[Bug 207667] New: power_dpm_force_performance_level set to "low" reduces CPU performance (Vega 8 / Ryzen 2200G)

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=207667

            Bug ID: 207667
           Summary: power_dpm_force_performance_level set to "low" reduces
                    CPU performance (Vega 8 / Ryzen 2200G)
           Product: Drivers
           Version: 2.5
    Kernel Version: 5.6.8
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: low
          Priority: P1
         Component: Video(DRI - non Intel)
          Assignee: drivers_video-dri@kernel-bugs.osdl.org
          Reporter: alekshs@hotmail.com
        Regression: No

I've been on a power saving quest lately, trying various different things. One
of the things that I don't particularly need in my Ryzen 2200G is GPU
performance. So it made sense to reduce the 400 MHz clock to lower levels to
gain in power efficiency.

I echoed "low" to power_dpm_force_performance_level and indeed my power
consumption dropped while the frequency went down from 400MHz to 200MHz. I now
see Vsoc went from 1.01v to 0.91v and Isoc went from 3.5A to 1.75A.

What I did not expect was that I would see performance issues in terms of CPU
tasks.

CPU tasks like SHA256 hashing are unaffected.
CPU tasks that involve RAM, like lrzip, cinebench 15 (under wine), etc, show a
considerable drop in performance. CB15 cpu score loses >10% while an lrzip file
compression went from 14 to 21 seconds. I also did a geekbench 5 comparison
(more of that in the end - with links).

I measured cache and memory bandwidth, it seems to be the same whether I have
power_dpm_force_performance_level set to low or auto, so downclocking the
integrated graphics of the ryzen didn't impact memory speeds. Maybe it's an
interactivity thing / irq thing or something that makes tasks wait? Or maybe
it's a power delay issue as the SOC has to increase watts on demand? I don't
know - someone has to find out.

I think it's abnormal behavior for the IGP clock to cause CPU applications to
slow down - especially by more than 50% in some cases:

I've run 2 geekbench 5.0 benchmarks, with the only difference being "low" and
"auto" on the power_dpm_force_performance_level:

Low: https://browser.geekbench.com/v5/cpu/2096508
Auto: https://browser.geekbench.com/v5/cpu/2096537

If you scroll down you see some tasks are very heavily affected while others
aren't.

For example multithreaded raytracing is at 4900 in both instances. But
multithreaded speech recognition is 900 vs 1800, while AES is 2100 vs 4500.
Huge differences.

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