machine_setup/nix/configuration/roles/kernel/default.nix

# Check current config:
#   nix build '/persist/machine_setup/nix/configuration#nixosConfigurations.hydra.pkgs.linux_me.configfile'
#   cat $(nix eval --raw '/persist/machine_setup/nix/configuration#nixosConfigurations.hydra.pkgs.linux_me.configfile') | less

{
  config,
  lib,
  pkgs,
  ...
}:

let
  preemption_type = with lib.kernel; {
    full = {
      PREEMPT_DYNAMIC = yes;
      PREEMPT = yes;
      PREEMPT_VOLUNTARY = lib.mkForce no;
      PREEMPT_LAZY = lib.mkForce no;
      PREEMPT_NONE = no;
    };
    lazy = {
      PREEMPT_DYNAMIC = yes;
      PREEMPT = no;
      PREEMPT_VOLUNTARY = lib.mkForce no;
      PREEMPT_LAZY = yes;
      PREEMPT_NONE = no;
    };
    voluntary = {
      PREEMPT_DYNAMIC = no;
      PREEMPT = no;
      PREEMPT_VOLUNTARY = yes;
      PREEMPT_LAZY = lib.mkForce no;
      PREEMPT_NONE = no;
    };
    none = {
      PREEMPT_DYNAMIC = no;
      PREEMPT = no;
      PREEMPT_VOLUNTARY = lib.mkForce no;
      PREEMPT_LAZY = lib.mkForce no;
      PREEMPT_NONE = yes;
    };
  };
  tick_hz =
    with lib.kernel;
    {
      "1000" = {
        HZ_1000 = yes;
        HZ = freeform "1000";
      };
    }
    // lib.genAttrs [ "100" "250" "300" "500" "600" "750" ] (hz: {
      HZ_1000 = no;
      "HZ_${hz}" = yes;
      HZ = freeform hz;
    });
  performance_governor = with lib.kernel; {
    default = {
      CPU_FREQ_DEFAULT_GOV_SCHEDUTIL = yes;
    };
    performance = {
      CPU_FREQ_DEFAULT_GOV_SCHEDUTIL = no;
      CPU_FREQ_DEFAULT_GOV_PERFORMANCE = yes;
    };
  };
  tick_rate = with lib.kernel; {
    # Always tick at the hz frequency.
    periodic = {
      NO_HZ_IDLE = no;
      NO_HZ_FULL = no;
      NO_HZ = no;
      NO_HZ_COMMON = no;
      HZ_PERIODIC = yes;
    };
    # Idle - Do not disturb the CPU when idle. This can save power but increase latency.
    idle = {
      HZ_PERIODIC = no;
      NO_HZ_FULL = no;
      NO_HZ_IDLE = yes;
      NO_HZ = yes;
      NO_HZ_COMMON = yes;
    };
    # Full dyntick system (tickless) - The kernel tries to shut down the tick whenever possible.
    tickless = {
      HZ_PERIODIC = no;
      NO_HZ_IDLE = no;
      NO_HZ_FULL = yes;
      NO_HZ = yes;
      NO_HZ_COMMON = yes;
      CONTEXT_TRACKING = yes;
    };
  };
  huge_page = with lib.kernel; {
    always = {
      TRANSPARENT_HUGEPAGE_MADVISE = no;
      TRANSPARENT_HUGEPAGE_ALWAYS = yes;
    };
    madvise = {
      TRANSPARENT_HUGEPAGE_ALWAYS = no;
      TRANSPARENT_HUGEPAGE_MADVISE = yes;
    };
  };
  common_config =
    with lib.kernel;
    {
      # Google's BBRv3 TCP congestion Control
      TCP_CONG_BBR = yes;
      DEFAULT_BBR = yes;
    };
  flavors = {
    server = lib.mkMerge [
      preemption_type.none
      tick_hz."300"
      performance_governor.default
      tick_rate.tickless
      huge_page.madvise
    ];
    interactive =
      with lib.kernel;
      lib.mkMerge [
        {
          # Enable RCU Lazy - Reduces power consumption when idle or lightly loaded. Useful for battery-powered devices like laptops.
          RCU_LAZY = yes;
        }
        preemption_type.lazy
        tick_hz."300"
        performance_governor.default
        tick_rate.tickless
        huge_page.madvise
      ];
  };
in
{
  imports = [ ];

  options.me = {
    kernel.enable = lib.mkOption {
      type = lib.types.bool;
      default = false;
      example = true;
      description = "Whether we want to install kernel.";
    };

    kernel.version = lib.mkOption {
      type = lib.types.str;
      default = "linux"; # LTS
      example = "linux_6_18";
      description = "What version of the kernl should we use.";
    };

    kernel.flavor = lib.mkOption {
      type = lib.types.str;
      default = "interactive";
      example = "server";
      description = "What type of kernel should be built.";
    };
  };

  config = lib.mkIf config.me.kernel.enable (
    lib.mkMerge [
      {
        boot.kernelPackages = pkgs.linuxPackagesFor pkgs.linux_me;
      }
      (lib.mkIf (!config.me.optimizations.enable) {
        nixpkgs.overlays = [
          (final: prev: {
            linux_me = final."${config.me.kernel.version}";
          })
        ];
      })
      (lib.mkIf (config.me.optimizations.enable) {
        nixpkgs.overlays = [
          (
            final: prev:
            let
              addConfig =
                additionalConfig: pkg:
                pkg.override (oldconfig: {
                  structuredExtraConfig = lib.mkMerge ([ pkg.structuredExtraConfig ] ++ additionalConfig);
                  # stdenv = pkgs.llvmPackages_latest.stdenv;
                  # stdenv = pkgs.clangStdenv;
                });
            in
            {
              linux_me = addConfig ([
                common_config
                flavors."${config.me.kernel.flavor}"
              ]) final."${config.me.kernel.version}";
            }
          )
        ];
      })
    ]
  );
}