Re: [Patch v5 0/6] Introduce Thermal Pressure

[Amit Kucheria]

On Wed, Nov 6, 2019 at 12:20 AM Thara Gopinath
<thara.gopinath@xxxxxxxxxx> 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 unaware of maximum capacity
> 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,

What contraints?

> 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 share the hack to get this behaviour as well so I can try to
reproduce on 845c?

> Test Results
>
> Hackbench: 1 group , 30000 loops, 10 runs
>                                                Result         SD
>                                                (Secs)     (% of mean)
>  No Thermal Pressure                            14.03       2.69%
>  Thermal Pressure PELT Algo. Decay : 32 ms      13.29       0.56%
>  Thermal Pressure PELT Algo. Decay : 64 ms      12.57       1.56%
>  Thermal Pressure PELT Algo. Decay : 128 ms     12.71       1.04%
>  Thermal Pressure PELT Algo. Decay : 256 ms     12.29       1.42%
>  Thermal Pressure PELT Algo. Decay : 512 ms     12.42       1.15%
>