linux-kernel - Re: [PATCH v3 2/5] sched: Take cpufreq feedback into account

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Date: Wed, 10 Jan 2024 14:51:04 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
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Subject: Re: [PATCH v3 2/5] sched: Take cpufreq feedback into account

On 09/01/2024 15:30, Vincent Guittot wrote:
> On Tue, 9 Jan 2024 at 12:22, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 08/01/2024 14:48, Vincent Guittot wrote:
>>> Aggregate the different pressures applied on the capacity of CPUs and
>>> create a new function that returns the actual capacity of the CPU:
>>>   get_actual_cpu_capacity()
>>
>>    function name                scaling
>>
>> (1) arch_scale_cpu_capacity() - uarch
>>
>> (2) get_actual_cpu_capacity() - hw + cpufreq/thermal of (1)
>>
>> (3) capacity_of()             - rt (rt/dl/irq) of (2) (used by fair)
>>
>> Although (1) - (3) are very close to each other from the functional
> 
> I don't get your point as name of (1) and (3) have not been changed by the patch

That's true. But with capacity_orig_of() for (1), we had some coherence
in the naming scheme of those cpu_capacity related functions (1) - (3).
which helps when trying to understand the code.

I can see that actual_capacity_of() (2) sounds awful though.

>> standpoint, their names are not very coherent.
>>
>> I assume this makes it hard to understand all of this when reading the
>> code w/o knowing these patches before.
>>
>> Why is (2) tagged with 'actual'?
> 
> This is the actual max compute capacity of the cpu at now  i.e.
> possibly reduced because of temporary frequency capping

Will the actual max compute capacity also depend on 'user space system
pressure' later, i.e. on 'permanent' frequency capping?

> So (2) equals (1) minus temporary performance capping and (3)
> additionally subtracts the time used by other class to (2)

OK.

A coherent set of those tags even reflected in those getters would help
but can be done later too.