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

[Benjamin GAIGNARD <benjamin.gaignard@st.com>]

On 4/30/20 11:03 AM, Rafael J. Wysocki wrote:
> On Thu, Apr 30, 2020 at 9:53 AM Benjamin GAIGNARD
> <benjamin.gaignard@st.com> wrote:
>>
>>
>> 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.
> That's not what I meant.
>
> I suppose that the interrupt processing in question takes place in
> process context and so you may set the lower clamp on the utilization
> of the task carrying that out.
I have try to add this code when starting streaming (before the first 
interrupt) the frames from the sensor:
const struct sched_attr sched_attr = {
         .sched_util_min = 10000, /* 100% of usage */
         .sched_flags = SCHED_FLAG_UTIL_CLAMP_MIN,
     };

sched_setattr(current, &sched_attr);

I don't see any benefices maybe there is some configuration flags to set.

How changing sched_util_min could impact cpufreq ondemand governor ?
Does it change the value returned when the governor check the idle time ?

>
> Alternatively, that task may be a deadline one.