linux-kernel - Re: [PATCH 3/3] ARM: exynos_defconfig: Enable Energy Model framework

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Date:   Wed, 5 Feb 2020 12:49:26 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     kgene@...nel.org, linux-arm-kernel@...ts.infradead.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree@...r.kernel.org, linux-pm@...r.kernel.org,
        myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
        Chanwoo Choi <cw00.choi@...sung.com>, robh+dt@...nel.org,
        mark.rutland@....com,
        Bartłomiej Żołnierkiewicz 
        <b.zolnierkie@...sung.com>, dietmar.eggemann@....com
Subject: Re: [PATCH 3/3] ARM: exynos_defconfig: Enable Energy Model framework

Hi Krzysztof,

On 1/31/20 8:41 PM, Krzysztof Kozlowski wrote:
> On Fri, Jan 31, 2020 at 05:30:46PM +0000, Lukasz Luba wrote:
>   
>>>
>>>>                   |-----------------------------------------------|---------------
>>>>                   | performance   | SchedUtil     | SchedUtil     | performance
>>>>                   | governor      | governor      | governor      | governor
>>>>                   |               | w/o EAS       | w/ EAS        |
>>>> ----------------|---------------|---------------|---------------|---------------
>>>> hackbench w/ PL | 12.7s         | 11.7s         | 12.0s         | 13.0s - 12.2s
>>>> hackbench w/o PL| 9.2s          | 8.1s          | 8.2s          | 9.2s - 8.4s
>>>
>>> Why does the performance different before and after this patch?
>>
>> Probably due to better locality and cache utilization. I can see that
>> there is ~700k context switches vs ~450k and ~160k migrations vs ~50k.
>> If you need to communicate two threads in different clusters, it will go
>> through CCI.
> 
> Mhmm... I was not specific - I mean, "performance governor". All this
> you mentioned should not differ between performance governor before and
> after. However once you have 12.7, then 13.0 - 12.2. Unless multi-core
> scheduler affects it... but then these numbers here are not showing
> only this change, but also the SCHED_MC effect.  In such case each of
> commits should be coming with their own numbers.

Agree, I should have not put 'this patch set' in the commit
msg. It should go into the cover letter and avoid this confusion.
You are right with ' Unless multi-core scheduler affects it...',
that's why when the SCHED_MC is missing, the decisions about task
placing might cause this variation and delay '13.0 - 12.2' seconds.

> 
>> As mentioned in response to patch 1/3. The fist patch would create MC
>> domain, something different than Energy Model or EAS. The decisions in
>> the scheduler would be different.
>>
>> I can merge 1/3 and 3/3 if you like, though.
> 
> I understand now that their independent. Still, they are part of one
> goal to tune the scheduler for Exynos platform. Splitting these looks
> too much, like enabling multiple drivers one after another.
> 
> However if you provide numbers for each of cases (before patches, multi
> core scheduler, energy model with DTS), then I see benefit of splitting
> it.  Each commit would have its own rationale.  I am not sure if it is
> worth such investigation - that's just defconfig... distros might ignore
> it anyway.

Good point, and I agree that it would require more investigation, for
which unfortunately I don't have currently spare cycles.

Should I merge patch 1/3 and 3/3 and send the v2 with a cover letter
which would have the test results?

Regards,
Lukasz