Date:   Thu, 31 Oct 2019 12:52:14 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        rui.zhang@...el.com, edubezval@...il.com, qperret@...gle.com,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, daniel.lezcano@...aro.org
Subject: Re: [Patch v4 0/6] Introduce Thermal Pressure

On 10/31/2019 12:41 PM, Thara Gopinath wrote:
> On 10/31/2019 05:44 AM, Ionela Voinescu wrote:
>> Hi Thara,
>>
>> On Tuesday 22 Oct 2019 at 16:34:19 (-0400), Thara Gopinath wrote:
>>> Thermal governors can respond to an overheat event of a cpu by
>>> capping the cpu's maximum possible frequency. This in turn
>>> means that the maximum available compute capacity of the
>>> cpu is restricted. But today in the kernel, task scheduler is 
>>> not notified of capping of maximum frequency of a cpu.
>>> In other words, scheduler is unware of maximum capacity
>>
>> Nit: s/unware/unaware
>>
>>> restrictions placed on a cpu due to thermal activity.
>>> This patch series attempts to address this issue.
>>> The benefits identified are better task placement among available
>>> cpus in event of overheating which in turn leads to better
>>> performance numbers.
>>>
>>> The reduction in the maximum possible capacity of a cpu due to a 
>>> thermal event can be considered as thermal pressure. Instantaneous
>>> thermal pressure is hard to record and can sometime be erroneous
>>> as there can be mismatch between the actual capping of capacity
>>> and scheduler recording it. Thus solution is to have a weighted
>>> average per cpu value for thermal pressure over time.
>>> The weight reflects the amount of time the cpu has spent at a
>>> capped maximum frequency. Since thermal pressure is recorded as
>>> an average, it must be decayed periodically. Exisiting algorithm
>>> in the kernel scheduler pelt framework is re-used to calculate
>>> the weighted average. This patch series also defines a sysctl
>>> inerface to allow for a configurable decay period.
>>>
>>> Regarding testing, basic build, boot and sanity testing have been
>>> performed on db845c platform with debian file system.
>>> Further, dhrystone and hackbench tests have been
>>> run with the thermal pressure algorithm. During testing, due to
>>> constraints of step wise governor in dealing with big little systems,
>>> trip point 0 temperature was made assymetric between cpus in little
>>> cluster and big cluster; the idea being that
>>> big core will heat up and cpu cooling device will throttle the
>>> frequency of the big cores faster, there by limiting the maximum available
>>> capacity and the scheduler will spread out tasks to little cores as well.
>>>
>>
>> Can you please share the changes you've made to sdm845.dtsi and a kernel
>> base on top of which to apply your patches? I would like to reproduce
>> your results and run more tests and it would be good if our setups were
>> as close as possible.
> Hi Ionela
> Thank you for the review.
> So I tested this on 5.4-rc1 kernel. The dtsi changes is to reduce the
> thermal trip points for the big CPUs to 60000 or 70000 from the default
> 90000. I did this for 2 reasons
> 1. I could never get the db845 to heat up sufficiently for my test cases
> with the default trip.
> 2. I was using the default step-wise governor for thermal. I did not
> want little and big to start throttling by the same % because then the
> task placement ratio will remain the same between little and big cores.
> 

So I am not sure though if this is the set up under which Daniel ran
glbench . I will let him comment on it.

> 
> 
> 


-- 
Warm Regards
Thara

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