Messages in this thread |  | | Subject | Re: [PATCH] drm/lima: Use delayed timer as default in devfreq profile | From | Lukasz Luba <> | Date | Thu, 4 Feb 2021 14:23:56 +0000 |
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On 2/4/21 1:39 PM, Robin Murphy wrote: > On 2021-02-03 02:01, Qiang Yu wrote: >> On Tue, Feb 2, 2021 at 10:02 PM Lukasz Luba <lukasz.luba@arm.com> wrote: >>> >>> >>> >>> On 2/2/21 1:01 AM, Qiang Yu wrote: >>>> Hi Lukasz, >>>> >>>> Thanks for the explanation. So the deferred timer option makes a >>>> mistake that >>>> when GPU goes from idle to busy for only one poll periodic, in this >>>> case 50ms, right? >>> >>> Not exactly. Driver sets the polling interval to 50ms (in this case) >>> because it needs ~3-frame average load (in 60fps). I have discovered the >>> issue quite recently that on systems with 2 CPUs or more, the devfreq >>> core is not monitoring the devices even for seconds. Therefore, we might >>> end up with quite big amount of work that GPU is doing, but we don't >>> know about it. Devfreq core didn't check <- timer didn't fired. Then >>> suddenly that CPU, which had the deferred timer registered last time, >>> is waking up and timer triggers to check our device. We get the stats, >>> but they might be showing load from 1sec not 50ms. We feed them into >>> governor. Governor sees the new load, but was tested and configured for >>> 50ms, so it might try to rise the frequency to max. The GPU work might >>> be already lower and there is no need for such freq. Then the CPU goes >>> idle again, so no devfreq core check for next e.g. 1sec, but the >>> frequency stays at max OPP and we burn power. >>> >>> So, it's completely unreliable. We might stuck at min frequency and >>> suffer the frame drops, or sometimes stuck to max freq and burn more >>> power when there is no such need. >>> >>> Similar for thermal governor, which is confused by this old stats and >>> long period stats, longer than 50ms. >>> >>> Stats from last e.g. ~1sec tells you nothing about real recent GPU >>> workload. >> Oh, right, I missed this case. >> >>> >>>> But delayed timer will wakeup CPU every 50ms even when system is >>>> idle, will this >>>> cause more power consumption for the case like phone suspend? >>> >>> No, in case of phone suspend it won't increase the power consumption. >>> The device won't be woken up, it will stay in suspend. >> I mean the CPU is waked up frequently by timer when phone suspend, >> not the whole device (like the display). >> >> Seems it's better to have deferred timer when device is suspended for >> power saving, >> and delayed timer when device in working state. User knows this and >> can use sysfs >> to change it. > > Doesn't devfreq_suspend_device() already cancel any timer work either > way in that case?
Correct, the governor should pause the monitoring mechanism (and timer).
Regards, Lukasz
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