Re: brocken devfreq simple_ondemand for Odroid XU3/4?

[Kamil Konieczny <k.konieczny@samsung.com>]

Hi,

On 24.06.2020 12:32, Lukasz Luba wrote:
> Hi Krzysztof and Willy
> 
> On 6/23/20 8:11 PM, Krzysztof Kozlowski wrote:
>> 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://protect2.fireeye.com/url?k=c364e88a-9eb6fe2f-c36563c5-0cc47a31bee8-631885f0a63a11a0&q=1&u=https%3A%2F%2Fgithub.com%2Fwwilly%2Fbenchmark.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.
> 
> Thanks Krzysztof for adding me here.
> 
>>
>> 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...
> 
> I had issues with devfreq governor which wasn't called by devfreq
> workqueue. The old DELAYED vs DEFERRED work discussions and my patches
> for it [1]. If the CPU which scheduled the next work went idle, the
> devfreq workqueue will not be kicked and devfreq governor won't check
> DMC status and will not decide to decrease the frequency based on low
> busy_time.
> The same applies for going up with the frequency. They both are
> done by the governor but the workqueue must be scheduled periodically.
> 
> I couldn't do much with this back then. I have given the example that
> this is causing issues with the DMC [2]. There is also a description
> of your situation staying at 633MHz for long time:
> ' When it is missing opportunity
> to change the frequency, it can either harm the performance or power
> consumption, depending of the frequency the device stuck on.'
> 
> The patches were not accepted because it will cause CPU wake-up from
> idle, which increases the energy consumption. I know that there were
> some other attempts, but I don't know the status.
> 
> I had also this devfreq workqueue issue when I have been working on
> thermal cooling for devfreq. The device status was not updated, because
> the devfreq workqueue didn't check the device [3].
> 
> Let me investigate if that is the case.
> 
> Regards,
> Lukasz
> 
> [1] https%3A%2F%2Flkml.org%2Flkml%2F2019%2F2%2F11%2F1146
> [2] https%3A%2F%2Flkml.org%2Flkml%2F2019%2F2%2F12%2F383
> [3] https%3A%2F%2Flwn.net%2Fml%2Flinux-kernel%2F20200511111912.3001-11-lukasz.luba%40arm.com%2F

and here was another try to fix wq: "PM / devfreq: add possibility for delayed work"

https://lkml.org/lkml/2019/12/9/486

-- 
Best regards,
Kamil Konieczny
Samsung R&D Institute Poland