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

[Lukasz Luba <lukasz.luba@arm.com>]

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