linux-kernel - Re: [PATCH v3 4/5] cpufreq: qcom-cpufreq-hw: Use new thermal pressure update function

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Date:   Mon, 8 Nov 2021 16:23:04 -0500
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, sudeep.holla@....com,
        will@...nel.org, catalin.marinas@....com, linux@...linux.org.uk,
        gregkh@...uxfoundation.org, rafael@...nel.org,
        viresh.kumar@...aro.org, amitk@...nel.org,
        daniel.lezcano@...aro.org, amit.kachhap@...il.com,
        bjorn.andersson@...aro.org, agross@...nel.org,
        Steev Klimaszewski <steev@...i.org>
Subject: Re: [PATCH v3 4/5] cpufreq: qcom-cpufreq-hw: Use new thermal pressure
 update function



On 11/8/21 9:12 AM, Lukasz Luba wrote:
...snip

>>
>>
> 
> Well, I think the issue is broader. Look at the code which
> calculate this 'capacity'. It's just a multiplication & division:
> 
> max_capacity = arch_scale_cpu_capacity(cpu); // =1024 in our case
> capacity = mult_frac(max_capacity, throttled_freq,
>          policy->cpuinfo.max_freq);
> 
> In the reported by Steev output from sysfs cpufreq we know
> that the value of 'policy->cpuinfo.max_freq' is:
> /sys/devices/system/cpu/cpu5/cpufreq/cpuinfo_max_freq:2956800
> 
> so when we put the values to the equation we get:
> capacity = 1024 * 2956800 / 2956800; // =1024
> The 'capacity' will be always <= 1024 and this check won't
> be triggered:
> 
> /* Don't pass boost capacity to scheduler */
> if (capacity > max_capacity)
>      capacity = max_capacity;
> 
> 
> IIUC you original code, you don't want to have this boost
> frequency to be treated as 1024 capacity. The reason is because
> the whole capacity machinery in arch_topology.c is calculated based
> on max freq value = 2841600,
> so the max capacity 1024 would be pinned to that frequency
> (according to Steeve's log:
> [   22.552273] THERMAL_PRESSURE: max_freq(2841) < capped_freq(2956) for 
> CPUs [4-7] )

Hi Lukasz,

Yes you are right in that I was using policy->cpuinfo.max_freq where as 
I should have used freq_factor. So now that you are using freq_factor, 
it makes sense to cap the capacity at the max capacity calulated by the 
scheduler.

I agree that the problem is complex because at some point we should look 
at rebuilding the topology based on changes to policy->cpuinfo.max_freq.

> 
> 
> Having all this in mind, the multiplication and division in your
> original code should be done:
> 
> capacity = 1024 * 2956800 / 2841600; // = 1065
> 
> then clamped to 1024 value.
> 
> My change just unveiled this division issue.
> 
> With that in mind, I tend to agree that I should have not
> rely on passed boost freq value and try to apply your suggestion check.
> Let me experiment with that...
> 
> Regards,
> Lukasz

-- 
Warm Regards
Thara (She/Her/Hers)