[PATCH 0/4] mm/damon/sysfs: support periodic and automated stats update

[PATCH 0/4] mm/damon/sysfs: support periodic and automated stats update

[SeongJae Park]

DAMON sysfs interface provides files for reading DAMON internal status
including auto-tuned monitoring intervals, DAMOS stats, DAMOS action
applied regions, and auto-tuned DAMOS effective quota.  Among those,
auto-tuned monitoring intervals, DAMOS stats and auto-tuned DAMOS
effective quota are essential for common DAMON/S use cases.

The content of the files are not automatically updated, though.  Users
should manually request updates of the contents by writing a special
command to 'state' file of each kdamond directory.  This interface is
good for minimizing overhead, but causes the below problems.

First, the usage is cumbersome.  This is arguably not a big problem,
since the user-space tool (damo) can do this instead of the user.

Second, it can be too slow.  The update request is not directly handled
by the sysfs interface but kdamond thread.  And kdamond threads wake up
only once per the sampling interval.  Hence if sampling interval is not
short, each update request could take too long time.  The recommended
sampling interval setup is asking DAMON to automatically tune it, within
a range between 5 milliseconds and 10 seconds.  On production systems it
is not very rare to have a few seconds sampling interval as a result of
the auto-tuning, so this can disturb observing DAMON internal status.

Finally, parallel update requests can conflict with each other.  When
parallel update requests are received, DAMON sysfs interface simply
returns -EBUSY to one of the requests.  DAMON user-space tool is hence
implementing its own backoff mechanism, but this can make the operation
even slower.

Introduce a new sysfs file, namely refresh_ms, for asking DAMON sysfs
interface to repeat the update of the above mentioned essential contents
with a user-specified time delay.  If non-zero value is written to the
file, DAMON sysfs interface does the updates for essential DAMON
internal status including auto-tuned monitoring intervals, DAMOS stats,
and auto-tuned DAMOS quotas using the user-written value as the time
delay.  In other words, it is similar to periodically writing
'update_schemes_stats', 'update_schemes_effective_quotas', and
'update_tuned_intervals' keywords to the 'state' file.  If zero is
written to the file, the automatic refresh is disabled.

Changes from RFC
(https://lore.kernel.or/20250712204650.155988-1-sj@kernel.org)
- Wordsmith cover letter to clarify what are updated

SeongJae Park (4):
  mm/damon/sysfs: implement refresh_ms file under kdamond directory
  mm/damon/sysfs: implement refresh_ms file internal work
  Docs/admin-guide/mm/damon/usage: document refresh_ms file
  Docs/ABI/damon: update for refresh_ms

 .../ABI/testing/sysfs-kernel-mm-damon         |  7 +++
 Documentation/admin-guide/mm/damon/usage.rst  | 13 ++++-
 mm/damon/sysfs.c                              | 58 +++++++++++++++++++
 3 files changed, 75 insertions(+), 3 deletions(-)


base-commit: 2951e548fd2e489bdaecf326e1c3d17d4b5663fe
-- 
2.39.5

[PATCH 1/4] mm/damon/sysfs: implement refresh_ms file under kdamond directory

[SeongJae Park]

Implement a new DAMON sysfs file named 'refresh_ms' under each kdamond
directory.  The file will be used as a control knob of automatic refresh
of a few DAMON internal status files.  This commit implements only
minimum file operations, though.  The automatic refresh feature will be
implemented by the following commit.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index cce2c8a296e2..4296dc201f4d 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1155,6 +1155,7 @@ struct damon_sysfs_kdamond {
 	struct kobject kobj;
 	struct damon_sysfs_contexts *contexts;
 	struct damon_ctx *damon_ctx;
+	unsigned int refresh_ms;
 };
 
 static struct damon_sysfs_kdamond *damon_sysfs_kdamond_alloc(void)
@@ -1690,6 +1691,30 @@ static ssize_t pid_show(struct kobject *kobj,
 	return sysfs_emit(buf, "%d\n", pid);
 }
 
+static ssize_t refresh_ms_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
+			struct damon_sysfs_kdamond, kobj);
+
+	return sysfs_emit(buf, "%u\n", kdamond->refresh_ms);
+}
+
+static ssize_t refresh_ms_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
+			struct damon_sysfs_kdamond, kobj);
+	unsigned int nr;
+	int err = kstrtouint(buf, 0, &nr);
+
+	if (err)
+		return err;
+
+	kdamond->refresh_ms = nr;
+	return count;
+}
+
 static void damon_sysfs_kdamond_release(struct kobject *kobj)
 {
 	struct damon_sysfs_kdamond *kdamond = container_of(kobj,
@@ -1706,9 +1731,13 @@ static struct kobj_attribute damon_sysfs_kdamond_state_attr =
 static struct kobj_attribute damon_sysfs_kdamond_pid_attr =
 		__ATTR_RO_MODE(pid, 0400);
 
+static struct kobj_attribute damon_sysfs_kdamond_refresh_ms_attr =
+		__ATTR_RW_MODE(refresh_ms, 0600);
+
 static struct attribute *damon_sysfs_kdamond_attrs[] = {
 	&damon_sysfs_kdamond_state_attr.attr,
 	&damon_sysfs_kdamond_pid_attr.attr,
+	&damon_sysfs_kdamond_refresh_ms_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_kdamond);
-- 
2.39.5

[PATCH 2/4] mm/damon/sysfs: implement refresh_ms file internal work

[SeongJae Park]

Only minimum file operations for refresh_ms file is implemented.
Further implement its designed behavior, the periodic essential files
content update,  using repeat mode damon_call().

If non-zero value is written to the file, update DAMON sysfs files for
auto-tuned monitoring intervals, DAMOS stats, and auto-tuned DAMOS quota
values, which are essential to be monitored in most DAMON use cases.
The user-written non-zero value becomes the time delay between the
update.  If zero is written to the file, the periodic refresh is
disabled.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 4296dc201f4d..6d2b0dab50cb 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1509,6 +1509,32 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 	return ctx;
 }
 
+static int damon_sysfs_repeat_call_fn(void *data)
+{
+	struct damon_sysfs_kdamond *sysfs_kdamond = data;
+	static unsigned long next_update_jiffies;
+
+	if (!sysfs_kdamond->refresh_ms)
+		return 0;
+	if (time_before(jiffies, next_update_jiffies))
+		return 0;
+	next_update_jiffies = jiffies +
+		msecs_to_jiffies(sysfs_kdamond->refresh_ms);
+
+	if (!mutex_trylock(&damon_sysfs_lock))
+		return 0;
+	damon_sysfs_upd_tuned_intervals(sysfs_kdamond);
+	damon_sysfs_upd_schemes_stats(sysfs_kdamond);
+	damon_sysfs_upd_schemes_effective_quotas(sysfs_kdamond);
+	mutex_unlock(&damon_sysfs_lock);
+	return 0;
+}
+
+static struct damon_call_control damon_sysfs_repeat_call_control = {
+	.fn = damon_sysfs_repeat_call_fn,
+	.repeat = true,
+};
+
 static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 {
 	struct damon_ctx *ctx;
@@ -1533,6 +1559,9 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 		return err;
 	}
 	kdamond->damon_ctx = ctx;
+
+	damon_sysfs_repeat_call_control.data = kdamond;
+	damon_call(ctx, &damon_sysfs_repeat_call_control);
 	return err;
 }
 
-- 
2.39.5

[PATCH 3/4] Docs/admin-guide/mm/damon/usage: document refresh_ms file

[SeongJae Park]

Document the new DAMON sysfs file, refresh_ms, on the usage document.

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 fc5c962353ed..ff3a2dda1f02 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -59,7 +59,7 @@ comma (",").
 
     :ref:`/sys/kernel/mm/damon <sysfs_root>`/admin
     │ :ref:`kdamonds <sysfs_kdamonds>`/nr_kdamonds
-    │ │ :ref:`0 <sysfs_kdamond>`/state,pid
+    │ │ :ref:`0 <sysfs_kdamond>`/state,pid,refresh_ms
     │ │ │ :ref:`contexts <sysfs_contexts>`/nr_contexts
     │ │ │ │ :ref:`0 <sysfs_context>`/avail_operations,operations
     │ │ │ │ │ :ref:`monitoring_attrs <sysfs_monitoring_attrs>`/
@@ -123,8 +123,8 @@ kdamond.
 kdamonds/<N>/
 -------------
 
-In each kdamond directory, two files (``state`` and ``pid``) and one directory
-(``contexts``) exist.
+In each kdamond directory, three files (``state``, ``pid`` and ``refresh_ms``)
+and one directory (``contexts``) exist.
 
 Reading ``state`` returns ``on`` if the kdamond is currently running, or
 ``off`` if it is not running.
@@ -161,6 +161,13 @@ Users can write below commands for the kdamond to the ``state`` file.
 
 If the state is ``on``, reading ``pid`` shows the pid of the kdamond thread.
 
+Users can ask the kernel to periodically update files showing auto-tuned
+parameters and DAMOS stats instead of manually writing
+``update_tuned_intervals`` like keywords to ``state`` file.  For this, users
+should write the desired update time interval in milliseconds to ``refresh_ms``
+file.  If the interval is zero, the periodic update is disabled.  Reading the
+file shows currently set time interval.
+
 ``contexts`` directory contains files for controlling the monitoring contexts
 that this kdamond will execute.
 
-- 
2.39.5

[PATCH 4/4] Docs/ABI/damon: update for refresh_ms

[SeongJae Park]

Document the new DAMON sysfs file, refresh_ms, on the ABI document.

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 e98974dfac7a..6791d879759e 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -44,6 +44,13 @@ Contact:	SeongJae Park <sj@kernel.org>
 Description:	Reading this file returns the pid of the kdamond if it is
 		running.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/refresh_ms
+Date:		Jul 2025
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Writing a value to this file sets the time interval for
+		automatic DAMON status file contents update.  Writing '0'
+		disables the update.  Reading this file returns the value.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/nr_contexts
 Date:		Mar 2022
 Contact:	SeongJae Park <sj@kernel.org>
-- 
2.39.5