Messages in this thread |  | | Subject | Re: [PATCH] drm/lima: Use delayed timer as default in devfreq profile | From | Robin Murphy <> | Date | Thu, 4 Feb 2021 13:39:00 +0000 |
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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?
Robin.
> Set the delayed timer as default is reasonable, so patch is: > Reviewed-by: Qiang Yu <yuq825@gmail.com> > > Regards, > Qiang > >> >> Regards, >> Lukasz >> >> >>> >>> Regards, >>> Qiang >>> >>> >>> On Mon, Feb 1, 2021 at 5:53 PM Lukasz Luba <lukasz.luba@arm.com> wrote: >>>> >>>> Hi Qiang, >>>> >>>> On 1/30/21 1:51 PM, Qiang Yu wrote: >>>>> Thanks for the patch. But I can't observe any difference on glmark2 >>>>> with or without this patch. >>>>> Maybe you can provide other test which can benefit from it. >>>> >>>> This is a design problem and has impact on the whole system. >>>> There is a few issues. When the device is not checked and there are >>>> long delays between last check and current, the history is broken. >>>> It confuses the devfreq governor and thermal governor (Intelligent Power >>>> Allocation (IPA)). Thermal governor works on stale stats data and makes >>>> stupid decisions, because there is no new stats (device not checked). >>>> Similar applies to devfreq simple_ondemand governor, where it 'tires' to >>>> work on a loooong period even 3sec and make prediction for the next >>>> frequency based on it (which is broken). >>>> >>>> How it should be done: constant reliable check is needed, then: >>>> - period is guaranteed and has fixed size, e.g 50ms or 100ms. >>>> - device status is quite recent so thermal devfreq cooling provides >>>> 'fresh' data into thermal governor >>>> >>>> This would prevent odd behavior and solve the broken cases. >>>> >>>>> >>>>> Considering it will wake up CPU more frequently, and user may choose >>>>> to change this by sysfs, >>>>> I'd like to not apply it. >>>> >>>> The deferred timer for GPU is wrong option, for UFS or eMMC makes more >>>> sense. It's also not recommended for NoC busses. I've discovered that >>>> some time ago and proposed to have option to switch into delayed timer. >>>> Trust me, it wasn't obvious to find out that this missing check has >>>> those impacts. So the other engineers or users might not know that some >>>> problems they faces (especially when the device load is changing) is due >>>> to this delayed vs deffered timer and they will change it in the sysfs. >>>> >>>> Regards, >>>> Lukasz >>>> >>>>> >>>>> Regards, >>>>> Qiang >>>>> > _______________________________________________ > dri-devel mailing list > dri-devel@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel >
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