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

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Date:   Tue, 4 Feb 2020 09:39:12 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thara Gopinath <thara.gopinath@...aro.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 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:
>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>>> index e35b28e..be4147b 100644
>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>> @@ -4376,6 +4376,11 @@
>>>>  			incurs a small amount of overhead in the scheduler
>>>>  			but is useful for debugging and performance tuning.
>>>>  
>>>> +	sched_thermal_decay_shift=
>>>> +			[KNL, SMP] Set decay shift for thermal pressure signal.
>>>> +			Format: integer between 0 and 10
>>>> +			Default is 0.
>>>> +
>>>
>>> That tells an admin [or any reader] almost nothing about this kernel parameter
>>> or what it does.  And nothing about what unit the value is in.
>>> Does the value 0 disable this feature?
>>
>> Thanks for the review. 0 does not disable "thermal pressure" feature. 0
>> means the default decay period for averaging PELT signals (which is
>> usually 32 but configurable) will also be applied for thermal pressure
>> signal. A shift will shift the default decay period.
>>
>> You are right. It needs more explanation here. I will fix it and send v10.
> 
> Or just send an update for this patch? I'm thinking most of this is
> looking good.

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

(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.