base/irqbalance: only start on multi-socket machines

base/services/irqbalance.nix

@@ -1,4 +1,22 @@
-{ ... }:
+{ config, pkgs, lib, ... }:
+let
+  cfg = config.services.irqbalance;
+in
 {
   services.irqbalance.enable = true;
-}
+
+  # irqbalance only has meaningful work to do on multi-socket machines, so
+  # skip starting it pointlessly everywhere else.
+  systemd.services.irqbalance.serviceConfig.ExecCondition = let
+    isMultiSocket = pkgs.writeShellApplication {
+      name = "irqbalance-is-multi-socket";
+      runtimeInputs = [ pkgs.coreutils ];
+      text = ''
+        sockets=$(cat /sys/devices/system/cpu/cpu*/topology/physical_package_id | sort -u | wc -l)
+        [ "$sockets" -gt 1 ]
+      '';
+    };
+  in lib.mkIf cfg.enable [
+    (lib.getExe isMultiSocket)
+  ];
+}