[PATCH 0/9] mm/damon: support pin-point targets removal

[PATCH 0/9] mm/damon: support pin-point targets removal

[SeongJae Park]

DAMON maintains the targets in a list, and allows committing only an
entire list of targets having the 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 target removal is really needed in some use cases,
though.  Monitoring access patterns on virtual address spaces of
processes that spawned from the 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means the matching destination target is obsolete.  Then the parameters
commit logic removes the destination target from the existing targets
list.  For sysfs ABI, add a new file under the target directory, namely
'obsolete_target'.  It is connected with the 'obsolete' field of the
commit source targets, so internally using the new API.

Also add a selftest for the new feature.  The related helper scripts for
manipulating the sysfs interface and dumping in-kernel DAMON status are
also extended for this.  Note that the selftest part was initially
posted as an individual RFC series [1], but now merged into this one.

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.

Changes from RFC
(https://lore.kernel.org/20251016214736.84286-1-sj@kernel.org)
- Wordsmith commit messages
- Add Reviewed-by: tags from Bijan
- Add a kselftest for the functionality of the new feature
  (https://lore.kernel.org/20251018204448.8906-1-sj@kernel.org)

[1] https://github.com/damonitor/damo/issues/36

SeongJae Park (9):
  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
  selftests/damon/_damon_sysfs: support obsolete_target file
  drgn_dump_damon_status: dump damon_target->obsolete
  sysfs.py: extend assert_ctx_committed() for monitoring targets
  selftests/damon/sysfs: add obsolete_target test

 .../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 ++++++++++++++++++-
 tools/testing/selftests/damon/_damon_sysfs.py | 11 +++-
 .../selftests/damon/drgn_dump_damon_status.py |  1 +
 tools/testing/selftests/damon/sysfs.py        | 48 +++++++++++++++++
 8 files changed, 140 insertions(+), 7 deletions(-)


base-commit: a3e008fdd7964bc3e6d876491c202d476406ed59
-- 
2.47.3

[PATCH 4/9] 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>
Reviewed-by: Bijan Tabatabai <bijan311@gmail.com>
---
 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