man: document systemd-oomd and related items

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+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1+ -->
+
+<refentry id="systemd-oomd.service" conditional='ENABLE_OOMD'>
+
+  <refentryinfo>
+    <title>systemd-oomd.service</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-oomd.service</refentrytitle>
+    <manvolnum>8</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-oomd.service</refname>
+    <refname>systemd-oomd</refname>
+    <refpurpose>A userspace out-of-memory (OOM) killer</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename>systemd-oomd.service</filename></para>
+    <para><filename>/usr/lib/systemd/systemd-oomd</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><command>systemd-oomd</command> is a system service that uses cgroups-v2 and pressure stall information (PSI)
+    to monitor and take action on processes before an OOM occurs in kernel space.</para>
+
+    <para>You can enable monitoring and actions on units by setting <varname>ManagedOOMSwap=</varname> and/or
+    <varname>ManagedOOMMemoryPressure=</varname> to the appropriate value. <command>systemd-oomd</command> will
+    periodically poll enabled units' cgroup data to detect when corrective action needs to occur. When an action needs
+    to happen, it will only be performed on the descendant cgroups of the enabled units. More precisely, only cgroups with
+    <filename>memory.oom.group</filename> set to <constant>1</constant> and leaf cgroup nodes are eligible candidates.
+    Action will be taken recursively on all of the processes under the chosen candidate.</para>
+
+    <para>See
+    <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for more information about the configuration of this service.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Setup Information</title>
+
+    <para>The system must be running systemd with a full unified cgroup hierarchy for the expected cgroups-v2 features.
+    Furthermore, resource accounting must be turned on for all units monitored by <command>systemd-oomd</command>.
+    The easiest way to turn on resource accounting is by ensuring the values for <varname>DefaultCPUAccounting</varname>,
+    <varname>DefaultIOAccounting</varname>, <varname>DefaultMemoryAccounting</varname>, and
+    <varname>DefaultTasksAccounting</varname> are set to <constant>true</constant> in
+    <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
+
+    <para>You will need a kernel compiled with PSI support. This is available in Linux 4.20 and above.</para>
+
+    <para>The system must also have swap enabled for <command>systemd-oomd</command> to function correctly. With swap
+    enabled, the system spends enough time swapping pages to let <command>systemd-oomd</command> react.
+    Without swap, the system enters a livelocked state much more quickly and may prevent <command>systemd-oomd</command>
+    from responding in a reasonable amount of time. See
+    <ulink url="https://chrisdown.name/2018/01/02/in-defence-of-swap.html">"In defence of swap: common misconceptions"</ulink>
+    for more details on swap.</para>
+
+    <para>Be aware that if you intend to enable monitoring and actions on <filename>user.slice</filename>,
+    <filename>user-$UID.slice</filename>, or their ancestor cgroups, it is highly recommended that your programs be
+    managed by the systemd user manager to prevent running too many processes under the same session scope (and thus
+    avoid a situation where memory intensive tasks trigger <command>systemd-oomd</command> to kill everything under the
+    cgroup). If you're using a desktop environment like GNOME, it already spawns many session components with the
+    systemd user manager.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Usage Recommendations</title>
+
+    <para><varname>ManagedOOMSwap=</varname> works with the system-wide swap values, so setting it on the root slice
+    <filename>-.slice</filename>, and allowing all descendant cgroups to be eligible candidates may make the most
+    sense.</para>
+
+    <para><varname>ManagedOOMMemoryPressure=</varname> tends to work better on the cgroups below the root slice
+    <filename>-.slice</filename>. For units which tend to have processes that are less latency sensitive (e.g.
+    <filename>system.slice</filename>), a higher limit like the default of 60% may be acceptable, as those processes
+    can usually ride out slowdowns caused by lack of memory without serious consequences. However, something like
+    <filename>user@$UID.service</filename> may prefer a much lower value like 40%.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>oomd.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>oomctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>