feat: introduce resource control over all machines

We were using over all our machines in the Lix infrastructure.
It still makes sense for all our machines.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

common/base-server.nix

@@ -1,6 +1,7 @@
 { lib, pkgs, ... }: {
   imports = [
     ./known-ssh-keys.nix
+    ./cgroups.nix
   ];
 
   nixpkgs.overlays = import ../overlays;

common/cgroups.nix

@@ -0,0 +1,83 @@
+# Relatively inspired by fbtax2:
+# https://facebookmicrosites.github.io/cgroup2/docs/fbtax-results.html
+#
+# See also the Chris Down talk at LISA'21:
+# https://www.usenix.org/conference/lisa21/presentation/down
+{ ... }:
+let
+  systemCriticalSliceConfig = {
+    ManagedOOMMemoryPressure = "kill";
+
+    # guarantee availability of memory
+    MemoryMin = "192M";
+    # default 100
+    IOWeight = 1000;
+    # default 100
+    CPUWeight = 1000;
+  };
+in
+{
+  systemd.oomd = {
+    enable = true;
+    # why not, we have cgroups at user level now so it'll just kill the
+    # terminal
+    enableRootSlice = true;
+    enableSystemSlice = true;
+    enableUserSlices = true;
+  };
+
+  systemd.enableCgroupAccounting = true;
+
+  systemd.services.nix-daemon = {
+    serviceConfig = {
+      # FIXME: how do i deprioritize this for memory
+      CPUWeight = 10;
+      IOWeight = 10;
+    };
+  };
+
+  systemd.slices.hostcritical = {
+    description = "Ensures that services to keep the system alive remain alive";
+
+    unitConfig = {
+      # required to avoid a dependency cycle on systemd-oomd. systemd will
+      # actually guess this right but we should fix it anyway.
+      DefaultDependencies = false;
+    };
+
+    sliceConfig = systemCriticalSliceConfig;
+  };
+
+  # make root logins higher priority for resources
+  systemd.slices."user-0" = {
+    sliceConfig = systemCriticalSliceConfig;
+  };
+
+
+  systemd.slices.system = {
+    sliceConfig = {
+      ManagedOOMMemoryPressure = "kill";
+      ManagedOOMMemoryPressureLimit = "50%";
+
+      IOWeight = 100;
+    };
+  };
+
+  systemd.services.sshd = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+
+  systemd.services.systemd-oomd = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+
+  systemd.services.systemd-journald = {
+    serviceConfig = {
+      Slice = "hostcritical.slice";
+    };
+  };
+}