[kwin] [Bug 524033] New: Kwin Wayland is too jittery compared to Xorg

https://bugs.kde.org/show_bug.cgi?id=524033

            Bug ID: 524033
           Summary: Kwin Wayland is too jittery compared to Xorg
    Classification: Plasma
           Product: kwin
      Version First 6.7.4
       Reported In:
          Platform: Arch Linux
                OS: Linux
            Status: REPORTED
          Keywords: wayland-only
          Severity: major
          Priority: NOR
         Component: performance
          Assignee: [email protected]
          Reporter: [email protected]
  Target Milestone: ---

DESCRIPTION
KDE desktop is moving to Wayland only and will move away from X11 future
versions, but Kwin as a standalone compositor has a major issue that Xorg
don't: Performance. Despite having higher average FPS in 3D applications/games
with Kwin Wayland, I have high amount of stutter/frame skip, including mouse
cursor, when interacting with Kwin.

I listed below some situations where Kwin struggles:

STEPS TO REPRODUCE
* When using native Wayland applications which uses CPU rendering such as Kate,
Dolphin, Konsole, Kwin is very jittery and the workaround is to launch
application with Xwayland with QT_QPA_PLATFORM=xcb. I previously opened a bug
about this issue (https://bugs.kde.org/show_bug.cgi?id=513295), but it was
closed without being solved and tested.
* Kwin also stutters when cursor shape changes
(https://bugs.kde.org/show_bug.cgi?id=509669).
* Opening display settings and clicking on "Identify screens" freezes Kwin for
~500 ms.
* Opening tab box with ALT+TAB also freezes Kwin for ~500 ms the first time
(https://bugs.kde.org/show_bug.cgi?id=479250).
* Moving a window the edges of a screen freezes Kwin for ~300 ms the first
time.
* Here is an example of user complaining about cursor lags using JetBrains
Toolbox:
https://youtrack.jetbrains.com/projects/JBR/issues/JBR-10036/Wayland-system-cursor-stutter-freezing

I applied to following patch on the top of Kwin to see when it hangs more than
100 ms:

```
--- a/src/backends/drm/drm_gpu.h
+++ b/src/backends/drm/drm_gpu.h
@@ -74,7 +74,7 @@ public:
     /**
      * This should always be longer than any real pageflip can take, even with
PSR and modesets
      */
-    static constexpr std::chrono::milliseconds s_pageflipTimeout =
std::chrono::seconds(1);
+    static constexpr std::chrono::milliseconds s_pageflipTimeout =
std::chrono::milliseconds(100);

     DrmGpu(DrmBackend *backend, int fd, std::unique_ptr<DrmDevice> &&device);
     ~DrmGpu();

```
And after I applied this patch, I saw the message "The main thread was hanging
temporarily!" many times even on normal desktop usage with an AMD Ryzen 7 3700X
CPU and a Nvidia GeForce RTX 2080 SUPER which are high end hardware from 2019.
With Xorg, even on a old laptop from 2009 with an Intel Core 2 Duo and GeForce
9600M GT, the cursor doesn't drop a single frame.

OBSERVED RESULT
Kwin is jittery and freezes.

EXPECTED RESULT
Kwin render smoothly.
The cursor shouldn't drop any frame.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 6.7.4
KDE Frameworks Version: 6.28.0
Qt Version: 6.11.1
Linux 7.1.6-arch1-1

ADDITIONAL INFORMATION
When my Nvidia GPU hangs due to a context switch timeout, I can still move the
cursor but when the cursor shape changes by hovering a link for example, the
cursor doesn't move anymore and the keyboard become unresponsive (Capslock LED
is stuck) and I have "The main thread was hanging temporarily!" and not
"Pageflip timed out! This is a bug in the nvidia_drm kernel driver", which
means this is a Kwin issue and not an issue with the display driver which is
still responding to atomic commits. On Xorg and GNOME Wayland, I can still move
the cursor in the same situation.

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