mesh-catalog/modules/kernel/module.json

{
  "module": "kernel",
  "version": "1",
  "capabilities": [
    "service-manager"
  ],
  "settings": {
    "transparent-huge-pages": {
      "kind": "preference",
      "default": "always",
      "why": "always is what the distribution's kernel boots with, so assigning the module changes nothing until a machine sets another value; madvise gives huge pages only to programs that ask for them, which stops a small write to a mapped file (the journal) dirtying and writing back a whole 2 MiB page on a machine whose disks are slow (novox/hq issue 746)"
    }
  },
  "tools": [
    "kernel_huge_pages"
  ],
  "resources": [
    {
      "id": "state",
      "type": "directory",
      "mode": "0700",
      "place": "."
    },
    {
      "id": "huge-pages",
      "type": "file",
      "path": "${dir:state}/huge-pages",
      "mode": "0755",
      "trusted": true,
      "content": "#!/bin/sh\n# Transparent huge pages as the mesh sets them on this machine (module kernel, novox/hq issue 746).\n# Written by the mesh from this machine's setting transparent-huge-pages and run by\n# mesh-kernel-huge-pages.service: `apply` at boot and whenever the setting changes, `restore` when the\n# unit stops (the module unassigned, or the machine shutting down). Replaced on every push; change the\n# setting, never this file. A change of the setting reloads the unit, which runs `apply` again: the mode\n# moves straight to the new value and never passes back through the one the machine was found with.\n#\n#   huge-pages apply|restore [<the kernel's transparent_hugepage directory> [<this module's state>]]\n#\n# The two directories are arguments only so the module's tests can run this against a stand-in tree.\nset -eu\n\nwant='${setting:transparent-huge-pages}'\nsys=/sys/kernel/mm/transparent_hugepage\nstate='${dir:state}'\n[ $# -ge 2 ] && sys=$2\n[ $# -ge 3 ] && state=$3\n\ncase \"$want\" in\nalways | madvise | never) ;;\n*)\n\techo \"the setting transparent-huge-pages is '$want'; the kernel takes always, madvise or never\" >&2\n\texit 2\n\t;;\nesac\n[ -f \"$sys/enabled\" ] || {\n\techo \"this kernel has no $sys/enabled: it was built without transparent huge pages, so there is nothing to set\" >&2\n\texit 1\n}\n\n# The mode in force is the word the kernel brackets: \"always [madvise] never\". A file holding one bare\n# word (the stand-in tree of the tests) is read as that word.\nmode() { sed -n -e 's/.*\\[\\([a-z]*\\)\\].*/\\1/p' -e t -e 's/^ *\\([a-z]*\\) *$/\\1/p' \"$sys/enabled\"; }\n\n[ $# -ge 1 ] || set -- usage\ncase \"$1\" in\napply)\n\tnow=$(mode)\n\t# What the machine had before the mesh first set it, kept once, so `restore` gives it back.\n\t[ -s \"$state/found\" ] || printf '%s\\n' \"$now\" >\"$state/found\"\n\tif [ \"$now\" = \"$want\" ]; then\n\t\techo \"transparent huge pages: $want, as set\"\n\t\texit 0\n\tfi\n\tprintf '%s\\n' \"$want\" >\"$sys/enabled\"\n\tgot=$(mode)\n\t[ \"$got\" = \"$want\" ] || {\n\t\techo \"wrote $want to $sys/enabled and the kernel says $got\" >&2\n\t\texit 1\n\t}\n\techo \"transparent huge pages: $now -> $want\"\n\t# A file folio already in the page cache keeps its size: the active journal files, mapped by journald\n\t# and by every reader, stay in 2 MiB folios until they are archived (issue 746). Rotating starts new\n\t# files that are mapped under the new mode. A failed rotation is said and does not fail the setting.\n\t# At boot nothing is in huge folios yet, and the persistent journal opens only after this unit: no rotation\n\t# while the service manager is still bringing the machine up.\n\tbooting=$(systemctl is-system-running 2>/dev/null || true)\n\tcase \"$booting\" in\n\tinitializing | starting) echo \"the machine is still starting: nothing to rotate yet\" ;;\n\t*)\n\t\tif [ \"$want\" != always ] && command -v journalctl >/dev/null 2>&1; then\n\t\t\tjournalctl --rotate || echo \"the journal could not be rotated: its active files keep their huge pages until they are archived\" >&2\n\t\tfi\n\t\t;;\n\tesac\n\t;;\nrestore)\n\tfound=$(cat \"$state/found\" 2>/dev/null || true)\n\tcase \"$found\" in\n\talways | madvise | never)\n\t\tprintf '%s\\n' \"$found\" >\"$sys/enabled\"\n\t\techo \"transparent huge pages: back to $found, as the machine had them\"\n\t\t;;\n\t*) echo \"transparent huge pages: nothing recorded as found, left at $(mode)\" ;;\n\tesac\n\t# Unassigned or shut down: nothing of the module is kept; the next start records what it finds again.\n\trm -f \"$state/found\"\n\t;;\n*)\n\techo \"usage: huge-pages apply|restore [<transparent_hugepage directory> [<state directory>]]\" >&2\n\texit 2\n\t;;\nesac\n"
    },
    {
      "id": "unit",
      "type": "file",
      "path": "/etc/systemd/system/mesh-kernel-huge-pages.service",
      "mode": "0644",
      "content": "[Unit]\nDescription=Transparent huge pages as the mesh sets them on this machine\n# Module kernel, novox/hq issue 746. Starting this unit sets the mode from the setting\n# transparent-huge-pages; reloading it (the setting changed) sets the new mode; stopping it gives back the\n# mode the machine had before. It runs early in boot, before the services that fill the page cache.\nDefaultDependencies=no\nAfter=local-fs.target\nBefore=sysinit.target shutdown.target\nConflicts=shutdown.target\n\n[Service]\nType=oneshot\nRemainAfterExit=yes\nExecStart=${dir:state}/huge-pages apply\nExecReload=${dir:state}/huge-pages apply\nExecStop=${dir:state}/huge-pages restore\n# It writes one file under /sys and its own state, and asks the journal daemon to rotate over its socket.\nProtectSystem=strict\nReadWritePaths=${dir:state}\nProtectHome=yes\nPrivateNetwork=yes\nNoNewPrivileges=yes\n\n[Install]\nWantedBy=sysinit.target\n"
    },
    {
      "id": "apply",
      "type": "service",
      "unit": "mesh-kernel-huge-pages.service",
      "state": "running",
      "boot": "enabled",
      "restart-on": [
        "unit"
      ],
      "reload-on": [
        "huge-pages"
      ],
      "health": {
        "kind": "unit"
      }
    }
  ],
  "build": {
    "artifacts": [
      {
        "name": "tools",
        "kind": "bundle",
        "language": "go",
        "system": "arch",
        "from": "cmd/kernel-tools",
        "binary": "kernel-tools",
        "loads": [
          "kernel-tools"
        ]
      }
    ]
  }
}