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

From: Benjamin GAIGNARD
Date: Wed May 27 2020 - 08:49:24 EST

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.
> 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):
>>>> 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(+)