[PATCH v2 6/7] docs: document shrinker debugfs

[Roman Gushchin <roman.gushchin@linux.dev>]

Add a document describing the shrinker debugfs interface.

Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
---
 Documentation/admin-guide/mm/index.rst        |  1 +
 .../admin-guide/mm/shrinker_debugfs.rst       | 90 +++++++++++++++++++
 2 files changed, 91 insertions(+)
 create mode 100644 Documentation/admin-guide/mm/shrinker_debugfs.rst

diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
index c21b5823f126..1bd11118dfb1 100644
--- a/Documentation/admin-guide/mm/index.rst
+++ b/Documentation/admin-guide/mm/index.rst
@@ -36,6 +36,7 @@ the Linux memory management.
    numa_memory_policy
    numaperf
    pagemap
+   shrinker_debugfs
    soft-dirty
    swap_numa
    transhuge
diff --git a/Documentation/admin-guide/mm/shrinker_debugfs.rst b/Documentation/admin-guide/mm/shrinker_debugfs.rst
new file mode 100644
index 000000000000..c2f3da534b70
--- /dev/null
+++ b/Documentation/admin-guide/mm/shrinker_debugfs.rst
@@ -0,0 +1,90 @@
+==========================
+Shrinker Debugfs Interface
+==========================
+
+Shrinker debugfs interface provides a visibility into the kernel memory
+shrinkers subsystem and allows to get statistics and interact with
+individual shrinkers.
+
+For each shrinker registered in the system a directory in <debugfs>/shrinker/
+is created. The directory is named like "kfree_rcu-0". Each name is composed
+from the shrinker's name and an unique id.
+
+Each shrinker directory contains "count" and "scan" files, which allow
+to trigger count_objects() and scan_objects() callbacks. For memcg-aware
+and numa-aware shrinkers count_memcg, scan_memcg, count_node, scan_node,
+count_memcg_node and scan_memcg_node are additionally provided. They allow
+to get per-memcg and/or per-node object count and shrink only a specific
+memcg/node.
+
+Usage examples:
+
+ 1. List registered shrinkers::
+      $ cd /sys/kernel/debug/shrinker/
+      $ ls
+      dqcache-16          sb-cgroup2-30    sb-hugetlbfs-33  sb-proc-41       sb-selinuxfs-22  sb-tmpfs-40    sb-zsmalloc-19
+      kfree_rcu-0         sb-configfs-23   sb-iomem-12      sb-proc-44       sb-sockfs-8      sb-tmpfs-42    shadow-18
+      sb-aio-20           sb-dax-11        sb-mqueue-21     sb-proc-45       sb-sysfs-26      sb-tmpfs-43    thp_deferred_split-10
+      sb-anon_inodefs-15  sb-debugfs-7     sb-nsfs-4        sb-proc-47       sb-tmpfs-1       sb-tmpfs-46    thp_zero-9
+      sb-bdev-3           sb-devpts-28     sb-pipefs-14     sb-pstore-31     sb-tmpfs-27      sb-tmpfs-49    xfs_buf-37
+      sb-bpf-32           sb-devtmpfs-5    sb-proc-25       sb-rootfs-2      sb-tmpfs-29      sb-tracefs-13  xfs_inodegc-38
+      sb-btrfs-24         sb-hugetlbfs-17  sb-proc-39       sb-securityfs-6  sb-tmpfs-35      sb-xfs-36      zspool-34
+
+ 2. Get information about a specific shrinker::
+      $ cd sb-btrfs-24/
+      $ ls
+      count  count_memcg  count_memcg_node  count_node  scan  scan_memcg  scan_memcg_node  scan_node
+
+ 3. Count objects on the system/root cgroup level::
+      $ cat count
+      212
+
+ 4. Count objects on the system/root cgroup level per numa node (on a 2-node machine)::
+      $ cat count_node
+      209 3
+
+ 5. Count objects for each memcg (output format: cgroup inode, count)::
+      $ cat count_memcg
+      1 212
+      20 96
+      53 817
+      2297 2
+      218 13
+      581 30
+      911 124
+      ...
+
+ 6. Same but with a per-node output::
+      $ cat count_memcg_node
+      1 209 3
+      20 96 0
+      53 810 7
+      2297 2 0
+      218 13 0
+      581 30 0
+      911 124 0
+      ...
+
+ 7. Scan system/root shrinker::
+      $ cat count
+      212
+      $ echo 100 > scan
+      $ cat scan
+      97
+      $ cat count
+      115
+
+ 8. Scan individual memcg::
+      $ echo "1868 500" > scan_memcg
+      $ cat scan_memcg
+      193
+
+ 9. Scan individual node::
+      $ echo "1 200" > scan_node
+      $ cat scan_node
+      2
+
+ 10. Scan individual memcg and node::
+     $ echo "1868 0 500" > scan_memcg_node
+     $ cat scan_memcg_node
+     435
-- 
2.35.1