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Date: Mon,  3 Jun 2024 19:05:49 -0700
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Subject: [PATCH v2 8/8] Docs/admin-guide/mm/workingset_report: document sysfs
 and memcg interfaces
From: Yuanchu Xie <yuanchu@google.com>
To: David Hildenbrand <david@redhat.com>, "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>, 
	Khalid Aziz <khalid.aziz@oracle.com>, Henry Huang <henry.hj@antgroup.com>, 
	Yu Zhao <yuzhao@google.com>, Dan Williams <dan.j.williams@intel.com>, 
	Gregory Price <gregory.price@memverge.com>, Huang Ying <ying.huang@intel.com>, 
	Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Kalesh Singh <kaleshsingh@google.com>, Wei Xu <weixugc@google.com>, 
	David Rientjes <rientjes@google.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, 
	Johannes Weiner <hannes@cmpxchg.org>, Michal Hocko <mhocko@kernel.org>, 
	Roman Gushchin <roman.gushchin@linux.dev>, Muchun Song <muchun.song@linux.dev>, 
	Shuah Khan <shuah@kernel.org>, Yosry Ahmed <yosryahmed@google.com>, 
	Matthew Wilcox <willy@infradead.org>, Sudarshan Rajagopalan <quic_sudaraja@quicinc.com>, 
	Kairui Song <kasong@tencent.com>, "Michael S. Tsirkin" <mst@redhat.com>, 
	Vasily Averin <vasily.averin@linux.dev>, Nhat Pham <nphamcs@gmail.com>, 
	Miaohe Lin <linmiaohe@huawei.com>, Qi Zheng <zhengqi.arch@bytedance.com>, 
	Abel Wu <wuyun.abel@bytedance.com>, "Vishal Moola (Oracle)" <vishal.moola@gmail.com>, 
	Kefeng Wang <wangkefeng.wang@huawei.com>, Yuanchu Xie <yuanchu@google.com>, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

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 1f883abf3f00..fba987de8997 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..f455ae93b30e
--- /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, i.e. 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_LRU_GEN_ENABLED=y``
+
+Sysfs interfaces
+================
+``/sys/devices/system/node/nodeX/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/page_age_interval`` 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
-- 
2.45.1.467.gbab1589fc0-goog