nixos/iosched: init module

nixos/modules/hardware/iosched.nix

@@ -0,0 +1,150 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+  cfg = config.hardware.block;
+  escape = lib.strings.escape [ ''"'' ];
+  udevValue = types.addCheck types.nonEmptyStr (x: builtins.match "[^\n\r]*" x != null) // {
+    name = "udevValue";
+    description = "udev rule value";
+    descriptionClass = "noun";
+  };
+
+  inherit (lib)
+    mkIf
+    mkOption
+    types
+
+    concatLines
+    concatStringsSep
+    mapAttrsToList
+    optional
+    ;
+in
+{
+  options.hardware.block = {
+    defaultScheduler = mkOption {
+      type = types.nullOr udevValue;
+      default = null;
+      description = ''
+        Default block I/O scheduler.
+
+        Unless `null`, the value is assigned through a udev rule matching all
+        block devices.
+      '';
+      example = "kyber";
+    };
+
+    defaultSchedulerRotational = mkOption {
+      type = types.nullOr udevValue;
+      default = null;
+      description = ''
+        Default block I/O scheduler for rotational drives (e.g. hard disks).
+
+        Unless `null`, the value is assigned through a udev rule matching all
+        rotational block devices.
+
+        This option takes precedence over
+        {option}`config.hardware.block.defaultScheduler`.
+      '';
+      example = "bfq";
+    };
+
+    scheduler = mkOption {
+      type = types.attrsOf udevValue;
+      default = { };
+      description = ''
+        Assign block I/O scheduler by device name pattern.
+
+        Names are matched using the {manpage}`udev(7)` pattern syntax:
+
+        `*`
+        :  Matches zero or more characters.
+
+        `?`
+        :  Matches any single character.
+
+        `[]`
+        :  Matches any single character specified in the brackets. Ranges are
+           supported via the `-` character.
+
+        `|`
+        :  Separates alternative patterns.
+
+
+        Please note that overlapping patterns may produce unexpected results.
+        More complex configurations requiring these should instead be specified
+        directly through custom udev rules, for example via
+        [{option}`config.services.udev.extraRules`](#opt-services.udev.extraRules),
+        to ensure correct ordering.
+
+        Available schedulers depend on the kernel configuration but modern
+        Linux systems typically support:
+
+        `none`
+        :  No‐operation scheduler with no re‐ordering of requests. Suitable
+           for devices with fast random I/O such as NVMe SSDs.
+
+        [`mq-deadline`](https://www.kernel.org/doc/html/latest/block/deadline-iosched.html)
+        :  Simple latency‐oriented general‐purpose scheduler.
+
+        [`kyber`](https://www.kernel.org/doc/html/latest/block/kyber-iosched.html)
+        :  Simple latency‐oriented scheduler for fast multi‐queue devices
+           like NVMe SSDs.
+
+        [`bfq`](https://www.kernel.org/doc/html/latest/block/bfq-iosched.html)
+        :  Complex fairness‐oriented scheduler. Higher processing overhead,
+           but good interactive response, especially with slower devices.
+
+
+        Schedulers assigned through this option take precedence over
+        {option}`config.hardware.block.defaultScheduler` and
+        {option}`config.hardware.block.defaultSchedulerRotational` but may be
+        overridden by other udev rules.
+      '';
+      example = {
+        "mmcblk[0-9]*" = "bfq";
+        "nvme[0-9]*" = "kyber";
+      };
+    };
+  };
+
+  config =
+    mkIf
+      (cfg.defaultScheduler != null || cfg.defaultSchedulerRotational != null || cfg.scheduler != { })
+      {
+        services.udev.packages = [
+          (pkgs.writeTextDir "etc/udev/rules.d/98-block-io-scheduler.rules" (
+            concatLines (
+              map (concatStringsSep ", ") (
+                optional (cfg.defaultScheduler != null) [
+                  ''SUBSYSTEM=="block"''
+                  ''ACTION=="add|change"''
+                  ''TEST=="queue/scheduler"''
+                  ''ATTR{queue/scheduler}="${escape cfg.defaultScheduler}"''
+                ]
+                ++ optional (cfg.defaultSchedulerRotational != null) [
+                  ''SUBSYSTEM=="block"''
+                  ''ACTION=="add|change"''
+                  ''ATTR{queue/rotational}=="1"''
+                  ''TEST=="queue/scheduler"''
+                  ''ATTR{queue/scheduler}="${escape cfg.defaultSchedulerRotational}"''
+                ]
+                ++ mapAttrsToList (name: sched: [
+                  ''SUBSYSTEM=="block"''
+                  ''ACTION=="add|change"''
+                  ''KERNEL=="${escape name}"''
+                  ''ATTR{queue/scheduler}="${escape sched}"''
+                ]) cfg.scheduler
+              )
+            )
+          ))
+        ];
+      };
+
+  meta.maintainers = with lib.maintainers; [ mvs ];
+}

nixos/modules/module-list.nix

@@ -72,6 +72,7 @@
   ./hardware/i2c.nix
   ./hardware/infiniband.nix
   ./hardware/inputmodule.nix
+  ./hardware/iosched.nix
   ./hardware/keyboard/qmk.nix
   ./hardware/keyboard/teck.nix
   ./hardware/keyboard/uhk.nix

nixos/tests/all-tests.nix

@@ -488,6 +488,7 @@ in {
   honk = runTest ./honk.nix;
   installed-tests = pkgs.recurseIntoAttrs (handleTest ./installed-tests {});
   invidious = handleTest ./invidious.nix {};
+  iosched = handleTest ./iosched.nix {};
   isolate = handleTest ./isolate.nix {};
   livebook-service = handleTest ./livebook-service.nix {};
   pyload = handleTest ./pyload.nix {};

nixos/tests/iosched.nix

@@ -0,0 +1,68 @@
+import ./make-test-python.nix (
+  {
+    pkgs,
+    ...
+  }:
+  let
+    qemu-img = pkgs.lib.getExe' pkgs.vmTools.qemu "qemu-img";
+    empty = pkgs.runCommand "empty.qcow2" { } ''
+      ${qemu-img} create -f qcow2 "$out" 32M
+    '';
+  in
+  {
+    name = "iosched";
+    meta.maintainers = with pkgs.lib.maintainers; [ mvs ];
+
+    nodes.machine = {
+      virtualisation.qemu.options = [
+        "-drive"
+        "id=sda,if=none,format=qcow2,readonly=on,file=${empty}"
+        "-drive"
+        "id=sdb,if=none,format=qcow2,readonly=on,file=${empty}"
+        "-drive"
+        "id=nvme0n1,if=none,format=qcow2,readonly=on,file=${empty}"
+        "-drive"
+        "id=mmcblk0,if=none,format=qcow2,file=./mmcblk0.qcow2"
+        "-device"
+        "virtio-scsi-pci,id=scsi0"
+        "-device"
+        "sdhci-pci"
+        "-device"
+        "scsi-hd,rotation_rate=1,bus=scsi0.0,drive=sda"
+        "-device"
+        "scsi-hd,rotation_rate=7200,bus=scsi0.0,drive=sdb"
+        "-device"
+        "sd-card,drive=mmcblk0"
+        "-device"
+        "nvme,serial=deadbeef,drive=nvme0n1"
+      ];
+
+      hardware.block = {
+        defaultScheduler = "none";
+        defaultSchedulerRotational = "mq-deadline";
+        scheduler = {
+          "nvme[0-9]*" = "kyber";
+          "mmcblk[0-9]*" = "bfq";
+        };
+      };
+    };
+
+    testScript = ''
+      import subprocess
+
+      def check_scheduler(dev, scheduler):
+        machine.succeed("grep -F -q '[{}]' /sys/block/{}/queue/scheduler".format(scheduler, dev))
+
+      subprocess.check_call([
+        "${qemu-img}", "create", "-f", "qcow2", "vm-state-machine/mmcblk0.qcow2", "32M"
+      ])
+
+      machine.start()
+      machine.succeed("udevadm verify --no-style")
+      check_scheduler("sda", "none")
+      check_scheduler("sdb", "mq-deadline")
+      check_scheduler("nvme0n1", "kyber")
+      check_scheduler("mmcblk0", "bfq")
+    '';
+  }
+)