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

From: Benjamin GAIGNARD
Date: Wed May 27 2020 - 10:54:37 EST

On 5/27/20 2:48 PM, Benjamin GAIGNARD wrote:
> On 5/27/20 2:22 PM, Vincent Guittot wrote:
>> On Wed, 27 May 2020 at 13:17, Benjamin GAIGNARD
>> <benjamin.gaignard@xxxxxx> wrote:
>>> On 5/27/20 12:09 PM, Valentin Schneider wrote:
>>>> Hi Benjamin,
>>>> On 26/05/20 16:16, Benjamin Gaignard wrote:
>>>>> A first round [1] of discussions and suggestions have already be
>>>>> done on
>>>>> this series but without found a solution to the problem. I resend
>>>>> it to
>>>>> progress on this topic.
>>>> Apologies for sleeping on that previous thread.
>>>> So what had been suggested over there was to use uclamp to boost the
>>>> frequency of the handling thread; however if you use threaded IRQs you
>>>> get RT threads, which already get the max frequency by default (at
>>>> least
>>>> with schedutil).
>>>> Does that not work for you, and if so, why?
>>> That doesn't work because almost everything is done by the hardware
>>> blocks
>>> without charge the CPU so the thread isn't running. I have done the
>>> tests with schedutil
>>> and ondemand scheduler (which is the one I'm targeting). I have no
>>> issues when using
>>> performance scheduler because it always keep the highest frequencies.
>> IMHO, the only way to ensure a min frequency for anything else than a
>> thread is to use freq_qos_add_request() just like cpufreq cooling
>> device but for the opposite QoS. This can be applied only on the
>> frequency domain of the CPU which handles the interrupt.
> I will give a try with this idea.
> Thanks.

Adding freq_qos_add_request(FREQ_QOS_MIN) when starting streaming frames
solve my problem. I remove the request at the end of the streaming to
the default value.


>> Have you also checked the wakeup latency of your idle state ?
> It just could go in WFI so latency should be minimal.
>>>>> 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.
>>>>> Benjamin
>>>>> [1]
>>>>> Benjamin Gaignard (3):
>>>>> ÂÂÂ PM: QoS: Introduce cpufreq minimum load QoS
>>>>> ÂÂÂ cpufreq: governor: Use minimum load QoS
>>>>> ÂÂÂ media: stm32-dcmi: Inform cpufreq governors about cpu load needs
>>>>> ÂÂ drivers/cpufreq/cpufreq_governor.cÂÂÂÂÂÂÂ |ÂÂ 5 +
>>>>> ÂÂ drivers/media/platform/stm32/stm32-dcmi.c |ÂÂ 8 ++
>>>>> ÂÂ include/linux/pm_qos.hÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ |Â 12 ++
>>>>> ÂÂ kernel/power/qos.cÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ | 213
>>>>> ++++++++++++++++++++++++++++++
>>>>> ÂÂ 4 files changed, 238 insertions(+)