[RFC PATCH 00/12] mm/damos/stat: introduce nr_snapshots, max_nr_snapshots and tracepoint

[RFC PATCH 00/12] mm/damos/stat: introduce nr_snapshots, max_nr_snapshots and tracepoint

[SeongJae Park]

Introduce three changes for improving DAMOS stat's provided information,
deterministic control, and reading usability.

DAMOS provides stats that are important for understanding its behavior.
It lacks information about how many DAMON-generated monitoring output
snapshots it has worked on.  Add a new stat, nr_snapshots, to show the
information.

Users can control DAMOS schemes in multiple ways.  Using the online
parameters commit feature, they can install and uninstall DAMOS schemes
whenever they want while keeping DAMON runs.  DAMOS quotas and
watermarks can be used for manually or automatically turning on/off or
adjusting the aggressiveness of the scheme.  DAMOS filters can be used
for applying the scheme to specific memory entities based on their types
and locations.  Some users want their DAMOS scheme to be applied to only
specific number of DAMON snapshots, for more deterministic control.  One
example use case is tracepoint based snapshot reading.  Add a new knob,
max_nr_snapshots, to support this.  If the nr_snapshots parameter
becomes same to or greater than the value of this parameter, the scheme
is deactivated.

Users can read DAMOS stats via DAMON's sysfs interface.  For deep level
investigations on environments having advanced tools like perf and
bpftrace, exposing the stats via a tracepoint can be useful.  Implement
a new tracepoint, namely damon:damos_stat_after_apply_interval.

First five patches (patches 1-5) of this series implement the new stat,
nr_snapshots, on the core layer (patch 1), expose on DAMON sysfs user
interface (patch 2), and update documents (patches 3-5).

Following six patches (patches 6-11) are for the new stat based DAMOS
deactivation (max_nr_snapshots).  The first one (patch 6) of this group
updates a kernel-doc comment before making further changes.  Then an
implementation of it on the core layer (patch 7), an introduction of a
new DAMON sysfs interface file for users of the feature (patch 8), and
three updates of the documents (patches 9-11) follow.

The final one (patch 12) introduces the new tracepoint that exposes the
DAMOS stat values for each scheme apply interval.

SeongJae Park (12):
  mm/damon/core: introduce nr_snapshots damos stat
  mm/damon/sysfs-schemes: introduce nr_snapshots damos stat file
  Docs/mm/damon/design: update for nr_snapshots damos stat
  Docs/admin-guide/mm/damon/usage: update for nr_snapshots damos stat
  Docs/ABI/damon: update for nr_snapshots damos stat
  mm/damon: update damos kerneldoc for stat field
  mm/damon/core: implement max_nr_snapshots
  mm/damon/sysfs-schemes: implement max_nr_snapshots file
  Docs/mm/damon/design: update for max_nr_snapshots
  Docs/admin-guide/mm/damon/usage: update for max_nr_snapshots
  Docs/ABI/damon: update for max_nr_snapshots
  mm/damon/core: add trace point for damos stat per apply interval

 .../ABI/testing/sysfs-kernel-mm-damon         | 13 ++++++
 Documentation/admin-guide/mm/damon/usage.rst  | 11 ++---
 Documentation/mm/damon/design.rst             |  7 +++
 include/linux/damon.h                         | 12 ++++--
 include/trace/events/damon.h                  | 41 ++++++++++++++++++
 mm/damon/core.c                               | 37 ++++++++++++++--
 mm/damon/sysfs-schemes.c                      | 43 +++++++++++++++++++
 7 files changed, 153 insertions(+), 11 deletions(-)


base-commit: 522f1a9fc5f1f0a11af39ba5bedae446322ca0a2
-- 
2.47.3

[RFC PATCH 01/12] mm/damon/core: introduce nr_snapshots damos stat

[SeongJae Park]

DAMON generates monitoring results snapshots for every sampling
interval.  DAMOS applies given schemes on the regions of the snapshots,
for every apply interval of the scheme.

DAMOS stat informs a given scheme has tried to how many memory entities
and applied, in the region and byte level.  In some use cases including
user-space oriented tuning and investigations, it is useful to know that
in the DAMON-snapshot level.  Introduce a new stat, namely nr_snapshots
for DAMON core API callers.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  3 +++
 mm/damon/core.c       | 13 ++++++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3813373a9200..1d8a1515e75a 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -330,6 +330,8 @@ struct damos_watermarks {
  * @sz_ops_filter_passed:
  *		Total bytes that passed ops layer-handled DAMOS filters.
  * @qt_exceeds: Total number of times the quota of the scheme has exceeded.
+ * @nr_snapshots:
+ *		Total number of DAMON snapshots that the scheme has tried.
  *
  * "Tried an action to a region" in this context means the DAMOS core logic
  * determined the region as eligible to apply the action.  The access pattern
@@ -355,6 +357,7 @@ struct damos_stat {
 	unsigned long sz_applied;
 	unsigned long sz_ops_filter_passed;
 	unsigned long qt_exceeds;
+	unsigned long nr_snapshots;
 };
 
 /**
diff --git a/mm/damon/core.c b/mm/damon/core.c
index f9fc0375890a..af922d5e500c 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -157,6 +157,12 @@ void damon_destroy_region(struct damon_region *r, struct damon_target *t)
 	damon_free_region(r);
 }
 
+static bool damon_is_last_region(struct damon_region *r,
+		struct damon_target *t)
+{
+	return list_is_last(&t->regions_list, &r->list);
+}
+
 /*
  * Check whether a region is intersecting an address range
  *
@@ -1949,10 +1955,11 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
 			continue;
 
-		if (!damos_valid_target(c, t, r, s))
-			continue;
+		if (damos_valid_target(c, t, r, s))
+			damos_apply_scheme(c, t, r, s);
 
-		damos_apply_scheme(c, t, r, s);
+		if (damon_is_last_region(r, t))
+			s->stat.nr_snapshots++;
 	}
 }
 
-- 
2.47.3

[RFC PATCH 02/12] mm/damon/sysfs-schemes: introduce nr_snapshots damos stat file

[SeongJae Park]

Introduce a new DAMON sysfs interface file for exposing the newly added
DAMOS stat, nr_snapshots.  The file has the name same to the stat name
(nr_snapshots) and placed under the damos stat sysfs directory.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs-schemes.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 30d20f5b3192..4d25469f957f 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -204,6 +204,7 @@ struct damon_sysfs_stats {
 	unsigned long sz_applied;
 	unsigned long sz_ops_filter_passed;
 	unsigned long qt_exceeds;
+	unsigned long nr_snapshots;
 };
 
 static struct damon_sysfs_stats *damon_sysfs_stats_alloc(void)
@@ -265,6 +266,15 @@ static ssize_t qt_exceeds_show(struct kobject *kobj,
 	return sysfs_emit(buf, "%lu\n", stats->qt_exceeds);
 }
 
+static ssize_t nr_snapshots_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_stats *stats = container_of(kobj,
+			struct damon_sysfs_stats, kobj);
+
+	return sysfs_emit(buf, "%lu\n", stats->nr_snapshots);
+}
+
 static void damon_sysfs_stats_release(struct kobject *kobj)
 {
 	kfree(container_of(kobj, struct damon_sysfs_stats, kobj));
@@ -288,6 +298,9 @@ static struct kobj_attribute damon_sysfs_stats_sz_ops_filter_passed_attr =
 static struct kobj_attribute damon_sysfs_stats_qt_exceeds_attr =
 		__ATTR_RO_MODE(qt_exceeds, 0400);
 
+static struct kobj_attribute damon_sysfs_stats_nr_snapshots_attr =
+		__ATTR_RO_MODE(nr_snapshots, 0400);
+
 static struct attribute *damon_sysfs_stats_attrs[] = {
 	&damon_sysfs_stats_nr_tried_attr.attr,
 	&damon_sysfs_stats_sz_tried_attr.attr,
@@ -295,6 +308,7 @@ static struct attribute *damon_sysfs_stats_attrs[] = {
 	&damon_sysfs_stats_sz_applied_attr.attr,
 	&damon_sysfs_stats_sz_ops_filter_passed_attr.attr,
 	&damon_sysfs_stats_qt_exceeds_attr.attr,
+	&damon_sysfs_stats_nr_snapshots_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_stats);
@@ -2761,6 +2775,7 @@ void damon_sysfs_schemes_update_stats(
 		sysfs_stats->sz_ops_filter_passed =
 			scheme->stat.sz_ops_filter_passed;
 		sysfs_stats->qt_exceeds = scheme->stat.qt_exceeds;
+		sysfs_stats->nr_snapshots = scheme->stat.nr_snapshots;
 	}
 }
 
-- 
2.47.3

[RFC PATCH 03/12] Docs/mm/damon/design: update for nr_snapshots damos stat

[SeongJae Park]

Update DAMON design document for the newly added damos stat,
nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/damon/design.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 2d8d8ca1e0a3..5cc7b7d662be 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -718,6 +718,8 @@ scheme's execution.
 - ``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.
+- ``nr_snapshots``: Total number of DAMON snapshots that the scheme is tried to
+  be applied.
 
 "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
-- 
2.47.3

[RFC PATCH 04/12] Docs/admin-guide/mm/damon/usage: update for nr_snapshots damos stat

[SeongJae Park]

Update DAMON usage document for the newly added damos stat,
nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/usage.rst | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index 9991dad60fcf..d0944bd78964 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -87,7 +87,7 @@ comma (",").
     │ │ │ │ │ │ │ │ 0/type,matching,allow,memcg_path,addr_start,addr_end,target_idx,min,max
     │ │ │ │ │ │ │ :ref:`dests <damon_sysfs_dests>`/nr_dests
     │ │ │ │ │ │ │ │ 0/id,weight
-    │ │ │ │ │ │ │ :ref:`stats <sysfs_schemes_stats>`/nr_tried,sz_tried,nr_applied,sz_applied,sz_ops_filter_passed,qt_exceeds
+    │ │ │ │ │ │ │ :ref:`stats <sysfs_schemes_stats>`/nr_tried,sz_tried,nr_applied,sz_applied,sz_ops_filter_passed,qt_exceeds,nr_snapshots
     │ │ │ │ │ │ │ :ref:`tried_regions <sysfs_schemes_tried_regions>`/total_bytes
     │ │ │ │ │ │ │ │ 0/start,end,nr_accesses,age,sz_filter_passed
     │ │ │ │ │ │ │ │ ...
@@ -543,9 +543,9 @@ online analysis or tuning of the schemes.  Refer to :ref:`design doc
 
 The statistics can be retrieved by reading the files under ``stats`` directory
 (``nr_tried``, ``sz_tried``, ``nr_applied``, ``sz_applied``,
-``sz_ops_filter_passed``, and ``qt_exceeds``), respectively.  The files are not
-updated in real time, so you should ask DAMON sysfs interface to update the
-content of the files for the stats by writing a special keyword,
+``sz_ops_filter_passed``, ``qt_exceeds`` and ``nr_snapshots``), respectively.
+The files are not updated in real time, so you should ask DAMON sysfs interface
+to update the content of the files for the stats by writing a special keyword,
 ``update_schemes_stats`` to the relevant ``kdamonds/<N>/state`` file.
 
 .. _sysfs_schemes_tried_regions:
-- 
2.47.3

[RFC PATCH 05/12] Docs/ABI/damon: update for nr_snapshots damos stat

[SeongJae Park]

Update DAMON ABI document for the newly added damos stat, nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/ABI/testing/sysfs-kernel-mm-damon | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-damon b/Documentation/ABI/testing/sysfs-kernel-mm-damon
index 4fb8b7a6d625..8ec469eb4471 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -516,6 +516,12 @@ Contact:	SeongJae Park <sj@kernel.org>
 Description:	Reading this file returns the number of the exceed events of
 		the scheme's quotas.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/stats/nr_snapshots
+Date:		Nov 2025
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Reading this file returns the total number of DAMON snapshots
+		that the scheme has tried to be applied.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/tried_regions/total_bytes
 Date:		Jul 2023
 Contact:	SeongJae Park <sj@kernel.org>
-- 
2.47.3

[RFC PATCH 06/12] mm/damon: update damos kerneldoc for stat field

[SeongJae Park]

Commit 0e92c2ee9f45 ("mm/damon/schemes: account scheme actions that
successfully applied") has replaced ->stat_count and ->stat_sz of
'struct damos' with ->stat.  The commit mistakenly did not update the
related kernel doc comment, though.  Update the comment.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 1d8a1515e75a..43dfbfe2292f 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -532,9 +532,7 @@ struct damos_migrate_dests {
  * unsets @last_applied when each regions walking for applying the scheme is
  * finished.
  *
- * After applying the &action to each region, &stat_count and &stat_sz is
- * updated to reflect the number of regions and total size of regions that the
- * &action is applied.
+ * After applying the &action to each region, &stat is updated.
  */
 struct damos {
 	struct damos_access_pattern pattern;
-- 
2.47.3

[RFC PATCH 07/12] mm/damon/core: implement max_nr_snapshots

[SeongJae Park]

There are DAMOS use cases that require user-space centric control of its
activation and deactivation.  Having the control plane on the
user-space, or using DAMOS as a way for monitoring results collection
are such examples.

DAMON parameters online commit, DAMOS quotas and watermarks can be
useful for this purpose.  However, those features work only at the
sub-DAMON-snapshot level.  In some use cases, the DAMON-snapshot level
control is required.  For example, in DAMOS-based monitoring results
collection use case, the user online-installs a DAMOS scheme with
DAMOS_STAT action, wait it be applied to whole regions of a single
DAMON-snapshot, retrieves the stats and tried regions information, and
online-uninstall the scheme.  It is efficient to ensure the lifetime of
the scheme as no more no less one snapshot consumption.

To support such use cases, introduce a new DAMOS core API per-scheme
parameter, namely max_nr_snapshots.  As the name implies, it is the
upper limit of nr_snapshots, which is a DAMOS stat that represents the
number of DAMON-snapshots that the scheme has fully applied.  If the
limit is set with a non-zero value and nr_snapshots reaches or exceeds
the limit, the scheme is deactivated.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h |  5 +++++
 mm/damon/core.c       | 11 ++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 43dfbfe2292f..a67292a2f09d 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -499,6 +499,7 @@ struct damos_migrate_dests {
  * @ops_filters:	ops layer handling &struct damos_filter objects list.
  * @last_applied:	Last @action applied ops-managing entity.
  * @stat:		Statistics of this scheme.
+ * @max_nr_snapshots:	Upper limit of nr_snapshots stat.
  * @list:		List head for siblings.
  *
  * For each @apply_interval_us, DAMON finds regions which fit in the
@@ -533,6 +534,9 @@ struct damos_migrate_dests {
  * finished.
  *
  * After applying the &action to each region, &stat is updated.
+ *
+ * If &max_nr_snapshots is set as non-zero and &stat.nr_snapshots be same to or
+ * greater than it, the scheme is deactivated.
  */
 struct damos {
 	struct damos_access_pattern pattern;
@@ -567,6 +571,7 @@ struct damos {
 	struct list_head ops_filters;
 	void *last_applied;
 	struct damos_stat stat;
+	unsigned long max_nr_snapshots;
 	struct list_head list;
 };
 
diff --git a/mm/damon/core.c b/mm/damon/core.c
index af922d5e500c..36313cd1ff1c 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -401,6 +401,7 @@ struct damos *damon_new_scheme(struct damos_access_pattern *pattern,
 	INIT_LIST_HEAD(&scheme->core_filters);
 	INIT_LIST_HEAD(&scheme->ops_filters);
 	scheme->stat = (struct damos_stat){};
+	scheme->max_nr_snapshots = 0;
 	INIT_LIST_HEAD(&scheme->list);
 
 	scheme->quota = *(damos_quota_init(quota));
@@ -1078,7 +1079,11 @@ static int damos_commit(struct damos *dst, struct damos *src)
 		return err;
 
 	err = damos_commit_filters(dst, src);
-	return err;
+	if (err)
+		return err;
+
+	dst->max_nr_snapshots = src->max_nr_snapshots;
+	return 0;
 }
 
 static int damon_commit_schemes(struct damon_ctx *dst, struct damon_ctx *src)
@@ -1955,6 +1960,10 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 		if (damos_skip_charged_region(t, &r, s, c->min_sz_region))
 			continue;
 
+		if (s->max_nr_snapshots &&
+				s->max_nr_snapshots <= s->stat.nr_snapshots)
+			continue;
+
 		if (damos_valid_target(c, t, r, s))
 			damos_apply_scheme(c, t, r, s);
 
-- 
2.47.3

[RFC PATCH 08/12] mm/damon/sysfs-schemes: implement max_nr_snapshots file

[SeongJae Park]

Add a new DAMON sysfs file for setting and getting the newly introduced
per-DAMON-snapshot level DAMOS deactivation control parameter,
max_nr_snapshots.  The file has a name same to the parameter and placed
under the damos stat directory.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs-schemes.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 4d25469f957f..bf1b637d4342 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -205,6 +205,7 @@ struct damon_sysfs_stats {
 	unsigned long sz_ops_filter_passed;
 	unsigned long qt_exceeds;
 	unsigned long nr_snapshots;
+	unsigned long max_nr_snapshots;
 };
 
 static struct damon_sysfs_stats *damon_sysfs_stats_alloc(void)
@@ -275,6 +276,28 @@ static ssize_t nr_snapshots_show(struct kobject *kobj,
 	return sysfs_emit(buf, "%lu\n", stats->nr_snapshots);
 }
 
+static ssize_t max_nr_snapshots_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_stats *stats = container_of(kobj,
+			struct damon_sysfs_stats, kobj);
+
+	return sysfs_emit(buf, "%lu\n", stats->max_nr_snapshots);
+}
+
+static ssize_t max_nr_snapshots_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_stats *stats = container_of(kobj,
+			struct damon_sysfs_stats, kobj);
+	unsigned long max_nr_snapshots, err = kstrtoul(buf, 0, &max_nr_snapshots);
+
+	if (err)
+		return err;
+	stats->max_nr_snapshots = max_nr_snapshots;
+	return count;
+}
+
 static void damon_sysfs_stats_release(struct kobject *kobj)
 {
 	kfree(container_of(kobj, struct damon_sysfs_stats, kobj));
@@ -301,6 +324,9 @@ static struct kobj_attribute damon_sysfs_stats_qt_exceeds_attr =
 static struct kobj_attribute damon_sysfs_stats_nr_snapshots_attr =
 		__ATTR_RO_MODE(nr_snapshots, 0400);
 
+static struct kobj_attribute damon_sysfs_stats_max_nr_snapshots_attr =
+		__ATTR_RW_MODE(max_nr_snapshots, 0600);
+
 static struct attribute *damon_sysfs_stats_attrs[] = {
 	&damon_sysfs_stats_nr_tried_attr.attr,
 	&damon_sysfs_stats_sz_tried_attr.attr,
@@ -309,6 +335,7 @@ static struct attribute *damon_sysfs_stats_attrs[] = {
 	&damon_sysfs_stats_sz_ops_filter_passed_attr.attr,
 	&damon_sysfs_stats_qt_exceeds_attr.attr,
 	&damon_sysfs_stats_nr_snapshots_attr.attr,
+	&damon_sysfs_stats_max_nr_snapshots_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_stats);
@@ -2731,6 +2758,7 @@ static struct damos *damon_sysfs_mk_scheme(
 		damon_destroy_scheme(scheme);
 		return NULL;
 	}
+	scheme->max_nr_snapshots = sysfs_scheme->stats->max_nr_snapshots;
 	return scheme;
 }
 
-- 
2.47.3

[RFC PATCH 09/12] Docs/mm/damon/design: update for max_nr_snapshots

[SeongJae Park]

Update DAMON design document for the newly added snapshot level DAMOS
deactivation feature, max_nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/mm/damon/design.rst | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index 5cc7b7d662be..7fd819b8bbf7 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -720,6 +720,7 @@ scheme's execution.
 - ``qt_exceeds``: Total number of times the quota of the scheme has exceeded.
 - ``nr_snapshots``: Total number of DAMON snapshots that the scheme is tried to
   be applied.
+- ``max_nr_snapshots``: Upper limit of ``nr_snapshots``.
 
 "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
@@ -741,6 +742,10 @@ 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.
 
+Unlike normal stats, ``max_nr_snapshots`` is set by users.  If it is set as
+non-zero and ``nr_snapshots`` be same to or greater than ``nr_snapshots``, the
+scheme is deactivated.
+
 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.
-- 
2.47.3

[RFC PATCH 10/12] Docs/admin-guide/mm/damon/usage: update for max_nr_snapshots

[SeongJae Park]

Update DAMON usage document for the newly added DAMON sysfs interface
file, max_nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/usage.rst | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index d0944bd78964..7da4c002cb39 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -87,7 +87,7 @@ comma (",").
     │ │ │ │ │ │ │ │ 0/type,matching,allow,memcg_path,addr_start,addr_end,target_idx,min,max
     │ │ │ │ │ │ │ :ref:`dests <damon_sysfs_dests>`/nr_dests
     │ │ │ │ │ │ │ │ 0/id,weight
-    │ │ │ │ │ │ │ :ref:`stats <sysfs_schemes_stats>`/nr_tried,sz_tried,nr_applied,sz_applied,sz_ops_filter_passed,qt_exceeds,nr_snapshots
+    │ │ │ │ │ │ │ :ref:`stats <sysfs_schemes_stats>`/nr_tried,sz_tried,nr_applied,sz_applied,sz_ops_filter_passed,qt_exceeds,nr_snapshots,max_nr_snapshots
     │ │ │ │ │ │ │ :ref:`tried_regions <sysfs_schemes_tried_regions>`/total_bytes
     │ │ │ │ │ │ │ │ 0/start,end,nr_accesses,age,sz_filter_passed
     │ │ │ │ │ │ │ │ ...
@@ -543,10 +543,11 @@ online analysis or tuning of the schemes.  Refer to :ref:`design doc
 
 The statistics can be retrieved by reading the files under ``stats`` directory
 (``nr_tried``, ``sz_tried``, ``nr_applied``, ``sz_applied``,
-``sz_ops_filter_passed``, ``qt_exceeds`` and ``nr_snapshots``), respectively.
-The files are not updated in real time, so you should ask DAMON sysfs interface
-to update the content of the files for the stats by writing a special keyword,
-``update_schemes_stats`` to the relevant ``kdamonds/<N>/state`` file.
+``sz_ops_filter_passed``, ``qt_exceeds``, ``nr_snapshots`` and
+``max_nr_snapshots``), respectively.  The files are not updated in real time,
+so you should ask DAMON sysfs interface to update the content of the files for
+the stats by writing a special keyword, ``update_schemes_stats`` to the
+relevant ``kdamonds/<N>/state`` file.
 
 .. _sysfs_schemes_tried_regions:
 
-- 
2.47.3

[RFC PATCH 11/12] Docs/ABI/damon: update for max_nr_snapshots

[SeongJae Park]

Update DAMON ABI document for the newly added DAMON sysfs interface
file, max_nr_snapshots.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/ABI/testing/sysfs-kernel-mm-damon | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-damon b/Documentation/ABI/testing/sysfs-kernel-mm-damon
index 8ec469eb4471..4504a0f8b446 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -522,6 +522,13 @@ Contact:	SeongJae Park <sj@kernel.org>
 Description:	Reading this file returns the total number of DAMON snapshots
 		that the scheme has tried to be applied.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/stats/max_nr_snapshots
+Date:		Nov 2025
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Writing a number to this file sets the upper limit of
+		nr_snapshots that deactivates the scheme when the limit is
+		reached or exceeded.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/tried_regions/total_bytes
 Date:		Jul 2023
 Contact:	SeongJae Park <sj@kernel.org>
-- 
2.47.3

[RFC PATCH 12/12] mm/damon/core: add trace point for damos stat per apply interval

[SeongJae Park]

DAMON users can read DAMOS stats via DAMON sysfs interface.  It enables
efficient, simple and flexible usages of the stats.  Especially for
systems not having advanced tools like perf or bpftrace, that can be
useful.  But if the advanced tools are available, exposing the stats via
tracepoint can reduce unnecessary reimplementation of the wheels.  Add a
new tracepoint for DAMOS stats, namely damos_stat_after_apply_interval.
The tracepoint is triggered for each scheme's apply interval and exposes
the whole stat values.  If the user needs sub-apply interval information
for any chance, damos_before_apply tracepoint could be used.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/trace/events/damon.h | 41 ++++++++++++++++++++++++++++++++++++
 mm/damon/core.c              | 15 +++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/trace/events/damon.h b/include/trace/events/damon.h
index 852d725afea2..24fc402ab3c8 100644
--- a/include/trace/events/damon.h
+++ b/include/trace/events/damon.h
@@ -9,6 +9,47 @@
 #include <linux/types.h>
 #include <linux/tracepoint.h>
 
+TRACE_EVENT(damos_stat_after_apply_interval,
+
+	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
+		struct damos_stat *stat),
+
+	TP_ARGS(context_idx, scheme_idx, stat),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, context_idx)
+		__field(unsigned int, scheme_idx)
+		__field(unsigned long, nr_tried)
+		__field(unsigned long, sz_tried)
+		__field(unsigned long, nr_applied)
+		__field(unsigned long, sz_applied)
+		__field(unsigned long, sz_ops_filter_passed)
+		__field(unsigned long, qt_exceeds)
+		__field(unsigned long, nr_snapshots)
+	),
+
+	TP_fast_assign(
+		__entry->context_idx = context_idx;
+		__entry->scheme_idx = scheme_idx;
+		__entry->nr_tried = stat->nr_tried;
+		__entry->sz_tried = stat->sz_tried;
+		__entry->nr_applied = stat->nr_applied;
+		__entry->sz_applied = stat->sz_applied;
+		__entry->sz_ops_filter_passed = stat->sz_ops_filter_passed;
+		__entry->qt_exceeds = stat->qt_exceeds;
+		__entry->nr_snapshots = stat->nr_snapshots;
+	),
+
+	TP_printk("ctx_idx=%u scheme_idx=%u nr_tried=%lu sz_tried=%lu "
+			"nr_applied=%lu sz_tried=%lu sz_ops_filter_passed=%lu "
+			"qt_exceeds=%lu nr_snapshots=%lu",
+			__entry->context_idx, __entry->scheme_idx,
+			__entry->nr_tried, __entry->sz_tried,
+			__entry->nr_applied, __entry->sz_applied,
+			__entry->sz_ops_filter_passed, __entry->qt_exceeds,
+			__entry->nr_snapshots)
+);
+
 TRACE_EVENT(damos_esz,
 
 	TP_PROTO(unsigned int context_idx, unsigned int scheme_idx,
diff --git a/mm/damon/core.c b/mm/damon/core.c
index 36313cd1ff1c..2f212a18e4a0 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -2256,6 +2256,19 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	quota->min_score = score;
 }
 
+static void damos_trace_stat(struct damon_ctx *c, struct damos *s)
+{
+	unsigned int cidx = 0, sidx = 0;
+	struct damos *siter;
+
+	damon_for_each_scheme(siter, c) {
+		if (siter == s)
+			break;
+		sidx++;
+	}
+	trace_damos_stat_after_apply_interval(cidx, sidx, &s->stat);
+}
+
 static void kdamond_apply_schemes(struct damon_ctx *c)
 {
 	struct damon_target *t;
@@ -2294,6 +2307,8 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 			(s->apply_interval_us ? s->apply_interval_us :
 			 c->attrs.aggr_interval) / sample_interval;
 		s->last_applied = NULL;
+		if (trace_damos_stat_after_apply_interval_enabled())
+			damos_trace_stat(c, s);
 	}
 	mutex_unlock(&c->walk_control_lock);
 }
-- 
2.47.3

Re: [RFC PATCH 00/12] mm/damos/stat: introduce nr_snapshots, max_nr_snapshots and tracepoint

[SeongJae Park]

On Sun, 23 Nov 2025 10:43:14 -0800 SeongJae Park <sj@kernel.org> wrote:

> Introduce three changes for improving DAMOS stat's provided information,
> deterministic control, and reading usability.

FYI, I'm not aiming to make this be merged in 6.19-rc1.


Thanks,
SJ

[...]

Re: [RFC PATCH 12/12] mm/damon/core: add trace point for damos stat per apply interval

[Steven Rostedt]

On Sun, 23 Nov 2025 10:43:26 -0800
SeongJae Park <sj@kernel.org> wrote:

> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -2256,6 +2256,19 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
>  	quota->min_score = score;
>  }
>  
> +static void damos_trace_stat(struct damon_ctx *c, struct damos *s)
> +{
> +	unsigned int cidx = 0, sidx = 0;
> +	struct damos *siter;
> +
> +	damon_for_each_scheme(siter, c) {
> +		if (siter == s)
> +			break;
> +		sidx++;
> +	}
> +	trace_damos_stat_after_apply_interval(cidx, sidx, &s->stat);
> +}
> +
>  static void kdamond_apply_schemes(struct damon_ctx *c)
>  {
>  	struct damon_target *t;
> @@ -2294,6 +2307,8 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
>  			(s->apply_interval_us ? s->apply_interval_us :
>  			 c->attrs.aggr_interval) / sample_interval;
>  		s->last_applied = NULL;
> +		if (trace_damos_stat_after_apply_interval_enabled())
> +			damos_trace_stat(c, s);
>  	}
>  	mutex_unlock(&c->walk_control_lock);
>  }


I wonder if the above would look better (and still produce good assembly)
if it was:

static inline void damos_trace_stat(struct damon_ctx *c, struct damos *s)
{
	unsigned int cidx = 0, sidx = 0;
	struct damos *siter;

	if (!trace_damos_stat_after_apply_interval_enabled())
		return;

	damon_for_each_scheme(siter, c) {
		if (siter == s)
			break;
		sidx++;
	}
	trace_damos_stat_after_apply_interval(cidx, sidx, &s->stat);
}

static void kdamond_apply_schemes(struct damon_ctx *c)
{
	struct damon_target *t;
> @@ -2294,6 +2307,8 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
			(s->apply_interval_us ? s->apply_interval_us :
			 c->attrs.aggr_interval) / sample_interval;
 		s->last_applied = NULL;
		damos_trace_stat(c, s);
	}
	mutex_unlock(&c->walk_control_lock);
}


I have no real preference. I just think keeping the "if ()" statement out
of the main code as a more aesthetic look. But the above should be
equivalent in actual functionality.

-- Steve

Re: [RFC PATCH 12/12] mm/damon/core: add trace point for damos stat per apply interval

[SeongJae Park]

On Mon, 24 Nov 2025 10:15:45 -0500 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sun, 23 Nov 2025 10:43:26 -0800
> SeongJae Park <sj@kernel.org> wrote:
> 
> > --- a/mm/damon/core.c
> > +++ b/mm/damon/core.c
[...]
> I wonder if the above would look better (and still produce good assembly)
> if it was:
[...]
> I have no real preference. I just think keeping the "if ()" statement out
> of the main code as a more aesthetic look. But the above should be
> equivalent in actual functionality.

Thank you for this suggestion, Steve!  I'll update this patch as you suggested
in the next version.


Thanks,
SJ

[...]