Hi Sylwester, On 6/25/20 12:11 AM, 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 It is good way to resolve the overflow issue. > that the interconnect bus clock control seems to work much better. > -- Best Regards, Chanwoo Choi Samsung Electronics
