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

From: Qiang Yu <yuq825@gmail.com>
To: Lukasz Luba <lukasz.luba@arm.com>
Cc: lima@lists.freedesktop.org, David Airlie <airlied@linux.ie>,
	Christian Hewitt <christianshewitt@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH] drm/lima: Use delayed timer as default in devfreq profile
Date: Sun, 7 Feb 2021 21:11:51 +0800	[thread overview]
Message-ID: <CAKGbVbvwQJxdM9d1oTgCC+dfppGBCbfA3r6hG24N2zMjmWo5sQ@mail.gmail.com> (raw)
In-Reply-To: <deb2c075-4177-d487-b1cd-1c60790ca625@arm.com>

Applied to drm-misc-next.

Regards,
Qiang

On Thu, Feb 4, 2021 at 10:24 PM Lukasz Luba <lukasz.luba@arm.com> wrote:
>
>
>
> 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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