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

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Date:   Wed, 27 May 2020 14:54:10 +0000
From:   Benjamin GAIGNARD <benjamin.gaignard@...com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
CC:     Valentin Schneider <valentin.schneider@....com>,
        "rjw@...ysocki.net" <rjw@...ysocki.net>,
        "viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
        Hugues FRUCHET <hugues.fruchet@...com>,
        "mchehab@...nel.org" <mchehab@...nel.org>,
        "mcoquelin.stm32@...il.com" <mcoquelin.stm32@...il.com>,
        Alexandre TORGUE <alexandre.torgue@...com>,
        "pavel@....cz" <pavel@....cz>,
        "len.brown@...el.com" <len.brown@...el.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC RESEND 0/3] Introduce cpufreq minimum load QoS



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@...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.

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 
restore
the default value.

Benjamin


>> 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(+)
>