linux-kernel - brocken devfreq simple_ondemand for Odroid XU3/4?

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Date:   Tue, 23 Jun 2020 18:47:33 +0200
From:   Willy Wolff <willy.mh.wolff.ml@...il.com>
To:     Chanwoo Choi <cw00.choi@...sung.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        linux-pm@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: brocken devfreq simple_ondemand for Odroid XU3/4?

Hi everybody,

Is DVFS for memory bus really working on Odroid XU3/4 board?
Using a simple microbenchmark that is doing only memory accesses, memory DVFS
seems to not working properly:

The microbenchmark is doing pointer chasing by following index in an array.
Indices in the array are set to follow a random pattern (cutting prefetcher),
and forcing RAM access.

git clone https://github.com/wwilly/benchmark.git \
  && cd benchmark \
  && source env.sh \
  && ./bench_build.sh \
  && bash source/scripts/test_dvfs_mem.sh

Python 3, cmake and sudo rights are required.

Results:
DVFS CPU with performance governor
mem_gov = simple_ondemand at 165000000 Hz in idle, should be bumped when the
benchmark is running.
- on the LITTLE cluster it takes 4.74308 s to run (683.004 c per memory access),
- on the big cluster it takes 4.76556 s to run (980.343 c per moemory access).

While forcing DVFS memory bus to use performance governor,
mem_gov = performance at 825000000 Hz in idle,
- on the LITTLE cluster it takes 1.1451 s to run (164.894 c per memory access),
- on the big cluster it takes 1.18448 s to run (243.664 c per memory access).

The kernel used is the last 5.7.5 stable with default exynos_defconfig.

Cheers,
Willy