Bug 101182

Summary: Reading amdgpu_pm_info causes stutter with Linux 4.12-rc
Product: DRI Reporter: Christoph Haag <haagch>
Component: DRM/AMDgpuAssignee: Default DRI bug account <dri-devel>
Status: RESOLVED WONTFIX QA Contact:
Severity: normal    
Priority: medium    
Version: DRI git   
Hardware: Other   
OS: All   
Whiteboard:
i915 platform: i915 features:
Attachments:
Description Flags
screenshot showing the stuttering in the HUD none

Description Christoph Haag 2017-05-25 06:59:12 UTC 
A stutter once wouldn't be so bad, but apps like radeon-profile repeatedly read this file, so rendering stutters constantly.

You will have a hard time spotting it in the average fps graph, but with my (still very inaccurate) low-fps patch at https://github.com/ChristophHaag/mesa-mesa/commit/e097b91a85c85718576589402e075e7f5668e68c it's very visible that there is at least one frame that's delayed a lot each time amdgpu_pm_info is read.
The GPU load does give an independent hint that something is wrong though.

To reproduce:
run  glxgears
run  sudo watch --interval=.3 cat /sys/kernel/debug/dri/0/amdgpu_pm_info
observe stuttering in glxgears.

RX 480, Linux 4.12-rc2.
I haven't tested 4.11 to confirm, but if I remember correctly, this happens in mainline since 4.12.
Comment 1 Christoph Haag 2017-05-25 07:00:35 UTC 
Created attachment 131501 [details]
screenshot showing the stuttering in the HUD
Comment 2 Alex Deucher 2017-05-25 13:16:18 UTC 
amdgpu_pm_info exposes power statistics now which causes a small delay to sample the power usage.  If you want to poll the current clocks, there is an ioctl interface to query this information.
Comment 3 Christoph Haag 2017-05-25 13:23:36 UTC 
Can this be done asynchronously in the driver so it doesn't impact the rendering of 3D applications?

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