Docs/mm/damon/design: add sections for basic parts of DAMOS

DAMOS is an important part of DAMON, but the design doc is not covering
it.  Add sections for covering the basic part of DAMOS.

Link: https://lkml.kernel.org/r/20230525214314.5204-8-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>

Documentation/mm/damon/design.rst

@@ -202,3 +202,73 @@ monitoring operations to check dynamic changes including memory mapping changes
 and applies it to monitoring operations-related data structures such as the
 abstracted monitoring target memory area only for each of a user-specified time
 interval (``update interval``).
+
+
+Operation Schemes
+-----------------
+
+One common purpose of data access monitoring is access-aware system efficiency
+optimizations.  For example,
+
+    paging out memory regions that are not accessed for more than two minutes
+
+or
+
+    using THP for memory regions that are larger than 2 MiB and showing a high
+    access frequency for more than one minute.
+
+One straightforward approach for such schemes would be profile-guided
+optimizations.  That is, getting data access monitoring results of the
+workloads or the system using DAMON, finding memory regions of special
+characteristics by profiling the monitoring results, and making system
+operation changes for the regions.  The changes could be made by modifying or
+providing advice to the software (the application and/or the kernel), or
+reconfiguring the hardware.  Both offline and online approaches could be
+available.
+
+Among those, providing advice to the kernel at runtime would be flexible and
+effective, and therefore widely be used.   However, implementing such schemes
+could impose unnecessary redundancy and inefficiency.  The profiling could be
+redundant if the type of interest is common.  Exchanging the information
+including monitoring results and operation advice between kernel and user
+spaces could be inefficient.
+
+To allow users to reduce such redundancy and inefficiencies by offloading the
+works, DAMON provides a feature called Data Access Monitoring-based Operation
+Schemes (DAMOS).  It lets users specify their desired schemes at a high
+level.  For such specifications, DAMON starts monitoring, finds regions having
+the access pattern of interest, and applies the user-desired operation actions
+to the regions as soon as found.
+
+
+Operation Action
+~~~~~~~~~~~~~~~~
+
+The management action that the users desire to apply to the regions of their
+interest.  For example, paging out, prioritizing for next reclamation victim
+selection, advising ``khugepaged`` to collapse or split, or doing nothing but
+collecting statistics of the regions.
+
+The list of supported actions is defined in DAMOS, but the implementation of
+each action is in the DAMON operations set layer because the implementation
+normally depends on the monitoring target address space.  For example, the code
+for paging specific virtual address ranges out would be different from that for
+physical address ranges.  And the monitoring operations implementation sets are
+not mandated to support all actions of the list.  Hence, the availability of
+specific DAMOS action depends on what operations set is selected to be used
+together.
+
+Applying an action to a region is considered as changing the region's
+characteristics.  Hence, DAMOS resets the age of regions when an action is
+applied to those.
+
+
+Target Access Pattern
+~~~~~~~~~~~~~~~~~~~~~
+
+The access pattern of the schemes' interest.  The patterns are constructed with
+the properties that DAMON's monitoring results provide, specifically the size,
+the access frequency, and the age.  Users can describe their access pattern of
+interest by setting minimum and maximum values of the three properties.  If a
+region's three properties are in the ranges, DAMOS classifies it as one of the
+regions that the scheme is having an interest in.