linux-kernel - Re: [PATCH v1] cpufreq: intel_pstate: Make it possible to avoid enabling CAS

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Message-ID: <CAJZ5v0jytAtj_rDSz52OznCNV5Wo3hb0bjXCUR+ox99YdSxJ_Q@mail.gmail.com>
Date: Fri, 7 Feb 2025 13:55:48 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc: Linux PM <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Zhang Rui <rui.zhang@...el.com>, "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v1] cpufreq: intel_pstate: Make it possible to avoid
 enabling CAS

On Mon, Jan 27, 2025 at 3:07 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> Capacity-aware scheduling (CAS) is enabled by default by intel_pstate on
> hybrid systems without SMT, but in some usage scenarios it may be more
> attractive to place tasks for maximum CPU performance regardless of the
> extra cost in terms of energy, which is the case on such systems when
> CAS is not enabled, so introduce a command line option to forbid
> intel_pstate to enable CAS.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Srinivas,

Any concerns or comments?

> ---
>  Documentation/admin-guide/kernel-parameters.txt |    3 +++
>  Documentation/admin-guide/pm/intel_pstate.rst   |    3 +++
>  drivers/cpufreq/intel_pstate.c                  |    9 +++++++++
>  3 files changed, 15 insertions(+)
>
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2295,6 +2295,9 @@
>                         per_cpu_perf_limits
>                           Allow per-logical-CPU P-State performance control limits using
>                           cpufreq sysfs interface
> +                       no_cas
> +                         Do not enable capacity-aware scheduling (CAS) on
> +                         hybrid systems
>
>         intremap=       [X86-64,Intel-IOMMU,EARLY]
>                         on      enable Interrupt Remapping (default)
> --- a/Documentation/admin-guide/pm/intel_pstate.rst
> +++ b/Documentation/admin-guide/pm/intel_pstate.rst
> @@ -696,6 +696,9 @@
>         Use per-logical-CPU P-State limits (see `Coordination of P-state
>         Limits`_ for details).
>
> +``no_cas``
> +       Do not enable capacity-aware scheduling (CAS) which is enabled by
> +       default on hybrid systems.
>
>  Diagnostics and Tuning
>  ======================
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -936,6 +936,8 @@
>         NULL,
>  };
>
> +static bool no_cas __ro_after_init;
> +
>  static struct cpudata *hybrid_max_perf_cpu __read_mostly;
>  /*
>   * Protects hybrid_max_perf_cpu, the capacity_perf fields in struct cpudata,
> @@ -1041,6 +1043,10 @@
>
>  static void hybrid_init_cpu_capacity_scaling(bool refresh)
>  {
> +       /* Bail out if enabling capacity-aware scheduling is prohibited. */
> +       if (no_cas)
> +               return;
> +
>         /*
>          * If hybrid_max_perf_cpu is set at this point, the hybrid CPU capacity
>          * scaling has been enabled already and the driver is just changing the
> @@ -3835,6 +3841,9 @@
>         if (!strcmp(str, "no_hwp"))
>                 no_hwp = 1;
>
> +       if (!strcmp(str, "no_cas"))
> +               no_cas = true;
> +
>         if (!strcmp(str, "force"))
>                 force_load = 1;
>         if (!strcmp(str, "hwp_only"))
>
>
>
>