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

Doug Smythies
2019-11-24T07:50:13Z

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.

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.
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, 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