Re: brocken devfreq simple_ondemand for Odroid XU3/4?

[Kamil Konieczny]

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

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