[PATCH v4 7/9] Docs/admin-guide/mm/workingset_report: document sysfs and memcg interfaces

[Yuanchu Xie <yuanchu@google.com>]

Add workingset reporting documentation for better discoverability of
its sysfs and memcg interfaces. Also document the required kernel
config to enable workingset reporting.

Signed-off-by: Yuanchu Xie <yuanchu@google.com>
---
 Documentation/admin-guide/mm/index.rst        |   1 +
 .../admin-guide/mm/workingset_report.rst      | 105 ++++++++++++++++++
 2 files changed, 106 insertions(+)
 create mode 100644 Documentation/admin-guide/mm/workingset_report.rst

diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
index 8b35795b664b..61a2a347fc91 100644
--- a/Documentation/admin-guide/mm/index.rst
+++ b/Documentation/admin-guide/mm/index.rst
@@ -41,4 +41,5 @@ the Linux memory management.
    swap_numa
    transhuge
    userfaultfd
+   workingset_report
    zswap
diff --git a/Documentation/admin-guide/mm/workingset_report.rst b/Documentation/admin-guide/mm/workingset_report.rst
new file mode 100644
index 000000000000..0969513705c4
--- /dev/null
+++ b/Documentation/admin-guide/mm/workingset_report.rst
@@ -0,0 +1,105 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Workingset Report
+=================
+Workingset report provides a view of memory coldness in user-defined
+time intervals, e.g. X bytes are Y milliseconds cold. It breaks down
+the user pages in the system per-NUMA node, per-memcg, for both
+anonymous and file pages into histograms that look like:
+::
+
+    1000 anon=137368 file=24530
+    20000 anon=34342 file=0
+    30000 anon=353232 file=333608
+    40000 anon=407198 file=206052
+    9223372036854775807 anon=4925624 file=892892
+
+The workingset reports can be used to drive proactive reclaim, by
+identifying the number of cold bytes in a memcg, then writing to
+``memory.reclaim``.
+
+Quick start
+===========
+Build the kernel with the following configurations. The report relies
+on Multi-gen LRU for page coldness.
+
+* ``CONFIG_LRU_GEN=y``
+* ``CONFIG_LRU_GEN_ENABLED=y``
+* ``CONFIG_WORKINGSET_REPORT=y``
+
+Optionally, the aging kernel daemon can be enabled with the following
+configuration.
+* ``CONFIG_WORKINGSET_REPORT_AGING=y``
+
+Sysfs interfaces
+================
+``/sys/devices/system/node/nodeX/workingset_report/page_age`` provides
+a per-node page age histogram, showing an aggregate of the node's lruvecs.
+Reading this file causes a hierarchical aging of all lruvecs, scanning
+pages and creates a new Multi-gen LRU generation in each lruvec.
+For example:
+::
+
+    1000 anon=0 file=0
+    2000 anon=0 file=0
+    100000 anon=5533696 file=5566464
+    18446744073709551615 anon=0 file=0
+
+``/sys/devices/system/node/nodeX/workingset_report/page_age_intervals``
+is a comma-separated list of time in milliseconds that configures what
+the page age histogram uses for aggregation. For the above histogram,
+the intervals are::
+
+    1000,2000,100000
+
+``/sys/devices/system/node/nodeX/workingset_report/refresh_interval``
+defines the amount of time the report is valid for in milliseconds.
+When a report is still valid, reading the ``page_age`` file shows
+the existing valid report, instead of generating a new one.
+
+``/sys/devices/system/node/nodeX/workingset_report/report_threshold``
+specifies how often the userspace agent can be notified for node
+memory pressure, in milliseconds. When a node reaches its low
+watermarks and wakes up kswapd, programs waiting on ``page_age`` are
+woken up so they can read the histogram and make policy decisions.
+
+Memcg interface
+===============
+While ``page_age_interval`` is defined per-node in sysfs, ``page_age``,
+``refresh_interval`` and ``report_threshold`` are available per-memcg.
+
+``/sys/fs/cgroup/.../memory.workingset.page_age``
+The memcg equivalent of the sysfs workingset page age histogram
+breaks down the workingset of this memcg and its children into
+page age intervals. Each node is prefixed with a node header and
+a newline. Non-proactive direct reclaim on this memcg can also
+wake up userspace agents that are waiting on this file.
+E.g.
+::
+
+    N0
+    1000 anon=0 file=0
+    2000 anon=0 file=0
+    3000 anon=0 file=0
+    4000 anon=0 file=0
+    5000 anon=0 file=0
+    18446744073709551615 anon=0 file=0
+
+``/sys/fs/cgroup/.../memory.workingset.refresh_interval``
+The memcg equivalent of the sysfs refresh interval. A per-node
+number of how much time a page age histogram is valid for, in
+milliseconds.
+E.g.
+::
+
+    echo N0=2000 > memory.workingset.refresh_interval
+
+``/sys/fs/cgroup/.../memory.workingset.report_threshold``
+The memcg equivalent of the sysfs report threshold. A per-node
+number of how often userspace agent waiting on the page age
+histogram can be woken up, in milliseconds.
+E.g.
+::
+
+    echo N0=1000 > memory.workingset.report_threshold
-- 
2.47.0.338.g60cca15819-goog