drivers installed (default 440) and I also suffer from up to minutes of waiting when switching users, which is a feature I rely on quite a bit with the kids needing the family computer while home schooled...
For me the mouse cursor is always there and not laggy though, but just a big black screen as soon as I click on the switch icon.

[Eddie B (fasteddieb216)]

Aloys, I have found ways to minimize the delays. It only happens when I use "Switch User" or when another user is logged in for a long time, so I use "lock" instead of "Switch User" when I'm not using the computer and make sure my wife always logs off when she is done and I log back in as soon as I notice she logged off. This gives me only slight delays and she doesn't have any delays.

I'm sure now that the issue is not with lightdm-gtk-greeter, since I have this issue on Ubuntu Studio (lightdm/xfce4), Ubuntu (gdm3/Gnome) and Kubuntu (sddm/Plasma). It is probably (mostly) an Nvidia bug, but I didn't have this issue on UbuntuStudio 18.04, so there is some new code in X11 or kernel that is causing it. And I have no other issues with the Nvidia drivers.