linux-kernel - Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period

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Date:   Mon, 10 Feb 2020 12:59:50 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Thara Gopinath <thara.gopinath@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>, mingo@...hat.com,
        ionela.voinescu@....com, vincent.guittot@...aro.org,
        rui.zhang@...el.com, qperret@...gle.com, daniel.lezcano@...aro.org,
        viresh.kumar@...aro.org, rostedt@...dmis.org, will@...nel.org,
        catalin.marinas@....com, sudeep.holla@....com,
        juri.lelli@...hat.com, corbet@....net,
        linux-kernel@...r.kernel.org, amit.kachhap@...il.com,
        javi.merino@...nel.org, amit.kucheria@...durent.com
Subject: Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period

On 07/02/2020 23:42, Thara Gopinath wrote:
> On 02/04/2020 03:39 AM, Dietmar Eggemann wrote:
>> On 03/02/2020 16:55, Peter Zijlstra wrote:
>>> On Mon, Feb 03, 2020 at 07:07:57AM -0500, Thara Gopinath wrote:
>>>> On 01/28/2020 06:56 PM, Randy Dunlap wrote:
>>>>> Hi,
>>>>>
>>>>> On 1/28/20 2:36 PM, Thara Gopinath wrote:

[...]

>> I do agree. IMHO, there are just two little things outstanding:
>>
>> (1) arch_scale_thermal_pressure() instead  of
>>     arch_cpu_thermal_pressure() in v8 4/7
> 
> The "scale_" part was discussed in v6. Ionela had suggested that having
> "scale" is not suited for this function because "thermal pressure" is
> not exactly scaled but subtracted. I actually agree with that.
> 
> https://lore.kernel.org/lkml/20191223175005.GA31446@arm.com/
> 
> Having said that if everyone feel the same about naming of this
> function, I can change it one last time.

I'm still in favor for this. And Vincent seems to be OK as well:

https://lore.kernel.org/r/CAKfTPtBzoLnvAJ7sjPogMYS=WwBbdzWO07Kj=KDFVpO4=Su5ow@mail.gmail.com

The 'v6->v7' change note:

- Renamed arch_scale_thermal_capacity to arch_cpu_thermal_pressure
  as per review comments from Peter, Dietmar and Ionela.

is really not saying from which review comment the individual changes in
the function name are coming from. And I don't see an answer to Ionela's
email saying that her proposal will manifest in a particular part of
this change.

>> (2) guarding of thermal pressure code in Arm's arch_topology driver  w/
>>     CONFIG_HAVE_SCHED_THERMAL_PRESSURE plus disabling it by default for
>>     Arm64.
> It was enabled by default as per your suggestion in v9.
> 
> The patch can be dropped.
> 
> I don't understand the need to guard arch_topology with
> CONFIG_HAVE_SCHED_THERMAL_PRESSURE. CONFIG_HAVE_SCHED_THERMAL_PRESSURE
> is for scheduler to enable/disable averaging of thermal pressure. We
> wanted to separate updating and retrieving of instantaneous thermal
> pressure from scheduler. Guarding it with
> CONFIG_HAVE_SCHED_THERMAL_PRESSURE is to me equivalent to putting back
> this whole code in the scheduler framework. I am against it. I also do
> not see other arch_ functions guarded similarly.

Cpu-invariant accounting can't be guarded with a kernel CONFIG switch.
Frequency-invariant accounting could be with CONFIG_CPU_FREQ but this is
enabled by default by Arm64 defconfig.
Thermal pressure (accounting) (CONFIG_HAVE_SCHED_THERMAL_PRESSURE) is
disabled by default so why should a per-cpu thermal_pressure be
maintained on such a system (CONFIG_CPU_THERMAL=y by default)?