Re: [Linux-stm32] [RFC 0/3] Introduce cpufreq minimum load QoS

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

On 5/4/20 11:17 AM, Benjamin GAIGNARD wrote:
>
> On 4/30/20 5:50 PM, Valentin Schneider wrote:
>> On 30/04/20 16:37, Benjamin GAIGNARD wrote:
>>> On 4/30/20 4:33 PM, Valentin Schneider wrote:
>>>> On 30/04/20 14:46, Benjamin GAIGNARD wrote:
>>>>>> 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 */
>>>> Unless you play with SCHED_CAPACITY_SHIFT, the max should be 1024 -
>>>> i.e. SCHED_CAPACITY_SCALE. That's a really big boost, but that's for you to
>>>> benchmark.
>>>>
>>>>>       .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 ?
>>>>>
>>>> You'll have to use the schedutil governor for uclamp to have an effect. And
>>>> arguably that's what you should be using, unless something explicitly
>>>> prevents you from doing that.
>>> Even with schedutil and SCHED_CAPACITY_SCALE that it doesn't work.
>>> cpufreq/cpuinfo_cur_freq values are always on the max value even if the
>>> stats show transitions between the available frequencies.
>>>
>>> I see two possibles reasons to explain that:
>>> - sched_setattr() is called in userland process context, but the
>>> threaded irq handler is running in another process.
>> Ah yes, this only works if the task you boost is the one that will handle
>> whatever work you care about (in this case handling the irq). That said, if
>> you do use threaded IRQs, that should give you a SCHED_FIFO thread, which
>> should drive the frequency to its max when using schedutil (unrelated to
>> uclamp).
> Can I conclude that sched_setattr() isn't the good way to solve this
> problem ?
> Does my patches make sense in this case ?
Gentle up on this series beacause I haven't found any other way to solve
this problem.

Thanks,
Benjamin
>
>>> - because this use case is almost running all in hardware the process
>>> isn't doing anything so the scheduler doesn't take care of it.
>>>
>>>>>> Alternatively, that task may be a deadline one.
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