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Date: Mon, 29 Jun 2020 12:41:54 +0100 From: Lukasz Luba <lukasz.luba@....com> To: Sylwester Nawrocki <s.nawrocki@...sung.com> Cc: Krzysztof Kozlowski <krzk@...nel.org>, Willy Wolff <willy.mh.wolff.ml@...il.com>, Chanwoo Choi <cw00.choi@...sung.com>, MyungJoo Ham <myungjoo.ham@...sung.com>, Kyungmin Park <kyungmin.park@...sung.com>, Kukjin Kim <kgene@...nel.org>, linux-pm@...r.kernel.org, "linux-samsung-soc@...r.kernel.org" <linux-samsung-soc@...r.kernel.org>, linux-arm-kernel@...ts.infradead.org, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: brocken devfreq simple_ondemand for Odroid XU3/4? On 6/26/20 6:50 PM, Sylwester Nawrocki wrote: > Hi Lukasz, > > On 25.06.2020 12:02, Lukasz Luba wrote: >> 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. > > Agreed, that's in case when we use just the performance counter (PMCNT) > overflow interrupts. In my experiments I used the (total) cycle counter > (CCNT) overflow interrupts. As that counter is clocked with fixed rate > between devfreq updates it can be used as a timer by pre-loading it with > initial value depending on current bus frequency. But we could as well > use some reliable system timer mechanism to generate periodic events. > I was hoping to use the cycle counter to generate low frequency monitor > events and the actual performance counters overflow interrupts to detect > any sudden changes of utilization. However, it seems it cannot be done > with as simple performance counters HW architecture as on Exynos4412. > It looks like on Exynos5422 we have all what is needed, there is more > flexibility in selecting the counter source signal, e.g. each counter > can be a clock cycle counter or can count various bus events related to > actual utilization. Moreover, we could configure the counter gating period > and alarm interrupts are available for when the counter value drops below > configured MIN threshold or exceeds configured MAX value. I see. I don't have TRM for Exynos5422 so couldn't see that. I also have to keep in mind other platforms which might not have this feature. > > So it should be possible to configure the HW to generate the utilization > monitoring events without excessive continuous CPU intervention. I agree, that would be desirable especially for low load in the system. > But I'm rather not going to work on the Exynos5422 SoC support at the moment. I see. > >> To address that, we still need to have another mechanism (like watchdog) >> which will be triggered just to check if the threshold needs adjustment. >> This mechanism can be a local timer in the driver or a framework >> timer running kind of 'for loop' on all this type of devices (like >> the scheduled workqueue). In both cases in the system there will be >> interrupts, timers (even at workqueues) and scheduling. >> The approach to force developers to implement their local watchdog >> timers (or workqueues) in drivers is IMHO wrong and that's why we have >> frameworks. > > Yes, it should be also possible in the framework to use the counter alarm > events where the hardware is advanced enough, in order to avoid excessive > SW polling. Looks promising, but that would need more plumbing I assume. Regards, Lukasz > > -- > Regards, > Sylwester >
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