linux-kernel - Re: [PATCH] x86: intel_epb: Add earlyparam option to keep bias at performance

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Date:   Tue, 5 Dec 2023 14:30:04 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Jack Allister <jalliste@...zon.com>
Cc:     rafael@...nel.org, Paul Durrant <pdurrant@...zon.com>,
        Jue Wang <juew@...zon.com>,
        Usama Arif <usama.arif@...edance.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: intel_epb: Add earlyparam option to keep bias at performance

On Tue, Dec 5, 2023 at 2:14 PM Jack Allister <jalliste@...zon.com> wrote:
>
> There are certain scenarios where it may be intentional that the EPB was
> set at to 0/ENERGY_PERF_BIAS_PERFORMANCE on kernel boot. For example, in
> data centers a kexec/live-update of the kernel may be performed regularly.
>
> Usually this live-update is time critical and defaulting of the bias back
> to ENERGY_PERF_BIAS_NORMAL may actually be detrimental to the overall
> update time if processors' time to ramp up/boost are affected.
>
> This patch introduces a kernel command line "intel_epb_keep_performance"
> which will leave the EPB at performance if during the restoration code path
> it is detected as such.
>
> Signed-off-by: Jack Allister <jalliste@...zon.com>
> Cc: Paul Durrant <pdurrant@...zon.com>
> Cc: Jue Wang <juew@...zon.com>
> Cc: Usama Arif <usama.arif@...edance.com>
> ---
>  arch/x86/kernel/cpu/intel_epb.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
> index e4c3ba91321c..cbe0e224b8d9 100644
> --- a/arch/x86/kernel/cpu/intel_epb.c
> +++ b/arch/x86/kernel/cpu/intel_epb.c
> @@ -50,7 +50,8 @@
>   * the OS will do that anyway.  That sometimes is problematic, as it may cause
>   * the system battery to drain too fast, for example, so it is better to adjust
>   * it on CPU bring-up and if the initial EPB value for a given CPU is 0, the
> - * kernel changes it to 6 ('normal').
> + * kernel changes it to 6 ('normal'). This however is overridable via
> + * intel_epb_no_override if required.
>   */

In the comment above I would say

"However, if it is desirable to retain the original initial EPB value,
intel_epb_no_override can be set to enforce it."

Otherwise

Acked-by: Rafael J. Wysocki <rafael@...nel.org>

>
>  static DEFINE_PER_CPU(u8, saved_epb);
> @@ -75,6 +76,8 @@ static u8 energ_perf_values[] = {
>         [EPB_INDEX_POWERSAVE] = ENERGY_PERF_BIAS_POWERSAVE,
>  };
>
> +static bool intel_epb_no_override __read_mostly;
> +
>  static int intel_epb_save(void)
>  {
>         u64 epb;
> @@ -106,7 +109,7 @@ static void intel_epb_restore(void)
>                  * ('normal').
>                  */
>                 val = epb & EPB_MASK;
> -               if (val == ENERGY_PERF_BIAS_PERFORMANCE) {
> +               if (!intel_epb_no_override && val == ENERGY_PERF_BIAS_PERFORMANCE) {
>                         val = energ_perf_values[EPB_INDEX_NORMAL];
>                         pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
>                 }
> @@ -213,6 +216,12 @@ static const struct x86_cpu_id intel_epb_normal[] = {
>         {}
>  };
>
> +static __init int intel_epb_no_override_setup(char *str)
> +{
> +       return kstrtobool(str, &intel_epb_no_override);
> +}
> +early_param("intel_epb_no_override", intel_epb_no_override_setup);
> +
>  static __init int intel_epb_init(void)
>  {
>         const struct x86_cpu_id *id = x86_match_cpu(intel_epb_normal);
> --
> 2.40.1
>