Re: [PATCH v3 00/21] cpufreq: introduce a new AMD CPU frequency control mechanism

[Matt McDonald <gardotd426@gmail.com>]

> > I've tested this driver and it seems the results are a little
> > underwhelming.
> > The test machine is a two sockets server with two AMD EPYC 7713,
> > family:model:stepping 25:1:1, 128 cores/256 threads, 256G of memory
> > and SSD
> > storage. On this system, the amd-pstate driver works only in
> > "shared memory support", not in "full MSR support",
> > meaning that frequency switches are triggered from a workqueue
> > instead of scheduler context (!fast_switch).

Huang, I've also done some detailed testing, and while many synthetic
benchmarks seem to show minimal differences between this new frequency
control mechanism and acpi_cpufreq, the general user experience seems a
bit degraded, but most of all, gaming performance in many instances (if
not all) is cut in half. Fully half. 

I have an RTX 3090 and a Ryzen 9 5900X, with 32GB (4x8) DDR4 3600. In
Control with DLSS and RT enabled, on 5.15.rc5 with acpi_cpufreq, I get
120-130 fps at 1440p. The same exact kernel with v3 of AMD_CPPC gives
me 50 fps. GPU usage is still at 100, but the CPU frequency is being
reported as like 5100Mhz*, and other assorted weirdness, but most
importantly the fps is stuck at 50. This is regardless of performance
scheduler (schedutil, ondemand, userspace or performance). 

*My CPU can indeed boost over 5GHz on a single core here and there, but
this was constant and on all cores, so clearly it wasn't accurate.

Also, from the documentation it looks like there's supposed to be a way
to fall back to acpi_cpufreq, but I found no such way to do that. If
AMD_CPPC was built into the kernel, I had to use amd-pstate, there was
no other option. Maybe I misinterpreted and acpi-cpufreq is only able
to be used as a fallback for CPUs that don't support amd-pstate.

I know that gaming on Linux hasn't historically been one of AMD's
priorities with their CPUs, but with the Steam Deck upcoming I would
imagine this is a pretty important use-case, and I've tested multiple
games and they all lose a full 50% performance. I'm happy to test any
revisions or even kernel parameters or whatever else to try and get
this sorted. 



> Would you mind that we add a module param or filter the known good
> processors (mobile parts) to load amd-pstate. And others can use the
> param
> to switch between amd-pstate and acpi-cpufreq manually? After we
> address the
> performance gap, then we can switch it back.


This would be something I would be interested to try.

> 
> It seems the issue mainly from the processors with big number of
> cores and
> threads. Let's find the similiar family threadripper or EYPC
> processors to
> duplicate the test results. Will contact at you for details. :-)

This may be an interesting route of investigation, I could potentially
try running a game with `taskset -c 0-7` or something similar. 

>