Docs/admin-guide/mm/damon/usage: document DAMON sysfs interface

This commit adds detailed usage of DAMON sysfs interface in the
admin-guide document for DAMON.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: SeongJae Park <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Xin Hao <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: sjp38

Documentation/admin-guide/mm/damon/usage.rst

@@ -4,7 +4,7 @@
 Detailed Usages
 ===============
 
-DAMON provides below three interfaces for different users.
+DAMON provides below interfaces for different users.
 
 - *DAMON user space tool.*
   `This <https://github.com/awslabs/damo>`_ is for privileged people such as
@@ -14,24 +14,362 @@ DAMON provides below three interfaces for different users.
   virtual and physical address spaces monitoring.  For more detail, please
   refer to its `usage document
   <https://github.com/awslabs/damo/blob/next/USAGE.md>`_.
-- *debugfs interface.*
-  :ref:`This <debugfs_interface>` is for privileged user space programmers who
+- *sysfs interface.*
+  :ref:`This <sysfs_interface>` is for privileged user space programmers who
   want more optimized use of DAMON.  Using this, users can use DAMON’s major
-  features by reading from and writing to special debugfs files.  Therefore,
-  you can write and use your personalized DAMON debugfs wrapper programs that
-  reads/writes the debugfs files instead of you.  The `DAMON user space tool
+  features by reading from and writing to special sysfs files.  Therefore,
+  you can write and use your personalized DAMON sysfs wrapper programs that
+  reads/writes the sysfs files instead of you.  The `DAMON user space tool
   <https://github.com/awslabs/damo>`_ is one example of such programs.  It
   supports both virtual and physical address spaces monitoring.  Note that this
   interface provides only simple :ref:`statistics <damos_stats>` for the
   monitoring results.  For detailed monitoring results, DAMON provides a
   :ref:`tracepoint <tracepoint>`.
+- *debugfs interface.*
+  :ref:`This <debugfs_interface>` is almost identical to :ref:`sysfs interface
+  <sysfs_interface>`.  This will be removed after next LTS kernel is released,
+  so users should move to the :ref:`sysfs interface <sysfs_interface>`.
 - *Kernel Space Programming Interface.*
   :doc:`This </vm/damon/api>` is for kernel space programmers.  Using this,
   users can utilize every feature of DAMON most flexibly and efficiently by
   writing kernel space DAMON application programs for you.  You can even extend
   DAMON for various address spaces.  For detail, please refer to the interface
   :doc:`document </vm/damon/api>`.
 
+.. _sysfs_interface:
+
+sysfs Interface
+===============
+
+DAMON sysfs interface is built when ``CONFIG_DAMON_SYSFS`` is defined.  It
+creates multiple directories and files under its sysfs directory,
+``<sysfs>/kernel/mm/damon/``.  You can control DAMON by writing to and reading
+from the files under the directory.
+
+For a short example, users can monitor the virtual address space of a given
+workload as below. ::
+
+    # cd /sys/kernel/mm/damon/admin/
+    # echo 1 > kdamonds/nr && echo 1 > kdamonds/0/contexts/nr
+    # echo vaddr > kdamonds/0/contexts/0/operations
+    # echo 1 > kdamonds/0/contexts/0/targets/nr
+    # echo $(pidof <workload>) > kdamonds/0/contexts/0/targets/0/pid
+    # echo on > kdamonds/0/state
+
+Files Hierarchy
+---------------
+
+The files hierarchy of DAMON sysfs interface is shown below.  In the below
+figure, parents-children relations are represented with indentations, each
+directory is having ``/`` suffix, and files in each directory are separated by
+comma (","). ::
+
+    /sys/kernel/mm/damon/admin
+    │ kdamonds/nr_kdamonds
+    │ │ 0/state,pid
+    │ │ │ contexts/nr_contexts
+    │ │ │ │ 0/operations
+    │ │ │ │ │ monitoring_attrs/
+    │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
+    │ │ │ │ │ │ nr_regions/min,max
+    │ │ │ │ │ targets/nr_targets
+    │ │ │ │ │ │ 0/pid_target
+    │ │ │ │ │ │ │ regions/nr_regions
+    │ │ │ │ │ │ │ │ 0/start,end
+    │ │ │ │ │ │ │ │ ...
+    │ │ │ │ │ │ ...
+    │ │ │ │ │ schemes/nr_schemes
+    │ │ │ │ │ │ 0/action
+    │ │ │ │ │ │ │ access_pattern/
+    │ │ │ │ │ │ │ │ sz/min,max
+    │ │ │ │ │ │ │ │ nr_accesses/min,max
+    │ │ │ │ │ │ │ │ age/min,max
+    │ │ │ │ │ │ │ quotas/ms,bytes,reset_interval_ms
+    │ │ │ │ │ │ │ │ weights/sz_permil,nr_accesses_permil,age_permil
+    │ │ │ │ │ │ │ watermarks/metric,interval_us,high,mid,low
+    │ │ │ │ │ │ │ stats/nr_tried,sz_tried,nr_applied,sz_applied,qt_exceeds
+    │ │ │ │ │ │ ...
+    │ │ │ │ ...
+    │ │ ...
+
+Root
+----
+
+The root of the DAMON sysfs interface is ``<sysfs>/kernel/mm/damon/``, and it
+has one directory named ``admin``.  The directory contains the files for
+privileged user space programs' control of DAMON.  User space tools or deamons
+having the root permission could use this directory.
+
+kdamonds/
+---------
+
+The monitoring-related information including request specifications and results
+are called DAMON context.  DAMON executes each context with a kernel thread
+called kdamond, and multiple kdamonds could run in parallel.
+
+Under the ``admin`` directory, one directory, ``kdamonds``, which has files for
+controlling the kdamonds exist.  In the beginning, this directory has only one
+file, ``nr_kdamonds``.  Writing a number (``N``) to the file creates the number
+of child directories named ``0`` to ``N-1``.  Each directory represents each
+kdamond.
+
+kdamonds/<N>/
+-------------
+
+In each kdamond directory, two files (``state`` and ``pid``) and one directory
+(``contexts``) exist.
+
+Reading ``state`` returns ``on`` if the kdamond is currently running, or
+``off`` if it is not running.  Writing ``on`` or ``off`` makes the kdamond be
+in the state.  Writing ``update_schemes_stats`` to ``state`` file updates the
+contents of stats files for each DAMON-based operation scheme of the kdamond.
+For details of the stats, please refer to :ref:`stats section
+<sysfs_schemes_stats>`.
+
+If the state is ``on``, reading ``pid`` shows the pid of the kdamond thread.
+
+``contexts`` directory contains files for controlling the monitoring contexts
+that this kdamond will execute.
+
+kdamonds/<N>/contexts/
+----------------------
+
+In the beginning, this directory has only one file, ``nr_contexts``.  Writing a
+number (``N``) to the file creates the number of child directories named as
+``0`` to ``N-1``.  Each directory represents each monitoring context.  At the
+moment, only one context per kdamond is supported, so only ``0`` or ``1`` can
+be written to the file.
+
+contexts/<N>/
+-------------
+
+In each context directory, one file (``operations``) and three directories
+(``monitoring_attrs``, ``targets``, and ``schemes``) exist.
+
+DAMON supports multiple types of monitoring operations, including those for
+virtual address space and the physical address space.  You can set and get what
+type of monitoring operations DAMON will use for the context by writing one of
+below keywords to, and reading from the file.
+
+ - vaddr: Monitor virtual address spaces of specific processes
+ - paddr: Monitor the physical address space of the system
+
+contexts/<N>/monitoring_attrs/
+------------------------------
+
+Files for specifying attributes of the monitoring including required quality
+and efficiency of the monitoring are in ``monitoring_attrs`` directory.
+Specifically, two directories, ``intervals`` and ``nr_regions`` exist in this
+directory.
+
+Under ``intervals`` directory, three files for DAMON's sampling interval
+(``sample_us``), aggregation interval (``aggr_us``), and update interval
+(``update_us``) exist.  You can set and get the values in micro-seconds by
+writing to and reading from the files.
+
+Under ``nr_regions`` directory, two files for the lower-bound and upper-bound
+of DAMON's monitoring regions (``min`` and ``max``, respectively), which
+controls the monitoring overhead, exist.  You can set and get the values by
+writing to and rading from the files.
+
+For more details about the intervals and monitoring regions range, please refer
+to the Design document (:doc:`/vm/damon/design`).
+
+contexts/<N>/targets/
+---------------------
+
+In the beginning, this directory has only one file, ``nr_targets``.  Writing a
+number (``N``) to the file creates the number of child directories named ``0``
+to ``N-1``.  Each directory represents each monitoring target.
+
+targets/<N>/
+------------
+
+In each target directory, one file (``pid_target``) and one directory
+(``regions``) exist.
+
+If you wrote ``vaddr`` to the ``contexts/<N>/operations``, each target should
+be a process.  You can specify the process to DAMON by writing the pid of the
+process to the ``pid_target`` file.
+
+targets/<N>/regions
+-------------------
+
+When ``vaddr`` monitoring operations set is being used (``vaddr`` is written to
+the ``contexts/<N>/operations`` file), DAMON automatically sets and updates the
+monitoring target regions so that entire memory mappings of target processes
+can be covered.  However, users could want to set the initial monitoring region
+to specific address ranges.
+
+In contrast, DAMON do not automatically sets and updates the monitoring target
+regions when ``paddr`` monitoring operations set is being used (``paddr`` is
+written to the ``contexts/<N>/operations``).  Therefore, users should set the
+monitoring target regions by themselves in the case.
+
+For such cases, users can explicitly set the initial monitoring target regions
+as they want, by writing proper values to the files under this directory.
+
+In the beginning, this directory has only one file, ``nr_regions``.  Writing a
+number (``N``) to the file creates the number of child directories named ``0``
+to ``N-1``.  Each directory represents each initial monitoring target region.
+
+regions/<N>/
+------------
+
+In each region directory, you will find two files (``start`` and ``end``).  You
+can set and get the start and end addresses of the initial monitoring target
+region by writing to and reading from the files, respectively.
+
+contexts/<N>/schemes/
+---------------------
+
+For usual DAMON-based data access aware memory management optimizations, users
+would normally want the system to apply a memory management action to a memory
+region of a specific access pattern.  DAMON receives such formalized operation
+schemes from the user and applies those to the target memory regions.  Users
+can get and set the schemes by reading from and writing to files under this
+directory.
+
+In the beginning, this directory has only one file, ``nr_schemes``.  Writing a
+number (``N``) to the file creates the number of child directories named ``0``
+to ``N-1``.  Each directory represents each DAMON-based operation scheme.
+
+schemes/<N>/
+------------
+
+In each scheme directory, four directories (``access_pattern``, ``quotas``,
+``watermarks``, and ``stats``) and one file (``action``) exist.
+
+The ``action`` file is for setting and getting what action you want to apply to
+memory regions having specific access pattern of the interest.  The keywords
+that can be written to and read from the file and their meaning are as below.
+
+ - ``willneed``: Call ``madvise()`` for the region with ``MADV_WILLNEED``
+ - ``cold``: Call ``madvise()`` for the region with ``MADV_COLD``
+ - ``pageout``: Call ``madvise()`` for the region with ``MADV_PAGEOUT``
+ - ``hugepage``: Call ``madvise()`` for the region with ``MADV_HUGEPAGE``
+ - ``nohugepage``: Call ``madvise()`` for the region with ``MADV_NOHUGEPAGE``
+ - ``stat``: Do nothing but count the statistics
+
+schemes/<N>/access_pattern/
+---------------------------
+
+The target access pattern of each DAMON-based operation scheme is constructed
+with three ranges including the size of the region in bytes, number of
+monitored accesses per aggregate interval, and number of aggregated intervals
+for the age of the region.
+
+Under the ``access_pattern`` directory, three directories (``sz``,
+``nr_accesses``, and ``age``) each having two files (``min`` and ``max``)
+exist.  You can set and get the access pattern for the given scheme by writing
+to and reading from the ``min`` and ``max`` files under ``sz``,
+``nr_accesses``, and ``age`` directories, respectively.
+
+schemes/<N>/quotas/
+-------------------
+
+Optimal ``target access pattern`` for each ``action`` is workload dependent, so
+not easy to find.  Worse yet, setting a scheme of some action too aggressive
+can cause severe overhead.  To avoid such overhead, users can limit time and
+size quota for each scheme.  In detail, users can ask DAMON to try to use only
+up to specific time (``time quota``) for applying the action, and to apply the
+action to only up to specific amount (``size quota``) of memory regions having
+the target access pattern within a given time interval (``reset interval``).
+
+When the quota limit is expected to be exceeded, DAMON prioritizes found memory
+regions of the ``target access pattern`` based on their size, access frequency,
+and age.  For personalized prioritization, users can set the weights for the
+three properties.
+
+Under ``quotas`` directory, three files (``ms``, ``bytes``,
+``reset_interval_ms``) and one directory (``weights``) having three files
+(``sz_permil``, ``nr_accesses_permil``, and ``age_permil``) in it exist.
+
+You can set the ``time quota`` in milliseconds, ``size quota`` in bytes, and
+``reset interval`` in milliseconds by writing the values to the three files,
+respectively.  You can also set the prioritization weights for size, access
+frequency, and age in per-thousand unit by writing the values to the three
+files under the ``weights`` directory.
+
+schemes/<N>/watermarks/
+-----------------------
+
+To allow easy activation and deactivation of each scheme based on system
+status, DAMON provides a feature called watermarks.  The feature receives five
+values called ``metric``, ``interval``, ``high``, ``mid``, and ``low``.  The
+``metric`` is the system metric such as free memory ratio that can be measured.
+If the metric value of the system is higher than the value in ``high`` or lower
+than ``low`` at the memoent, the scheme is deactivated.  If the value is lower
+than ``mid``, the scheme is activated.
+
+Under the watermarks directory, five files (``metric``, ``interval_us``,
+``high``, ``mid``, and ``low``) for setting each value exist.  You can set and
+get the five values by writing to the files, respectively.
+
+Keywords and meanings of those that can be written to the ``metric`` file are
+as below.
+
+ - none: Ignore the watermarks
+ - free_mem_rate: System's free memory rate (per thousand)
+
+The ``interval`` should written in microseconds unit.
+
+.. _sysfs_schemes_stats:
+
+schemes/<N>/stats/
+------------------
+
+DAMON counts the total number and bytes of regions that each scheme is tried to
+be applied, the two numbers for the regions that each scheme is successfully
+applied, and the total number of the quota limit exceeds.  This statistics can
+be used for online analysis or tuning of the schemes.
+
+The statistics can be retrieved by reading the files under ``stats`` directory
+(``nr_tried``, ``sz_tried``, ``nr_applied``, ``sz_applied``, and
+``qt_exceeds``), respectively.  The files are not updated in real time, so you
+should ask DAMON sysfs interface to updte the content of the files for the
+stats by writing a special keyword, ``update_schemes_stats`` to the relevant
+``kdamonds/<N>/state`` file.
+
+Example
+~~~~~~~
+
+Below commands applies a scheme saying "If a memory region of size in [4KiB,
+8KiB] is showing accesses per aggregate interval in [0, 5] for aggregate
+interval in [10, 20], page out the region.  For the paging out, use only up to
+10ms per second, and also don't page out more than 1GiB per second.  Under the
+limitation, page out memory regions having longer age first.  Also, check the
+free memory rate of the system every 5 seconds, start the monitoring and paging
+out when the free memory rate becomes lower than 50%, but stop it if the free
+memory rate becomes larger than 60%, or lower than 30%". ::
+
+    # cd <sysfs>/kernel/mm/damon/admin
+    # # populate directories
+    # echo 1 > kdamonds/nr_kdamonds; echo 1 > kdamonds/0/contexts/nr_contexts;
+    # echo 1 > kdamonds/0/contexts/0/schemes/nr_schemes
+    # cd kdamonds/0/contexts/0/schemes/0
+    # # set the basic access pattern and the action
+    # echo 4096 > access_patterns/sz/min
+    # echo 8192 > access_patterns/sz/max
+    # echo 0 > access_patterns/nr_accesses/min
+    # echo 5 > access_patterns/nr_accesses/max
+    # echo 10 > access_patterns/age/min
+    # echo 20 > access_patterns/age/max
+    # echo pageout > action
+    # # set quotas
+    # echo 10 > quotas/ms
+    # echo $((1024*1024*1024)) > quotas/bytes
+    # echo 1000 > quotas/reset_interval_ms
+    # # set watermark
+    # echo free_mem_rate > watermarks/metric
+    # echo 5000000 > watermarks/interval_us
+    # echo 600 > watermarks/high
+    # echo 500 > watermarks/mid
+    # echo 300 > watermarks/low
+
+Please note that it's highly recommended to use user space tools like `damo
+<https://github.com/awslabs/damo>`_ rather than manually reading and writing
+the files as above.  Above is only for an example.
 
 .. _debugfs_interface: