linux-kernel - Re: [PATCH V1 5/6] Documentation: amd-pstate: introduce AMD Pstate Preferred Core

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Date:   Tue, 8 Aug 2023 10:40:51 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Meng Li <li.meng@....com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Huang Rui <ray.huang@....com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-acpi@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        linux-kselftest@...r.kernel.org,
        Nathan Fontenot <nathan.fontenot@....com>,
        Deepak Sharma <deepak.sharma@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Shimmer Huang <shimmer.huang@....com>,
        Perry Yuan <Perry.Yuan@....com>,
        Xiaojian Du <Xiaojian.Du@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH V1 5/6] Documentation: amd-pstate: introduce AMD Pstate
 Preferred Core

On 8/8/2023 03:10, Meng Li wrote:
> Introduce AMD Pstate Preferred Core.
> 
> check preferred core state:
> $ cat /sys/devices/system/cpu/amd-pstate/prefcore_state
> 
> Signed-off-by: Meng Li <li.meng@....com>
> ---
>   Documentation/admin-guide/pm/amd-pstate.rst | 55 +++++++++++++++++++++
>   1 file changed, 55 insertions(+)
> 
> diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
> index 1cf40f69278c..4a30cf235425 100644
> --- a/Documentation/admin-guide/pm/amd-pstate.rst
> +++ b/Documentation/admin-guide/pm/amd-pstate.rst
> @@ -353,6 +353,49 @@ is activated.  In this mode, driver requests minimum and maximum performance
>   level and the platform autonomously selects a performance level in this range
>   and appropriate to the current workload.
>   
> +AMD Pstate Preferred Core
> +=================================
> +
> +The core frequency is subjected to the process variation in semiconductors.
> +Not all cores are able to reach the maximum frequency respecting the
> +infrastructure limits. Consequently, AMD has redefined the concept of
> +maximum frequency of a part. This means that a fraction of cores can reach
> +maximum frequency. To find the best process scheduling policy for a given
> +scenario, OS needs to know the core ordering informed by the platform through
> +highest performance capability register of the CPPC interface.
> +
> +``AMD Pstate Preferred Core`` use ITMT arch provides functions and data structures
> +for enabling the scheduler to favor scheduling on cores can be get a higher frequency
> +with lower voltage under preferred core. 

This sentence was useful for the commit message, but I don't think it 
should be in the user facing documentation.

> And it has the ability to dynamically
> +change the preferred core based on the workload and platform conditions and
> +accounting for thermals and aging.
> +
> +The priority metric will be initialized by the AMD Pstate driver. The AMD Pstate
> +driver will also determine whether or not ``AMD Pstate Preferred Core`` is
> +supported by the platform.
> +
> +AMD Pstate driver will provide an initial core ordering when the system boots.
> +The platform uses the CPPC interfaces to communicate the core ranking to the
> +operating system and scheduler to make sure that OS is choosing the cores
> +with highest performance firstly for scheduling the process. When AMD Pstate
> +driver receives a message with the highest performance change, it will
> +update the core ranking and set the cpu's priority.
> +
> +AMD Preferred Core Switch
> +=================================
> +Kernel Parameters
> +-----------------
> +
> +``AMD Pstate Preferred Core`` has two states: enable and disable.
> +Enable/disable states can be chosen by different kernel parameters.
> +Default disable ``AMD Pstate Preferred Core``.

Why default disable?

> +
> +``amd_prefcore=enable``
> +
> +If ``amd_prefcore=enable`` is passed to kernel command line option
> +then enable ``AMD Pstate Preferred Core`` if the processor and power
> +firmware can support preferred core feature.

This can be simplified as "platform can support the preferred core feature".

> +
>   User Space Interface in ``sysfs`` - General
>   ===========================================
>   
> @@ -385,6 +428,18 @@ control its functionality at the system level.  They are located in the
>           to the operation mode represented by that string - or to be
>           unregistered in the "disable" case.
>   
> +``prefcore_state``
> +	Preferred Core state of the driver: "enabled" or "disabled".
> +
> +	"enabled"
> +		Enable the AMD Preferred Core.
> +
> +	"disabled"
> +		Disable the AMD Preferred Core
> +
> +
> +        This attribute is read-only to check the state of Preferred Core.

As the attribute is read only and won't change at runtime, I don't think 
it makes sense to include the word "state" in the sysfs file name.

You can just rename it to "prefcore".

> +
>   ``cpupower`` tool support for ``amd-pstate``
>   ===============================================
>