linux-kernel - Re: [PATCH V4 6/7] Documentation: amd-pstate: introduce AMD Pstate Preferred Core

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Date:   Tue, 29 Aug 2023 16:07:52 +0800
From:   Huang Rui <ray.huang@....com>
To:     "Meng, Li (Jassmine)" <Li.Meng@....com>
Cc:     "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Fontenot, Nathan" <Nathan.Fontenot@....com>,
        "Sharma, Deepak" <Deepak.Sharma@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        "Limonciello, Mario" <Mario.Limonciello@....com>,
        "Huang, Shimmer" <Shimmer.Huang@....com>,
        "Yuan, Perry" <Perry.Yuan@....com>,
        "Du, Xiaojian" <Xiaojian.Du@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH V4 6/7] Documentation: amd-pstate: introduce AMD Pstate
 Preferred Core

On Tue, Aug 29, 2023 at 02:43:39PM +0800, Meng, Li (Jassmine) 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 | 54 +++++++++++++++++++++
>  1 file changed, 54 insertions(+)
> 
> diff --git a/Documentation/admin-guide/pm/amd-pstate.rst b/Documentation/admin-guide/pm/amd-pstate.rst
> index 1cf40f69278c..400264d52007 100644
> --- a/Documentation/admin-guide/pm/amd-pstate.rst
> +++ b/Documentation/admin-guide/pm/amd-pstate.rst
> @@ -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

Please align with ``amd-pstate`` in the whole documentation.

> +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 same here.

> +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 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``.

As the comment in previos patch, we would better to let developers know how
to detect this function in the hardware.

Thanks,
Ray

> +
>  User Space Interface in ``sysfs`` - General
>  ===========================================
>  
> @@ -385,6 +427,18 @@ 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
>