Re: [PATCH] drm/lima: Use delayed timer as default in devfreq profile

[Robin Murphy]

On 2021-02-03 02:01, Qiang Yu wrote:
> On Tue, Feb 2, 2021 at 10:02 PM Lukasz Luba <lukasz.luba@xxxxxxx> 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@xxxxxxxxx>
>
> Regards,
> Qiang
>
> > Regards,
> > Lukasz
> >
> >
> > > Regards,
> > > Qiang
> > >
> > >
> > > On Mon, Feb 1, 2021 at 5:53 PM Lukasz Luba <lukasz.luba@xxxxxxx> 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
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