KWin freezing about kwin-lowlatency

[tildearrow]

I probably have fixed this issue now! By just using glFinish instead of glXWaitVideoSyncSGI, we achieve a similar effect (no extra latency). May you please test the changes I just pushed to the Plasma/5.15 branch?

[sanderboom]

Unfortunately, with the build of yesterday, I experienced two more hangs. The compositor dialog complains that OpenGL has crashed. Is there any package I can downgrade/upgrade (mesa?) to prevent hangs at the moment? Next week I will be on-site at a client and hangs would be unfortunate. I know I can go back to stock kwin, but I rather not as your changes make the WM smoother than ever - it has positive effect on my workflow (I especially hate input lag).

[sanderboom]

This reddit post seems the describe the problem I had. Maybe this is a hint that the problem is not due to kwin-lowlatency but other factors (someone suggests kernel 5.1).
I will install kwin-lowlatency again and run kernel 5.0.15 and see whether I have problems. When I have my other machine at hand I can try the offscreen debug session.

[tildearrow]

Linux 5.1...? I haven't upgraded to that version because in my case anything after 5.0.2 is borked (I upgraded once to 5.0.6, and next day I had filesystem corruption).

[sanderboom]

I had the same thing, but plasma packages on my system were still at 5.14. Now that I've update those to 5.15 I haven't had any freezes anymore.

[An4cl3t0]

I can reproduce the issue on Plasma 5.15.5-1

[tildearrow]

Confirmed. After checking the code, I may be able to conclude this is more of a Mesa bug in the glXWaitVideoSyncSGI function which sometimes causes the function to never return. I'll be trying the OML replacement for a while, and if it still reproduces, this isn't going to be good...

[tildearrow]

Nope, OML doesn't make a difference. I'm opening a bug in the Mesa bug tracker...

[tildearrow]

OK, so indeed Mesa 19.0 causes freezing, even on my AMD GPU. I'm going to try using the Intel swap event instead of glXWaitVideoSyncSGI, but if it doesn't work, this will be even more depressing...

[sanderboom]

Awesome! Compiling now. Yesterday and today I indeed experienced hangs on 5.15.5-1 as well. Will let you know the results!

[tildearrow]

I'm not sure which package may be able to fix the hangs. I tried downgrading X.Org and bisecting Mesa to go back to a point where in my system it no longer hangs, but neither helped.

Can you provide a backtrace at the time it does? Here are some instructions for doing so:
In an off-screen terminal (e.g. SSH):

$ cd kwin-lowlatency/build
$ QT_PLUGIN_PATH="$PWD/bin" gdb ./bin/kwin_x11

After the debugger loads:

(gdb) run --replace

Then, try getting the compositor to hang, and when it does, press Ctrl-Z in the debugger to break. Then do:

(gdb) backtrace

and copy/paste the result here.

[sanderboom]

I definitely will help, but this week is madness for work, so I reinstalled stock kwin for now :sad: Will report back when I have the opportunity!

[tildearrow]

Sure. I will be patiently waiting for a response. (sidenote: you can try openbox in the meanwhile)

(By the way, @An4cl3t0 are you still here?)

[An4cl3t0]

Yes, I'm following the report but I can't test the system until the weekend. Sorry :/

[tildearrow]

That's fine. I thought you abandoned the bug report.

[lucasslima]

Yo, Just wanted to give an Up, I'm experiencing the same issue.I haven't seen it in the last couple of days, but let me know if there's any way I can test it more precisely. I'm also on Intel Graphics.

[tildearrow]

@lucasslima Have you updated to 5.15.6? If so, and if it still reproduces, then may you provide me with a backtrace at the time of the hang? (instructions above)

[light0ear]

Can confirm that it works exactly as intended on kernel 5.0.18 with nvidia 418.74 without any freezing.
5.1 introduces freezes with a standard Kwin for me. The new kwin breaks Vsync so KDE devs should definitely find a way to fix their GLX swap buffers "fix". Legacy method of forcing triple buffering used to work, even at the cost of extra latency. Now it doesn't.

@tildearrow Thanks a lot for your effort and work! Right now, it is virtually the only way to get Vsync without stuttering, when running an Nvidia GPU.

[lucasslima]

I'm on versions:

kwin 5.15.6
kernel 5.1.5-2
mesa 19.0.5-1

I'm trying to reproduce the issue as hard as possible but didn't find that issue in the last couple of days. I'll try to find a case that reliably reproduces it.

By the way, just to send a message of appreciation for the project, this thing improved my Plasma experience quite a lot!

[tildearrow]

Sure. If you do reproduce the hang while not debugging, you may still be able to attach a debugger and provide a backtrace:

$ sudo gdb -p `pgrep kwin_x11`

(gdb) backtrace

(thanks for the message!)

[An4cl3t0]

Been using for 2 days and completely stable on kwin 5.15.6 and linux 5.0.18-hardened. Thanks.

[fvguilherme]

I am using Manjaro this kwin with:

KDE Plasma: 5.15.5
KDE Frameworks: 5.58.0
Qt: 5.12.3
Kernel: 5.1.4-1-MANJARO
NVidia 418.74-1 (MX150)

I can reproduce the bug and nothing has fix it. I found just a palliative: disable nvidia drm (options nvidia_drm modeset=1). That, however, cause some vertical tearing. So... It's tearing or hangs when using Edges. I choose the second one.

I already tried OpenGL 3.1 and 2.0 with VSync as "Never", but no success. A guy on Manjaro's official forum has solved the problem just installing kwin-lowlattency. So, I am confuse.

[lucasslima]

@fvguilherme , I assume you are using the Nvidia PRIME Sync? I had all sorts of problem with it when I was using it on my laptop with regular kwin (including absurd amounts of latency). You sure that doesn't happen on vanilla kwin?

[fvguilherme]

yes, you're right, I am using PRIME. And yes, the issue happen here with kwin-lowlatency. I tried with kernels 5.1.x, 5.0.x and 4.19.x-LTS, OpenGL 3.1 and 2.0 with VSync as "Never". Just two things has solved the problem: disable Compositor or set nvidia_drm modeset =0.

What worked for me (as palliative) to stop the freeze:

1 - nvidia_drm modeset =0 + OpenGL
2 - nvidia_drm modeset =1 + Compositor off
3 - nvidia_drm modeset =1 + Compositor on + XRender

The number 1 decrease performance on games. I got this when executing nvidia-settings:

$ nvidia-settings 

(nvidia-settings:3373): GLib-GObject-CRITICAL **: 23:10:08.370: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
GPU at BusId 0x1 doesn't have a supported video decoder

Maybe this can be the reason why kwin-lowlatency doesn't work for me.