On 2020-06-29-12-52-10, Lukasz Luba wrote: > Hi Chanwoo, > > On 6/29/20 2:43 AM, Chanwoo Choi wrote: > > Hi, > > > > Sorry for late reply because of my perfornal issue. I count not check the email. > > I hope you are good now. > > > > > On 6/26/20 8:22 PM, Bartlomiej Zolnierkiewicz wrote: > > > > > > 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@xxxxxxxxxxx/) > > > > > > "After posting my suggestion, we can discuss it" > > > (https://lore.kernel.org/linux-pm/f5c5cd64-b72c-2802-f6ea-ab3d28483260@xxxxxxxxxxx/) > > > > > > so we have been waiting on the patch to be posted.. > > > > Sorry for this. I'll send it within few days. > > > Feel free to add me on CC, I can review&test the patches if you like. Please CC me too. > > Stay safe and healthy. > > Regards, > Lukasz > Cheers, Willy
