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

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Message-ID: <099f5b6c-aa81-be4a-19bf-52a2fff7b3db@st.com>
Date:   Wed, 27 May 2020 13:11:42 +0000
From:   Benjamin GAIGNARD <benjamin.gaignard@...com>
To:     Valentin Schneider <valentin.schneider@....com>
CC:     "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>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "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 2:14 PM, Valentin Schneider wrote:
> On 27/05/20 12:17, Benjamin GAIGNARD 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'm not sure I follow; the frequency of the CPU doesn't matter while
> your hardware blocks are spinning, right? AIUI what matters is running
> your interrupt handler / action at max freq, which you get if you use
> threaded IRQs and schedutil.
Yes but not limited to schedutil.
Given the latency needed to change of frequencies I think it could 
already too late
to change the CPU frequency when handling the threaded interrupt.
>
> I think it would help if you could clarify which tasks / parts of your
> pipeline you need running at high frequencies. The point is that setting
> a QoS request affects all tasks, whereas we could be smarter and only
> boost the required tasks.
What make us drop frames is that the threaded IRQ is scheduled too late.
The not thread part of the interrupt handler where we clear the 
interrupt flags
is going fine but the thread part not.
>
>> 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.
>>
>>
>>>> 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(+)