[RFC PATCH 0/5] mm/damon: support pin-point targets removal

[RFC PATCH 0/5] mm/damon: support pin-point targets removal

[SeongJae Park]

DAMON maintains the targets in a list, and allows committing only and
entire list of targets having new parameters.  Targets having same index
on the lists are treated as matching source and destination targets.  If
an existing target cannot find a matching one in the sources list, the
target is removed.  This means that there is no way to remove only a
specific monitoring target in the middle of the current targets list.

Such pin-point targets removal is really needed in some use cases,
though.  Monitoring accesses to virtual address spaces of processes that
spawned from a same ancestor is one example.  If a process of the group
is terminated, the user may want to remove the matching DAMON target as
soon as possible, to save in-kernel memory usage for the unnecessary
target data.  The user may also want to do that without turning DAMON
off or removing unnecessary targets, to keep the current monitoring
results for other active processes.

Extend DAMON kernel API and sysfs ABI to support the pin-point removal
in the following way.  For API, add a new damon_target field, namely
'obsolete'.  If the field on parameters commit source target is set, it
means the matching destination target is obsolete.  Than the parameters
commit logic removes the destination target from the existing targets
list.  For sysfs ABI, add a new file under target directory, namely
'obsolete_target'.  It is connected with the 'obsolete' field of the
commit source targets, so internally using the new API.

Bijan Tabatabai (bijan311@gmail.com) has originally reported this issue,
and participated in this solution design on a GitHub issue [1] for DAMON
user-space tool.

SeongJae Park (5):
  mm/damon/core: add damon_target->obsolete for pin-point removal
  mm/damon/sysfs: test commit input against realistic destination
  mm/damon/sysfs: implement obsolete_target file
  Docs/admin-guide/mm/damon/usage: document obsolete_target file
  Docs/ABI/damon: document obsolete_target sysfs file

 .../ABI/testing/sysfs-kernel-mm-damon         |  7 +++
 Documentation/admin-guide/mm/damon/usage.rst  | 13 +++--
 include/linux/damon.h                         |  6 +++
 mm/damon/core.c                               | 10 +++-
 mm/damon/sysfs.c                              | 51 ++++++++++++++++++-
 5 files changed, 82 insertions(+), 5 deletions(-)


base-commit: f82c3c7358c244b2ba14dadcae822c8d36cf2163
-- 
2.47.3

[RFC PATCH 4/5] Docs/admin-guide/mm/damon/usage: document obsolete_target file

[SeongJae Park]

Document the newly added obsolete_target DAMON sysfs file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/usage.rst | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index 98958975604d..d8a3d6e740b3 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -67,7 +67,7 @@ comma (",").
     │ │ │ │ │ │ │ intervals_goal/access_bp,aggrs,min_sample_us,max_sample_us
     │ │ │ │ │ │ nr_regions/min,max
     │ │ │ │ │ :ref:`targets <sysfs_targets>`/nr_targets
-    │ │ │ │ │ │ :ref:`0 <sysfs_target>`/pid_target
+    │ │ │ │ │ │ :ref:`0 <sysfs_target>`/pid_target,obsolete_target
     │ │ │ │ │ │ │ :ref:`regions <sysfs_regions>`/nr_regions
     │ │ │ │ │ │ │ │ :ref:`0 <sysfs_region>`/start,end
     │ │ │ │ │ │ │ │ ...
@@ -264,13 +264,20 @@ to ``N-1``.  Each directory represents each monitoring target.
 targets/<N>/
 ------------
 
-In each target directory, one file (``pid_target``) and one directory
-(``regions``) exist.
+In each target directory, two files (``pid_target`` and ``obsolete_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.
 
+Users can selectively remove targets in the middle of the targets array by
+writing non-zero value to ``obsolete_target`` file and committing it (writing
+``commit`` to ``state`` file).  DAMON will remove the matching targets from its
+internal targets array.  Users are responsible to construct target directories
+again, so that those correctly represent the changed internal targets array.
+
+
 .. _sysfs_regions:
 
 targets/<N>/regions
-- 
2.47.3