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Subject: Re: [Patch v9 7/8] sched/fair: Enable tuning of decay period
To:     Peter Zijlstra <peterz@infradead.org>,
        Thara Gopinath <thara.gopinath@linaro.org>
Cc:     Randy Dunlap <rdunlap@infradead.org>, mingo@redhat.com,
        ionela.voinescu@arm.com, vincent.guittot@linaro.org,
        rui.zhang@intel.com, qperret@google.com, daniel.lezcano@linaro.org,
        viresh.kumar@linaro.org, rostedt@goodmis.org, will@kernel.org,
        catalin.marinas@arm.com, sudeep.holla@arm.com,
        juri.lelli@redhat.com, corbet@lwn.net,
        linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
        javi.merino@kernel.org, amit.kucheria@verdurent.com
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From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <cc83634f-b3af-6024-7f89-9b231b153070@arm.com>
Date:   Tue, 4 Feb 2020 09:39:12 +0100
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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.