Docs/mm/damon/design: add 'statistics' section

DAMOS stats are important feature for tuning of DAMOS-based access-aware
system operation, and efficient access pattern monitoring.  But not well
documented on the design document.  Add a section on the document.

Link: https://lkml.kernel.org/r/20250106193401.109161-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 38 insertions, 0 deletions

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index a577ae40e71c..d0f3bd92ed13 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -539,6 +539,44 @@ To know how user-space can set the watermarks via :ref:`DAMON sysfs interface
 <sysfs_interface>`, refer to :ref:`filters <sysfs_filters>` part of the
 documentation.
 
+Statistics
+~~~~~~~~~~
+
+The statistics of DAMOS behaviors that designed to help monitoring, tuning and
+debugging of DAMOS.
+
+DAMOS accounts below statistics for each scheme, from the beginning of the
+scheme's execution.
+
+- ``nr_tried``: Total number of regions that the scheme is tried to be applied.
+- ``sz_trtied``: Total size of regions that the scheme is tried to be applied.
+- ``nr_applied``: Total number of regions that the scheme is applied.
+- ``sz_applied``: Total size of regions that the scheme is applied.
+- ``qt_exceeds``: Total number of times the quota of the scheme has exceeded.
+
+"A scheme is tried to be applied to a region" means DAMOS core logic determined
+the region is eligible to apply the scheme's :ref:`action
+<damon_design_damos_action>`.  The :ref:`access pattern
+<damon_design_damos_access_pattern>`, :ref:`quotas
+<damon_design_damos_quotas>`, :ref:`watermarks
+<damon_design_damos_watermarks>`, and :ref:`filters
+<damon_design_damos_filters>` that handled on core logic could affect this.
+The core logic will only ask the underlying :ref:`operation set
+<damon_operations_set>` to do apply the action to the region, so whether the
+action is really applied or not is unclear.  That's why it is called "tried".
+
+"A scheme is applied to a region" means the :ref:`operation set
+<damon_operations_set>` has applied the action to at least a part of the
+region.  The :ref:`filters <damon_design_damos_filters>` that handled by the
+operation set, and the types of the :ref:`action <damon_design_damos_action>`
+and the pages of the region can affect this.  For example, if a filter is set
+to exclude anonymous pages and the region has only anonymous pages, or if the
+action is ``pageout`` while all pages of the region are unreclaimable, applying
+the action to the region will fail.
+
+To know how user-space can read the stats via :ref:`DAMON sysfs interface
+<sysfs_interface>`, refer to :ref:s`stats <sysfs_stats>` part of the
+documentation.
 
 Regions Walking
 ~~~~~~~~~~~~~~~