Re: [RFC 0/3] Introduce cpufreq minimum load QoS

From: Benjamin GAIGNARD <benjamin.gaignard@st.com>
To: Valentin Schneider <valentin.schneider@arm.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: "viresh.kumar@linaro.org" <viresh.kumar@linaro.org>,
	Hugues FRUCHET <hugues.fruchet@st.com>,
	"mchehab@kernel.org" <mchehab@kernel.org>,
	"mcoquelin.stm32@gmail.com" <mcoquelin.stm32@gmail.com>,
	Alexandre TORGUE <alexandre.torgue@st.com>,
	"pavel@ucw.cz" <pavel@ucw.cz>,
	"len.brown@intel.com" <len.brown@intel.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-media@vger.kernel.org" <linux-media@vger.kernel.org>,
	"linux-stm32@st-md-mailman.stormreply.com" 
	<linux-stm32@st-md-mailman.stormreply.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Patrick Bellasi <patrick.bellasi@arm.com>
Subject: Re: [RFC 0/3] Introduce cpufreq minimum load QoS
Date: Thu, 30 Apr 2020 07:52:57 +0000	[thread overview]
Message-ID: <6b5cde14-58b3-045d-9413-223e66b87bf0@st.com> (raw)
In-Reply-To: <70e743cf-b88e-346a-5114-939b8724c83d@arm.com>


On 4/29/20 6:12 PM, Valentin Schneider wrote:
> On 29/04/2020 16:57, Benjamin GAIGNARD wrote:
>>
>> On 4/29/20 5:50 PM, Rafael J. Wysocki wrote:
>>> On Friday, April 24, 2020 1:40:55 PM CEST Benjamin Gaignard wrote:
>>>> When start streaming from the sensor the CPU load could remain very low
>>>> because almost all the capture pipeline is done in hardware (i.e. without
>>>> using the CPU) and let believe to cpufreq governor that it could use lower
>>>> frequencies. If the governor decides to use a too low frequency that
>>>> becomes a problem when we need to acknowledge the interrupt during the
>>>> blanking time.
>>>> The delay to ack the interrupt and perform all the other actions before
>>>> the next frame is very short and doesn't allow to the cpufreq governor to
>>>> provide the required burst of power. That led to drop the half of the frames.
>>>>
>>>> To avoid this problem, DCMI driver informs the cpufreq governors by adding
>>>> a cpufreq minimum load QoS resquest.
>>> This seems to be addressing a use case that can be addressed with the help of
>>> utilization clamps with less power overhead.
>> Do mean clamping the policy frequencies ? I may have miss the API to do
>> that...
> IIUC Rafael is referring to uclamp, i.e. scheduler utilization clamping, see:
>
>    https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html#cpu
>
> The above describes the cgroup interface, note that you can also set clamps
> per task (via sched_setattr()).
>
> One thing that comes to mind however is that schedutil only "sees" the clamps
> of runnable tasks, and from reading your changelog you may not have moments
> without any (i.e. gears are grinding in HW). You'd have to try boosting
> (setting a high uclamp.min) whatever tasks you have on the software side and
> see how it all behaves.
Relying on userland side means that various applications need to be aware
of this specific hardware case and fix it. I was hoping to find a 
solution in side the kernel
to not impact the software side.

>
>>> Thanks!
>>>
>>>
>>>