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

Rafael J. Wysocki
2023-12-05T12:48:25Z

On Mon, Dec 4, 2023 at 6:30 PM Jack Allister <jalliste@amazon.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@amazon.com>
> Cc: Paul Durrant <pdurrant@amazon.com>
> Cc: Jue Wang <juew@amazon.com>
> Cc: Usama Arif <usama.arif@bytedance.com>
> ---
> arch/x86/kernel/cpu/intel_epb.c | 19 ++++++++++++++++---
> 1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
> index e4c3ba91321c..0c7dd092f723 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_keep_performance if required.
> */
>
> 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_keep_performance __read_mostly;
> +
> static int intel_epb_save(void)
> {
> u64 epb;
> @@ -107,8 +110,12 @@ static void intel_epb_restore(void)
> */
> val = epb & EPB_MASK;
> if (val == ENERGY_PERF_BIAS_PERFORMANCE) {
> - val = energ_perf_values[EPB_INDEX_NORMAL];
> - pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
> + if (!intel_epb_keep_performance) {

if (!intel_epb_keep_performance && val == ENERGY_PERF_BIAS_PERFORMANCE) {

and you need not notify the sysadmin that the original value has
returned - they have set the command line switch for this purpose
after all.

> + val = energ_perf_values[EPB_INDEX_NORMAL];
> + pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
> + } else {
> + pr_warn_once("ENERGY_PERF_BIAS: Kept at 'performance', no change\n");
> + }
> }
> }
> wrmsrl(MSR_IA32_ENERGY_PERF_BIAS, (epb & ~EPB_MASK) | val);
> @@ -213,6 +220,12 @@ static const struct x86_cpu_id intel_epb_normal[] = {
> {}
> };
>
> +static __init int intel_epb_keep_performance_setup(char *str)
> +{
> + return kstrtobool(str, &intel_epb_keep_performance);
> +}
> +early_param("intel_epb_keep_performance", intel_epb_keep_performance_setup);
> +
> static __init int intel_epb_init(void)
> {
> const struct x86_cpu_id *id = x86_match_cpu(intel_epb_normal);
> --
> 2.40.1
>