[Bug 107111] New: "Avoid waitboosting" now prevents all GPU boosting (e9af4ea2b9e7)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=107111

            Bug ID: 107111
           Summary: "Avoid waitboosting" now prevents all GPU boosting
                    (e9af4ea2b9e7)
           Product: DRI
           Version: DRI git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: freedesktop at nmacleod.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Created attachment 140459
  --> https://bugs.freedesktop.org/attachment.cgi?id=140459&action=edit
Monitoring script and results

We've had reports of the Kodi GUI "stuttering" since LibreELEC switched from
kernel 4.14.49 to 4.17.3.

I ran a script that monitors /sys/kernel/debug/dri/0/i915_rps_boost_info and
noticed that with 4.17.3 the GPU never boosts out of the "low power" window.
It's the same when watching a video - the GPU is now almost always stuck in the
"low power" window.

git bisecting the kernel between 4.14.49 and 4.17.3 reveals
e9af4ea2b9e7e5d3caa6354be14de06b678ed0fa[1] ("drm/i915: Avoid waitboosting on
the active request") as the bad commit.

In the attached file you can see the script, and also the results of testing
many different LibreELEC/Kodi builds (corresponding to the builds here[2]).

With this script the first 30 seconds of a 4K video (Elysium trailer, on a
Skylake NUC6i5SYH outputting at 1080p) is played from local storage in Kodi. At
the end of the script a summary is output which includes the percentage of
"LOW" (ie. low power) and "!LOW" (ie. high + mixed) window samples. The script
is executed 5 times per build.

What is very clear is that from build 180616 (ie. #0616[3]) onwards - which is
when the LibreELEC kernel switched from 4.14.49 to 4.17.3 - the number of
"!LOW" samples is reduced to almost zero - ie. the GPU no longer boosts,
remaining locked at the "idle" frequency.

To rule out any changes in Kodi that might also be responsible I rebuilt build
180702 replacing only the 4.17.3 kernel with 4.14.49, creating build 180702a,
where now the GPU boosts as expected.

Reverting the bad commit from kernel 4.17.3[4] resulted in build 180702b, and
again the GPU is now boosting as expected.

Kernel 4.18-rc1 also has this non-boosting issue (same as 4.17.3).

With this commit reverted, the reports of "stutter" in Kodi with 4.17.3 (and
now also 4.17.4) are resolved[5, 6, 7].

Can this commit be reverted, or is there a better solution that doesn't hobble
the GPU?

Thanks
Neil

1.
https://github.com/torvalds/linux/commit/e9af4ea2b9e7e5d3caa6354be14de06b678ed0fa
2. https://forum.kodi.tv/showthread.php?tid=298462
3. http://forum.kodi.tv/showthread.php?tid=298462&pid=2744031#pid2744031
4.
https://github.com/LibreELEC/LibreELEC.tv/pull/2798/commits/914cadbafd09861f05244d4b985476414760823b
5. https://forum.kodi.tv/showthread.php?tid=298462&pid=2748603#pid2748603
6. https://forum.kodi.tv/showthread.php?tid=298462&pid=2748688#pid2748688
7. https://forum.kodi.tv/showthread.php?tid=298462&pid=2748696#pid2748696

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