linux-kernel - Re: [Patch v4 6/6] sched: thermal: Enable tuning of decay period

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Date:   Mon, 28 Oct 2019 16:42:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thara Gopinath <thara.gopinath@...aro.org>
Cc:     mingo@...hat.com, ionela.voinescu@....com,
        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 6/6] sched: thermal: Enable tuning of decay period

On Tue, Oct 22, 2019 at 04:34:25PM -0400, Thara Gopinath wrote:
> Thermal pressure follows pelt signas which means the
> decay period for thermal pressure is the default pelt
> decay period. Depending on soc charecteristics and thermal
> activity, it might be beneficial to decay thermal pressure
> slower, but still in-tune with the pelt signals.
> One way to achieve this is to provide a command line parameter
> to set the decay coefficient to an integer between 0 and 10.
> 
> Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
> ---
> v3->v4:
> 	- Removed the sysctl setting to tune decay period and instead
> 	  introduced a command line parameter to control it. The rationale
> 	  here being changing decay period of a PELT signal runtime can
> 	  result in a skewed average value for atleast some cycles.

TBH, I don't care too much about that. If you touch a knob, you'd better
know what it does anyway.

>  void trigger_thermal_pressure_average(struct rq *rq)
>  {
> -	update_thermal_load_avg(rq_clock_task(rq), rq,
> +	update_thermal_load_avg(rq_clock_task(rq) >>
> +				sched_thermal_decay_coeff, rq,

You see, 'coefficient' means 'multiplicative factor', but what we have
here is a negative exponent. More specifically it is a power-of-2
exponent, and we typically call them '_shift', given how we use them
with 'shift' operators.