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@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?
Correct, the governor should pause the monitoring mechanism (and timer).
Regards,
Lukasz
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