[Bug 109055] ~10% perf drop in Sascha Willems Vulkan Multithreading demo

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=109055

            Bug ID: 109055
           Summary: ~10% perf drop in Sascha Willems Vulkan Multithreading
                    demo
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Vulkan/intel
          Assignee: intel-3d-bugs at lists.freedesktop.org
          Reporter: eero.t.tamminen at intel.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org
                CC: jason at jlekstrand.net

Setup:
* SKL or KBL device (don't have data from others)
* Ubuntu 18.04 / Unity
* drm-tip v4.19 or newer kernel
* git version of X with modifiers (dmabuf capable) enabled
* Mesa git version

Test-case:
* multithreading --fullscreen --benchmark --benchwarmup 3 --benchruntime 20

Result:
* FPS drops by 5-10%

Between following Mesa commits:
41c8f991379d1a 2018-11-12 18:28:04 util: Fix warning in u_cpu_detect on non-x86
25b48e3df93dee 2018-11-14 12:12:09 st/xa: Bump minor

Along with the perf drop, one can see both CPU & GPU power usage drops
(according to RAPL), and GPU spending 30-40% in RC6 instead of 0%.

I.e. there's a Mesa change that makes this GPU test CPU bound.

I didn't see any significant perf changes in other benchmarks (Vulkan or GL).

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