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

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Message-ID: <a20c5214-211b-1f70-1162-57b32e60549b@st.com>
Date:   Thu, 30 Apr 2020 15:37:17 +0000
From:   Benjamin GAIGNARD <benjamin.gaignard@...com>
To:     Valentin Schneider <valentin.schneider@....com>
CC:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <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>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [RFC 0/3] Introduce cpufreq minimum load QoS



On 4/30/20 4:33 PM, Valentin Schneider wrote:
> On 30/04/20 14:46, Benjamin GAIGNARD wrote:
>>> That's not what I meant.
>>>
>>> I suppose that the interrupt processing in question takes place in
>>> process context and so you may set the lower clamp on the utilization
>>> of the task carrying that out.
>> I have try to add this code when starting streaming (before the first
>> interrupt) the frames from the sensor:
>> const struct sched_attr sched_attr = {
>>     .sched_util_min = 10000, /* 100% of usage */
> Unless you play with SCHED_CAPACITY_SHIFT, the max should be 1024 -
> i.e. SCHED_CAPACITY_SCALE. That's a really big boost, but that's for you to
> benchmark.
>
>>     .sched_flags = SCHED_FLAG_UTIL_CLAMP_MIN,
>>    };
>>
>> sched_setattr(current, &sched_attr);
>>
>> I don't see any benefices maybe there is some configuration flags to set.
>>
>> How changing sched_util_min could impact cpufreq ondemand governor ?
>> Does it change the value returned when the governor check the idle time ?
>>
> You'll have to use the schedutil governor for uclamp to have an effect. And
> arguably that's what you should be using, unless something explicitly
> prevents you from doing that.
Even with schedutil and SCHED_CAPACITY_SCALE that it doesn't work.
cpufreq/cpuinfo_cur_freq values are always on the max value even if the 
stats show transitions between the available frequencies.

I see two possibles reasons to explain that:
- sched_setattr() is called in userland process context, but the 
threaded irq handler is running in another process.
- because this use case is almost running all in hardware the process 
isn't doing anything so the scheduler doesn't take care of it.

>
>>> Alternatively, that task may be a deadline one.