RE: [PATCH v4 1/6] x86,sched: Add support for frequency invariance

Doug Smythies
2019-11-25T08:16:27Z

On 2019.11.23 23:50 Doug Smythies wrote:

> Hi all,
>
> The address list here is likely incorrect,
> and this e-mail is really about a kernel 5.4
> bisected regression.
>
> It had been since mid September, and kernel 5.3-rc8 since
> I had tried this, so I wanted to try it again. Call it due diligence.
> I focused on my own version of the "gitsource" test.
>
> Kernel 5.4-rc8 (as a baseline reference).
>
> My results were extremely surprising.
>
> As it turns out, at least on my test computer, both the
> acpi-cpufreq and intel_cpufreq CPU frequency scaling drivers
> using the schedutil governor are broken. For the tests that
> I ran, there is negligible difference between them and the
> performance governor. So, one might argue that they are not
> broken, but rather working incredibly well, which if true
> then this patch is no longer needed.

Should be able to gain better insight here with the
intel_pstate_tracer.py utility, watching for differences
in rates of rotation between CPUs. Too late tonight.

>
> I bisected the kernel and got:
>
> first bad commit: [04cbfba6208592999d7bfe6609ec01dc3fde73f5]
> Merge tag 'dmaengine-5.4-rc1' of git://git.infradead.org/users/vkoul/slave-dma
>
> Which did not make any sense at all. I don't even know how
> this is being pulled into my kernel compile.
> O.K., I often (usually) make a mistake
> during bisection, so I did it again, and got the same result.
>
> Relevant excerpt from the commit:
>
> diff --cc drivers/dma/Kconfig
> index 413efef,03fa0c5..7c511e3
> --- a/drivers/dma/Kconfig
> +++ b/drivers/dma/Kconfig
> @@@ -294,8 -294,8 +294,8 @@@ config INTEL_IOATDM
> If unsure, say N.
>
> config INTEL_IOP_ADMA
> - tristate "Intel IOP ADMA support"
> - depends on ARCH_IOP32X || ARCH_IOP33X || ARCH_IOP13XX
> + tristate "Intel IOP32x ADMA support"
> - depends on ARCH_IOP32X
> ++ depends on ARCH_IOP32X || COMPILE_TEST
> select DMA_ENGINE
> select ASYNC_TX_ENABLE_CHANNEL_SWITCH
> help
>
> If I revert the above, manually, then everything behaves
> as expected (minimally tested only, so far).
>
> Are others seeing the schedutil governors not working as
> expected with any of kernels 5.4-rc1 - 5.4-rc8?
>
> I do have a pretty graph of my method of doing the
> "gitsource" test, but am not ready to post it yet.

Graphs and write up (mostly the same as herein) are now
here:

http://www.smythies.com/~doug/linux/single-threaded/k54regression/index.html

The graphs are rather crowded, sorry.

> Here is some gitsource test data, 6 runs of "make test",
> the first run is discarded:
>
> "gg 6" means this 6 patch set.
>
> Kernel 5.4-rc8 + revert, intel_cpufreq/schedutil: 3899 seconds
> Kernel 5.4-rc8 + gg 6 + revert, intel_cpufreq/schedutil: 2740.7 seconds
> Ratio: 0.70 (as expected)

Kernel 5.4-rc8 + gg 6 + revert, forced CPU affinity performance: 2106.5 seconds

> Kernel 5.4-rc8, intel_cpufreq/schedutil: 2334.7 seconds (faster than expected)
> Kernel 5.4-rc8 + gg 6 patch set, intel_cpufreq/schedutil: 2275.0 seconds (faster than expected)
> Ratio: 0.97 (not as expected)
> Kernel 5.4-rc8, intel_cpufreq/performance: 2215.3 seconds
> Kernel 5.4-rc8, intel_cpufreq/ondemand: 3286.3 seconds
> Re-stated from previous e-mail:
> Kernel 5.3-rc8, intel_cpufreq/schedutil: ratio: 0.69 (I don't have the original times)

... Doug