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From:   Wyes Karny <wyes.karny@amd.com>
To:     Meng Li <li.meng@amd.com>
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Subject: Re: [PATCH V8 6/7] Documentation: amd-pstate: introduce amd-pstate preferred core
Date:   Mon, 9 Oct 2023 11:53:48 +0530
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On 09 Oct 10:49, Meng Li wrote:
> Introduce amd-pstate preferred core.
> 
> check preferred core state set by the kernel parameter:
> $ cat /sys/devices/system/cpu/amd-pstate/prefcore
> 
> Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
> Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Wyes Karny <wyes.karny@amd.com>
> Signed-off-by: Meng Li <li.meng@amd.com>
> ---
>  Documentation/admin-guide/pm/amd-pstate.rst | 59 ++++++++++++++++++++-
>  1 file changed, 57 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
> index 1cf40f69278c..0b832ff529db 100644
> --- a/Documentation/admin-guide/pm/amd-pstate.rst
> +++ b/Documentation/admin-guide/pm/amd-pstate.rst
> @@ -300,8 +300,8 @@ platforms. The AMD P-States mechanism is the more performance and energy
>  efficiency frequency management method on AMD processors.
>  
>  
> -AMD Pstate Driver Operation Modes
> -=================================
> +``amd-pstate`` Driver Operation Modes
> +======================================
>  
>  ``amd_pstate`` CPPC has 3 operation modes: autonomous (active) mode,
>  non-autonomous (passive) mode and guided autonomous (guided) mode.
> @@ -353,6 +353,48 @@ 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 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-pstate`` Preferred Core Switch
> +=================================
> +Kernel Parameters
> +-----------------
> +
> +``amd-pstate`` peferred 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``
> +
> +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``.
> +
>  User Space Interface in ``sysfs`` - General
>  ===========================================
>  
> @@ -385,6 +427,19 @@ 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-pstate`` preferred core.
> +
> +	"disabled"
> +		Disable the ``amd-pstate`` preferred core
> +
> +
> +        This attribute is read-only to check the state of preferred core set
> +        by the kernel parameter.
> +
>  ``cpupower`` tool support for ``amd-pstate``
>  ===============================================
>  
> -- 
> 2.34.1
>