linux-kernel - Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

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Message-ID: <CAJZ5v0gOBH7OKF3KXwxYfWkGkC45rzDguR4VmSnoZDKpm+KPSg@mail.gmail.com>
Date: Wed, 19 Jun 2024 19:09:35 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Aaron Rainbolt <arainbolt@...cus.org>
Cc: Mario Limonciello <mario.limonciello@....com>, linux-acpi@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rafael@...nel.org, lenb@...nel.org, 
	mmikowski@...cus.org, Perry.Yuan@....com
Subject: Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

On Wed, Jun 19, 2024 at 6:33 AM Aaron Rainbolt <arainbolt@...cus.org> wrote:
>
> acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter
>
> The _OSC is supposed to contain a bit indicating whether the hardware
> supports CPPC v2 or not. This bit is not always set, causing CPPC v2 to
> be considered absent. This results in severe single-core performance
> issues with the EEVDF scheduler on heterogenous-core Intel processors.

While some things can be affected by this, I don't immediately see a
connection between CPPC v2, Intel hybrid processors and EEVDF.

In particular, why would EEVDF alone be affected?

Care to explain this?

> To work around this, provide a new kernel parameter, "ignore_osc_cppc_bit",
> which may be used to ignore the _OSC CPPC v2 bit and act as if the bit was
> enabled. This allows CPPC to be properly detected even if not "enabled" by
> _OSC, allowing users with problematic hardware to obtain decent single-core
> performance.
>
> Tested-by: Michael Mikowski <mmikowski@...cus.org>
> Signed-off-by: Aaron Rainbolt <arainbolt@...cus.org>
>
> ---
>
> V2 -> V3: Move bit ignore to before switch.
> V1 -> V2: Rewrite to work in cpc_supported_by_cpu.
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b600df82669d..af2d8973ba3a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2063,6 +2063,12 @@
>                         could change it dynamically, usually by
>                         /sys/module/printk/parameters/ignore_loglevel.
>
> +       ignore_osc_cppc_bit
> +                       Assume CPPC is present and ignore the CPPC v2 bit from
> +                       the ACPI _OSC method. This is useful for working
> +                       around buggy firmware where CPPC is supported, but
> +                       _OSC incorrectly reports it as being absent.
> +
>         ignore_rlimit_data
>                         Ignore RLIMIT_DATA setting for data mappings,
>                         print warning at first misuse.  Can be changed via
> diff --git a/arch/x86/kernel/acpi/cppc.c b/arch/x86/kernel/acpi/cppc.c
> index ff8f25faca3d..0ca1eac826af 100644
> --- a/arch/x86/kernel/acpi/cppc.c
> +++ b/arch/x86/kernel/acpi/cppc.c
> @@ -11,8 +11,20 @@
>
>  /* Refer to drivers/acpi/cppc_acpi.c for the description of functions */
>
> +static bool ignore_osc_cppc_bit;
> +static int __init parse_ignore_osc_cppc_bit(char *arg)
> +{
> +       ignore_osc_cppc_bit = true;
> +       return 0;
> +}
> +early_param("ignore_osc_cppc_bit", parse_ignore_osc_cppc_bit);
> +
>  bool cpc_supported_by_cpu(void)
>  {
> +       if (ignore_osc_cppc_bit) {
> +               return true;
> +       }
> +
>         switch (boot_cpu_data.x86_vendor) {
>         case X86_VENDOR_AMD:
>         case X86_VENDOR_HYGON:
>