linux-kernel - Re: [RFC v3 6/6] arm64/amu: use capacity_ref

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Date:   Thu, 26 Oct 2023 16:30:53 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>, linux@...linux.org.uk,
        catalin.marinas@....com, will@...nel.org, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, sudeep.holla@....com,
        gregkh@...uxfoundation.org, rafael@...nel.org, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, viresh.kumar@...aro.org, lenb@...nel.org,
        robert.moore@...el.com, lukasz.luba@....com,
        pierre.gondois@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-pm@...r.kernel.org, linux-acpi@...r.kernel.org,
        acpica-devel@...ts.linuxfoundation.org, conor.dooley@...rochip.com,
        suagrfillet@...il.com, ajones@...tanamicro.com, lftan@...nel.org
Subject: Re: [RFC v3 6/6] arm64/amu: use capacity_ref_freq to set AMU ratio

On Thu, 26 Oct 2023 at 13:19, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 24/10/2023 11:58, Vincent Guittot wrote:
> > On Mon, 23 Oct 2023 at 22:58, Ionela Voinescu <ionela.voinescu@....com> wrote:
> >>
> >> Hi,
> >>
> >> On Wednesday 18 Oct 2023 at 18:25:40 (+0200), Vincent Guittot wrote:
> >>> Use the new capacity_ref_freq to set the ratio that is used by AMU for
> >>> computing the arch_scale_freq_capacity().
> >>> This helps to keep everything aligned using the same reference for
> >>> computing CPUs capacity.
> >>>
> >>> The default value of the ratio ensures that arch_scale_freq_capacity()
> >>> returns max capacity until it is set to its correct value with the
> >>> cpu capacity and capacity_ref_freq.
>
> Nitpick: Could you mention that arch_max_freq_scale is the default value
> for this ratio? Took me a while to recreate the (not so simple) story
> for this change, i.e. make the connection between ratio and
> arch_max_freq_scale.

something like :
"
The default value of the ratio (saved in per_cpu(arch_max_freq_scale)
ensures that arch_scale_freq_capacity() returns max capacity until it
is set to its correct value with the cpu capacity and
capacity_ref_freq.
"

Or I can rename the variable per_cpu(arch_max_freq_ratio)

>
> init_cpu_capacity_callback()
>
>   freq_inv_set_max_ratio()
>
>     u64 ratio
>     ...
>     per_cpu(arch_max_freq_scale, cpu) = (unsigned long)ratio
>             ^^^^^^^^^^^^^^^^^^^
>
>
> static struct scale_freq_data amu_sfd = {
>         .set_freq_scale = amu_scale_freq_tick,
> }
>
> #define arch_scale_freq_tick topology_scale_freq_tick
>
> topology_scale_freq_tick()
>
>   sfd->set_freq_scale()
>
>
> amu_scale_freq_tick()
>
>   ...
>   scale *= this_cpu_read(arch_max_freq_scale)
>                          ^^^^^^^^^^^^^^^^^^^
>   ...
>   this_cpu_write(arch_freq_scale, (unsigned long)scale);
>
>
> #define arch_scale_freq_capacity topology_get_freq_scale
>
> topology_get_freq_scale(cpu)
>
>   return per_cpu(arch_freq_scale, cpu)
>
> [...]
>