linux-kernel - Re: [PATCH] base: arch_topology: Use policy->max to calculate freq

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Message-ID: <7f077790-da4c-35b8-0eea-cbdc630f9d2a@arm.com>
Date:   Wed, 17 Nov 2021 15:08:04 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Thara Gopinath <thara.gopinath@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH] base: arch_topology: Use policy->max to calculate
 freq_factor



On 11/17/21 12:49 PM, Rafael J. Wysocki wrote:
> On Wed, Nov 17, 2021 at 11:46 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> Hi Rafael,
>>
>> On 11/16/21 7:05 PM, Rafael J. Wysocki wrote:
>>> On Mon, Nov 15, 2021 at 9:10 PM Thara Gopinath
>>> <thara.gopinath@...aro.org> wrote:
>>>>
>>>> cpuinfo.max_freq can reflect boost frequency if enabled during boot.  Since
>>>> we don't consider boost frequencies while calculating cpu capacities, use
>>>> policy->max to populate the freq_factor during boot up.
>>>
>>> I'm not sure about this.  schedutil uses cpuinfo.max_freq as the max frequency.
>>
>> Agree it's tricky how we treat the boost frequencies and also combine
>> them with thermal pressure.
>> We probably would have consider these design bits:
>> 1. Should thermal pressure include boost frequency?
> 
> Well, I guess so.
> 
> Running at a boost frequency certainly increases thermal pressure.
> 
>> 2. Should max capacity 1024 be a boost frequency so scheduler
>>      would see it explicitly?
> 
> That's what it is now if cpuinfo.max_freq is a boost frequency.
> 
>> - if no, then schedutil could still request boost freq thanks to
>>     map_util_perf() where we add 25% to the util and then
>>     map_util_freq() would return a boost freq when util was > 1024
>>
>>
>> I can see in schedutil only one place when cpuinfo.max_freq is used:
>> get_next_freq(). If the value stored in there is a boost,
>> then don't we get a higher freq value for the same util?
> 
> Yes. we do, which basically is my point.
> 
> The schedutil's response is proportional to cpuinfo.max_freq and that
> needs to be taken into account for the results to be consistent.
> 

This boost thing wasn't an issue for us, because we didn't have
platforms which come with it (till recently). I've checked that you have
quite a few CPUs which support huge boost freq, e.g. 5GHz vs. 3.6GHz
nominal max freq [1]. Am I reading this correctly as kernel boost freq?
Do you represent this 5GHz as 1024 capacity?
 From this schedutil get_next_freq() I would guess yes.

I cannot find if you use thermal pressure, could you help me with this,
please?


[1] 
https://ark.intel.com/content/www/us/en/ark/products/186605/intel-core-i99900k-processor-16m-cache-up-to-5-00-ghz.html