linux-kernel - Re: [PATCH V3 6/7] Documentation: amd-pstate: introduce AMD Pstate Preferred Core

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Message-ID: <20230824111319.vyyaxnlpw6wxkni6@BLR-5CG13462PL.amd.com>
Date:   Thu, 24 Aug 2023 16:48:45 +0530
From:   Wyes Karny <wyes.karny@....com>
To:     Meng Li <li.meng@....com>
CC:     Wyes Karny <wyes.karny@....com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Huang Rui <ray.huang@....com>, <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>,
        Mario Limonciello <mario.limonciello@....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 V3 6/7] Documentation: amd-pstate: introduce AMD Pstate Preferred Core

On 24 Aug 10:12, Meng Li wrote:
> Introduce AMD Pstate Preferred Core.
> 
> check preferred core state:
> $ cat /sys/devices/system/cpu/amd-pstate/prefcore
> 
> Signed-off-by: Meng Li <li.meng@....com>
> ---
>  Documentation/admin-guide/pm/amd-pstate.rst | 53 +++++++++++++++++++++
>  1 file changed, 53 insertions(+)
> 
> diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
> index 1cf40f69278c..2369b58a3521 100644
> --- a/Documentation/admin-guide/pm/amd-pstate.rst
> +++ b/Documentation/admin-guide/pm/amd-pstate.rst
> @@ -353,6 +353,47 @@ 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.
> +

---------------------------->8----------------------------------
> +``AMD Pstate Preferred Core`` enable the scheduler to favor scheduling on cores
> +can be get a higher frequency with lower voltage under preferred core.
> +And it has the ability to dynamically change the preferred core based on the
> +workload and platform conditions and accounting for thermals and aging.

Can we reword this like below:

``AMD Pstate Preferred Core`` enables the scheduler to prefer scheduling
on cores that can achieve a higher frequency with lower voltage.  The
preferred core rankings can dynamically change based on the workload,
platform conditions, thermals and ageing.

> +
> +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
> +-----------------
> +

----------------------------->8------------------------------
> +``AMD Pstate Preferred Core`` has two states: enable and disable.
> +Enable/disable states can be chosen by different kernel parameters.
> +Default enable ``AMD Pstate Preferred Core``.
> +
> +``amd_prefcore=disable``
> +
> +``AMD Pstate Preferred Core`` will be enabled if the underlying platform
> +supports it. It can be disabled by kernerl parameter: ``amd_prefcore=disable``.

Can we re-word this like below:

``amd_prefcore=disable``

For systems that support ``AMD Pstate Preferred Core``, the core
rankings will always be advertised by the platform. But OS can choose to
ignore that via the kernel parameter ``amd_prefcore=disable``.


Thanks,
Wyes
> +
>  User Space Interface in ``sysfs`` - General
>  ===========================================
>  
> @@ -385,6 +426,18 @@ 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``
> +	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.
> +
>  ``cpupower`` tool support for ``amd-pstate``
>  ===============================================
>  
> -- 
> 2.34.1
>