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

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Date: Wed, 10 Jan 2024 18:25:15 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org, 
	sudeep.holla@....com, rafael@...nel.org, viresh.kumar@...aro.org, 
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Subject: Re: [PATCH v3 2/5] sched: Take cpufreq feedback into account

On Wed, 10 Jan 2024 at 14:51, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> 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?

yes it will


>
> > 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.