Re: brocken devfreq simple_ondemand for Odroid XU3/4?

[Krzysztof Kozlowski <krzk@kernel.org>]

On Tue, Jun 23, 2020 at 09:02:38PM +0200, Krzysztof Kozlowski wrote:
> On Tue, 23 Jun 2020 at 18:47, Willy Wolff <willy.mh.wolff.ml@gmail.com> wrote:
> >
> > 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.
> 
> Thanks for the report. Few thoughts:
> 1. What trans_stat are saying? Except DMC driver you can also check
> all other devfreq devices (e.g. wcore) - maybe the devfreq events
> (nocp) are not properly assigned?
> 2. Try running the measurement for ~1 minutes or longer. The counters
> might have some delay (which would require probably fixing but the
> point is to narrow the problem).
> 3. What do you understand by "mem_gov"? Which device is it?

+Cc Lukasz who was working on this.

I just run memtester and more-or-less ondemand works (at least ramps
up):

Before:
/sys/class/devfreq/10c20000.memory-controller$ cat trans_stat
     From  :   To
           : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000   time(ms)
* 165000000:         0         0         0         0         0         0         0         0   1795950
  206000000:         1         0         0         0         0         0         0         0      4770
  275000000:         0         1         0         0         0         0         0         0     15540
  413000000:         0         0         1         0         0         0         0         0     20780
  543000000:         0         0         0         1         0         0         0         1     10760
  633000000:         0         0         0         0         2         0         0         0     10310
  728000000:         0         0         0         0         0         0         0         0         0
  825000000:         0         0         0         0         0         2         0         0     25920
Total transition : 9


$ sudo memtester 1G

During memtester:
/sys/class/devfreq/10c20000.memory-controller$ cat trans_stat
     From  :   To
           : 165000000 206000000 275000000 413000000 543000000 633000000 728000000 825000000   time(ms)
  165000000:         0         0         0         0         0         0         0         1   1801490
  206000000:         1         0         0         0         0         0         0         0      4770
  275000000:         0         1         0         0         0         0         0         0     15540
  413000000:         0         0         1         0         0         0         0         0     20780
  543000000:         0         0         0         1         0         0         0         2     11090
  633000000:         0         0         0         0         3         0         0         0     17210
  728000000:         0         0         0         0         0         0         0         0         0
* 825000000:         0         0         0         0         0         3         0         0    169020
Total transition : 13

However after killing memtester it stays at 633 MHz for very long time
and does not slow down. This is indeed weird...

Best regards,
Krzysztof

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