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Subject[PATCH 0/1] cpufreq: pcc-cpufreq: Re-introduce deadband effect to reduce number of frequency changes
Hello,

I've observed performance degradation with different workloads on HP
ProLiant systems that use pcc-cpufreq driver between older and more
recent kernels. Bisection showed that commit 6393d6a102 (cpufreq:
ondemand: Eliminate the deadband effect) caused a significant
performance drop. This patch was introduced in v3.17.

I am not too familiar with the PCC stuff but I think that elimination
of the deadband effect causes a significant increase of requested
frequency changes which in turn will have to be served by pcc-cpufreq
and slow down corresponding systems significantly.

AFAIK there is no frequency table for this driver, instead the driver
just asks firmware to set any requested frequency for a CPU (if its in
the min-max-range of allowed frequencies). Thus I think the
probability that a requested target frequency is matching the current
frequency of a CPU is lower in comparison to drivers that use a
fixed set of frequencies.

This is with two exceptions:

(1) when a CPU is under full load -- maximum frequency already set and
maximum frequency is requested.

(2) CPU has just "minor load" -- minimum frequency already set and
minimum frequency is requested.

I think what commit 6393d6a102 caused is, that case (2) occurs only if
the CPU is fully idle. Whereas before commit 6393d6a102 this case
occurred in all situations when

load < freq_min/freq_max * 100

I suggest to introduce the old behaviour (re-introduce the deadband effect)
for pcc-cpufreq with the patch that follows.

Here are typical numbers from kernel compilation tests with varying
number of compile jobs:

v4.8.0-rc2 4.8.0-rc2-pcc-cpufreq-deadband
# of jobst user sys elapsed CPU user sys elapsed CPU
2 440.39 116.49 4:33.35 203% 404.85 109.10 4:10.35 205%
4 436.87 133.39 2:22.88 399% 381.83 128.00 2:06.84 401%
8 475.49 157.68 1:22.24 769% 344.36 149.08 1:04.29 767%
16 620.69 188.33 0:54.74 1477% 374.60 157.40 0:36.76 1447%
32 815.79 209.58 0:37.22 2754% 490.46 160.22 0:24.87 2616%
64 394.13 60.55 0:13.54 3355% 386.54 60.33 0:12.79 3493%
120 398.24 61.55 0:14.60 3148% 390.44 61.19 0:13.07 3453%

As expected under full load (64, 120 jobs) performance is "almost"
comparable. But with partial load (esp. 32 jobs) length of kernel
build is just 2/3 with patched kernel in comparison to current
mainline. Similar behaviour is observable since introduction of commit
6393d6a102 (ie. since v3.17).

Numbers are from an HP ProLiant DL580 Gen8 system:
- Intel(R) Xeon(R) CPU E7-4890 v2 @ 2.80GHz
- 60 CPUs, 128GB RAM

PCC info of this system

PCCH header (virtual) addr: 0xffffc9000d840000
PCCH header is at physical address: 0x7ac59000, signature: 0x24504343,
length: 64 bytes, major: 1, minor: 0, supported features: 0x1,
command field: 0x1, status field: 0x0, nominal latency: 300 us
min time between commands: 50000 us, max time between commands: 1000000 us,
nominal CPU frequency: 2800 MHz, minimum CPU frequency: 150 MHz,
minimum CPU frequency without throttling: 1200 MHz

Kernel message when driver loads:
pcc-cpufreq: (v1.10.00) driver loaded with frequency limits: 1200 MHz, 2800 MHz

My patch adds a debug message, which on this system looks like
pcc-cpufreq: setting deadband limit to 1885000 kHz


Regards,

Andreas

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