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

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

On 5/27/20 2:22 PM, Vincent Guittot wrote:
> On Wed, 27 May 2020 at 13:17, Benjamin GAIGNARD
> <benjamin.gaignard@st.com> 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.
> 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] https://lkml.org/lkml/2020/4/24/360
>>>>
>>>> Benjamin Gaignard (3):
>>>>     PM: QoS: Introduce cpufreq minimum load QoS
>>>>     cpufreq: governor: Use minimum load QoS
>>>>     media: stm32-dcmi: Inform cpufreq governors about cpu load needs
>>>>
>>>>    drivers/cpufreq/cpufreq_governor.c        |   5 +
>>>>    drivers/media/platform/stm32/stm32-dcmi.c |   8 ++
>>>>    include/linux/pm_qos.h                    |  12 ++
>>>>    kernel/power/qos.c                        | 213 ++++++++++++++++++++++++++++++
>>>>    4 files changed, 238 insertions(+)