Re: brocken devfreq simple_ondemand for Odroid XU3/4?

[Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>]

On 6/25/20 2:12 PM, Kamil Konieczny wrote:
> On 25.06.2020 14:02, Lukasz Luba wrote:
>>
>>
>> On 6/25/20 12:30 PM, Kamil Konieczny wrote:
>>> Hi Lukasz,
>>>
>>> On 25.06.2020 12:02, Lukasz Luba wrote:
>>>> Hi Sylwester,
>>>>
>>>> On 6/24/20 4:11 PM, Sylwester Nawrocki wrote:
>>>>> Hi All,
>>>>>
>>>>> On 24.06.2020 12:32, Lukasz Luba wrote:
>>>>>> 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.
>>>>>
>>>>> As I have been working on resolving the video mixer IOMMU fault issue
>>>>> described here: https://patchwork.kernel.org/patch/10861757
>>>>> I did some investigation of the devfreq operation, mostly on Odroid U3.
>>>>>
>>>>> My conclusions are similar to what Lukasz says above. I would like to add
>>>>> that broken scheduling of the performance counters read and the devfreq
>>>>> updates seems to have one more serious implication. In each call, which
>>>>> normally should happen periodically with fixed interval we stop the counters,
>>>>> read counter values and start the counters again. But if period between
>>>>> calls becomes long enough to let any of the counters overflow, we will
>>>>> get wrong performance measurement results. My observations are that
>>>>> the workqueue job can be suspended for several seconds and conditions for
>>>>> the counter overflow occur sooner or later, depending among others
>>>>> on the CPUs load.
>>>>> Wrong bus load measurement can lead to setting too low interconnect bus
>>>>> clock frequency and then bad things happen in peripheral devices.
>>>>>
>>>>> I agree the workqueue issue needs to be fixed. I have some WIP code to use
>>>>> the performance counters overflow interrupts instead of SW polling and with
>>>>> that the interconnect bus clock control seems to work much better.
>>>>>
>>>>
>>>> Thank you for sharing your use case and investigation results. I think
>>>> we are reaching a decent number of developers to maybe address this
>>>> issue: 'workqueue issue needs to be fixed'.
>>>> I have been facing this devfreq workqueue issue ~5 times in different
>>>> platforms.
>>>>
>>>> Regarding the 'performance counters overflow interrupts' there is one
>>>> thing worth to keep in mind: variable utilization and frequency.
>>>> For example, in order to make a conclusion in algorithm deciding that
>>>> the device should increase or decrease the frequency, we fix the period
>>>> of observation, i.e. to 500ms. That can cause the long delay if the
>>>> utilization of the device suddenly drops. For example we set an
>>>> overflow threshold to value i.e. 1000 and we know that at 1000MHz
>>>> and full utilization (100%) the counter will reach that threshold
>>>> after 500ms (which we want, because we don't want too many interrupts
>>>> per sec). What if suddenly utilization drops to 2% (i.e. from 5GB/s
>>>> to 250MB/s (what if it drops to 25MB/s?!)), the counter will reach the
>>>> threshold after 50*500ms = 25s. It is impossible just for the counters
>>>> to predict next utilization and adjust the threshold. [...]
>>>
>>> irq triggers for underflow and overflow, so driver can adjust freq
>>>
>>
>> Probably possible on some platforms, depends on how many PMU registers
>> are available, what information can be can assign to them and type of
>> interrupt. A lot of hassle and still - platform and device specific.
>> Also, drivers should not adjust the freq, governors (different types
>> of them with different settings that they can handle) should do it.
>>
>> What the framework can do is to take this responsibility and provide
>> generic way to monitor the devices (or stop if they are suspended).
>> That should work nicely with the governors, which try to predict the
>> next best frequency. From my experience the more fluctuating intervals
>> the governors are called, the more odd decisions they make.
>> That's why I think having a predictable interval i.e. 100ms is something
>> desirable. Tuning the governors is easier in this case, statistics
>> are easier to trace and interpret, solution is not to platform specific,
>> etc.
>>
>> Kamil do you have plans to refresh and push your next version of the
>> workqueue solution?
> 
> I do not, as Bartek takes over my work,
> +CC Bartek

Hi Lukasz,

As you remember in January Chanwoo has proposed another idea (to allow
selecting workqueue type by devfreq device driver):

"I'm developing the RFC patch and then I'll send it as soon as possible."
(https://lore.kernel.org/linux-pm/6107fa2b-81ad-060d-89a2-d8941ac4d17e@samsung.com/)

"After posting my suggestion, we can discuss it"
(https://lore.kernel.org/linux-pm/f5c5cd64-b72c-2802-f6ea-ab3d28483260@samsung.com/)

so we have been waiting on the patch to be posted..

Similarly we have been waiting on (any) feedback for exynos-bus/nocp
fixes for Exynos5422 support (which have been posted by Kamil also in
January):

https://lore.kernel.org/linux-pm/8f82d8d5-927b-afb4-272f-45c16b5a23b9@samsung.com/

Considering the above and how hard it has been to push the changes
through review/merge process last year we are near giving up when it
comes to upstream devfreq contributions. Sylwester is still working on
exynos-bus & interconnect integration (continuation of Artur Swigon's
work from last year) & related issues (IRQ support for PPMU)  but
I'm seriously considering putting it all on-hold..

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics