brocken devfreq simple_ondemand for Odroid XU3/4?

[Kamil Konieczny]

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

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