[Bug 106175] amdgpu.dc=1 shows performance issues with Xorg compositors when moving windows

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=106175

--- Comment #5 from Mariusz Ceier <mceier+freedesktop at gmail.com> ---

amdgpu.dc=1 also causes performance issue with 2 games I own: "Rise of the
Tomb Raider" and "Helium Rain" (UE4 game with sources publicly available on 
github).

I have 2 monitors (1st is 60Hz, 2nd in 144Hz). During tests I was using
1920x1080 resolution in both games which is 60Hz on both monitors.

1. With amdgpu.dc=0 everything is fine.

2. With amdgpu.dc=1:

The issue was showing up only in menus when cursor was visible and was in the
game window e.g. when Helium Rain was running on 2nd monitor (fullscreen)
and I was moving mouse over its window - mouse was lagging/stuttering/not 
responding and Xorg server was producing messages like below  in Xorg.0.log:

   (II) event5  - USB Gaming Mouse: SYN_DROPPED event - some input events have
been lost.
   (EE) client bug: timer event5 debounce short: offset negative (-7ms)     


When I moved mouse to the 1st monitor the messages were not produced and mouse
was not lagging.

I tried few things (including switching from full dyntick system to idle
dyntick) until finally MrCooper at #xorg-devel suggested trying amdgpu.dc=0
which fixed the issue.

With amdgpu.dc=1 latencytop was showing that drm_modeset_backoff was taking
~20ms.

I was using kernel 4.17.0-rc1 and 4.17.0-rc2, Mesa 18.2.0-devel 
(git-d136a5fad9) and X.Org X Server 1.19.99.904 (1.20.0 RC 4) with
xf86-video-amdgpu and radeonsi.

I'm using 1000Hz gaming mouse.

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