[RFC PATCH 00/10] mm/damon: let DAMON be paused and resumed

[RFC PATCH 00/10] mm/damon: let DAMON be paused and resumed

[SeongJae Park]

DAMON utilizes a few mechanisms that enhance itself over time. Adaptive
regions adjustment, goal-based DAMOS quota auto-tuning and monitoring
intervals auto-tuning like self-training mechanisms are such examples.
It also adds access frequency stability information (age) to the
monitoring results, which makes it enhanced over time.

Sometimes users have to stop DAMON.  In this case, DAMON internal state
that enhanced over the time of the last execution simply goes away.
Restarted DAMON have to train itself and enhance its output from the
scratch.  This makes DAMON less useful in such cases.  Introducing three
such use cases below.

Investigation of DAMON.  It is best to do the investigation online,
especially when it is a production environment.  DAMON therefore
provides features for such online investigations, including DAMOS stats,
monitoring result snapshot exposure, and multiple tracepoints.  When
those are insufficient, and there are additional clues that could be
interfered by DAMON, users have to temporarily stop DAMON to collect the
additional clues.  It is not very useful since many of DAMON internal
clues are gone when DAMON is stopped.  The loss of the monitoring
results that improved over time is also problematic, especially in
production environments.

Monitoring of workloads that have different user-known phases.  For
example, in Android, applications are known to have very different
access patterns and behaviors when they are running on the foreground
and the background.  It can therefore be useful to separate monitoring
of apps based on whether they are running on the foreground and on the
background.  Having two DAMON threads per application that paused and
resumed for the apps foreground/background switches can be useful for
the purpose.  But such pause/resume of the execution is not supported.

Tests of DAMON.  A few DAMON selftests are using drgn to dump the
internal DAMON status.  The tests show if the dumped status is the same
as what the test code expected.  Because DAMON keeps running and
modifying its internal status, there are chances of data races that can
cause false test results.  Stopping DAMON can avoid the race.  But,
since the internal state of DAMON is dropped, the test coverage will be
limited.

Let DAMON execution be paused and resumed without loss of the internal
state, to overhaul the limitations.  For this, introduce a new DAMON
context parameter, namely 'pause'.  API callers can update it while the
context is running, using the online parameters update functions
(damon_commit_ctx() and damon_call()).  Once it is set, kdamond_fn()
main loop will do only limited works excluding the monitoring and DAMOS
works, while sleeping sampling intervals per the work.  The limited
works include handling of the online parameters update.  Hence users can
unset the 'pause' parameter again.  Once it is unset, kdamond_fn() main
loop will do all the work again (resumed).  Under the paused state, it
also does stop condition checks and handling of it, so that paused DAMON
can also be stopped if needed.  Expose the feature to the user space via
DAMON sysfs interface.  Also, update existing drgn-based tests to test
and use the feature.

Tests
=====

I confirmed the feature functionality using real time tracing ('perf
trace' or 'trace-cmd stream') of damon:damon_aggregated DAMON
tracepoint.  By pausing and resuming the DAMON execution, I was able to
see the trace stops and continued as expected.  Note that the pause
feature support is added to DAMON user-space tool (damo) after v3.1.9.
Users can use '--pause_ctx' command line option of damo for that, and I
actually used it for my test.  The extended drgn-based selftests are
also testing a part of the functionality.

Patches Sequence
================

Patch 1 introduces the new core API for the pause feature.  Patch 2
extend DAMON sysfs interface for the new parameter.  Patches 3-5 update
design, usage and ABI documents for the new sysfs file, respectively.
The following five patches are for tests.  Patch 6 implements a new
kunit test for the pause parameter online commitment.  Patches 7 and 8
extend DAMON selftest helpers to support the new feature.  Patch 9
extends selftest to test the commitment of the feature.  Finally, patch
10 updates existing selftest to be safe from the race condition using
the pause/resume feature.

SeongJae Park (10):
  mm/damon/core: introduce damon_ctx->paused
  mm/damon/sysfs: add pause file under context dir
  Docs/mm/damon/design: update for context pause/resume feature
  Docs/admin-guide/mm/damon/usage: update for pause file
  Docs/ABI/damon: update for pause sysfs file
  mm/damon/tests/core-kunit: test pause commitment
  selftests/damon/_damon_sysfs: support pause file staging
  selftests/damon/drgn_dump_damon_status: dump pause
  selftests/damon/sysfs.py: check pause on assert_ctx_committed()
  selftets/damon/sysfs.py: pause DAMON before dumping status

 .../ABI/testing/sysfs-kernel-mm-damon         |  7 +++++
 Documentation/admin-guide/mm/damon/usage.rst  | 12 +++++---
 Documentation/mm/damon/design.rst             |  7 +++++
 include/linux/damon.h                         |  2 ++
 mm/damon/core.c                               |  8 +++++
 mm/damon/sysfs.c                              | 30 +++++++++++++++++++
 mm/damon/tests/core-kunit.h                   |  4 +++
 tools/testing/selftests/damon/_damon_sysfs.py | 10 ++++++-
 .../selftests/damon/drgn_dump_damon_status.py |  1 +
 tools/testing/selftests/damon/sysfs.py        | 28 +++++++++++++++++
 10 files changed, 104 insertions(+), 5 deletions(-)


base-commit: 027cb70b08db6e3df632c49a8c7590ea830f7a96
-- 
2.47.3

[RFC PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

DAMON supports only start and stop of the execution.  When it is
stopped, its internal data that it self-trained goes away.  It will be
useful if the execution can be paused and resumed with the previous
self-trained data.

Introduce per-context API parameter, 'paused', for the purpose.  The
parameter can be set and unset while DAMON is running and paused, using
the online parameters commit helper functions (damon_commit_ctx() and
damon_call()).  Once 'paused' is set, the kdamond_fn() main loop does
only limited works with sampling interval sleep during the works.  The
limited works include the handling of the online parameters update, so
that users can unset the 'pause' and resume the execution when they
want.  It also keep checking DAMON stop conditions and handling of it,
so that DAMON can be stopped while paused if needed.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h | 2 ++
 mm/damon/core.c       | 8 ++++++++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 3a441fbca170d..421e51eff3bd2 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -811,6 +811,8 @@ struct damon_ctx {
 	 * intervals tuning
 	 */
 	unsigned long next_intervals_tune_sis;
+	/* pause kdamond main loop */
+	bool pause;
 	/* for waiting until the execution of the kdamond_fn is started */
 	struct completion kdamond_started;
 	/* for scheme quotas prioritization */
diff --git a/mm/damon/core.c b/mm/damon/core.c
index f9854aedc42d1..1e9f6aa569fd2 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1331,6 +1331,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
 		if (err)
 			return err;
 	}
+	dst->pause = src->pause;
 	dst->ops = src->ops;
 	dst->addr_unit = src->addr_unit;
 	dst->min_region_sz = src->min_region_sz;
@@ -2978,6 +2979,13 @@ static int kdamond_fn(void *data)
 		 * kdamond_merge_regions() if possible, to reduce overhead
 		 */
 		kdamond_call(ctx, false);
+		while (ctx->pause) {
+			if (kdamond_need_stop(ctx))
+				goto done;
+			kdamond_usleep(ctx->attrs.sample_interval);
+			/* allow caller unset pause via damon_call() */
+			kdamond_call(ctx, false);
+		}
 		if (!list_empty(&ctx->schemes))
 			kdamond_apply_schemes(ctx);
 		else
-- 
2.47.3

[RFC PATCH 02/10] mm/damon/sysfs: add pause file under context dir

[SeongJae Park]

Add pause DAMON sysfs file under the context directory.  It exposes the
damon_ctx->pause API parameter to the users so that they can use the
pause/resume feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 576d1ddd736bf..4cbb8b9aaba3c 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -866,6 +866,7 @@ struct damon_sysfs_context {
 	struct damon_sysfs_attrs *attrs;
 	struct damon_sysfs_targets *targets;
 	struct damon_sysfs_schemes *schemes;
+	bool pause;
 };
 
 static struct damon_sysfs_context *damon_sysfs_context_alloc(
@@ -1053,6 +1054,30 @@ static ssize_t addr_unit_store(struct kobject *kobj,
 	return count;
 }
 
+static ssize_t pause_show(struct kobject *kobj, struct kobj_attribute *attr,
+		char *buf)
+{
+	struct damon_sysfs_context *context = container_of(kobj,
+			struct damon_sysfs_context, kobj);
+
+	return sysfs_emit(buf, "%c\n", context->pause ? 'Y' : 'N');
+}
+
+static ssize_t pause_store(struct kobject *kobj, struct kobj_attribute *attr,
+		const char *buf, size_t count)
+{
+	struct damon_sysfs_context *context = container_of(kobj,
+			struct damon_sysfs_context, kobj);
+	bool pause;
+	int err = kstrtobool(buf, &pause);
+
+	if (err)
+		return err;
+	context->pause = pause;
+	return count;
+}
+
+
 static void damon_sysfs_context_release(struct kobject *kobj)
 {
 	kfree(container_of(kobj, struct damon_sysfs_context, kobj));
@@ -1067,10 +1092,14 @@ static struct kobj_attribute damon_sysfs_context_operations_attr =
 static struct kobj_attribute damon_sysfs_context_addr_unit_attr =
 		__ATTR_RW_MODE(addr_unit, 0600);
 
+static struct kobj_attribute damon_sysfs_context_pause_attr =
+		__ATTR_RW_MODE(pause, 0600);
+
 static struct attribute *damon_sysfs_context_attrs[] = {
 	&damon_sysfs_context_avail_operations_attr.attr,
 	&damon_sysfs_context_operations_attr.attr,
 	&damon_sysfs_context_addr_unit_attr.attr,
+	&damon_sysfs_context_pause_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_context);
@@ -1470,6 +1499,7 @@ static int damon_sysfs_apply_inputs(struct damon_ctx *ctx,
 	if (sys_ctx->ops_id == DAMON_OPS_PADDR)
 		ctx->min_region_sz = max(
 				DAMON_MIN_REGION_SZ / sys_ctx->addr_unit, 1);
+	ctx->pause = sys_ctx->pause;
 	err = damon_sysfs_set_attrs(ctx, sys_ctx->attrs);
 	if (err)
 		return err;
-- 
2.47.3

[RFC PATCH 03/10] Docs/mm/damon/design: update for context pause/resume feature

[SeongJae Park]

Update DAMON design document for the context execution pause/resume
feature.

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

diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst
index dc37402c0fee9..5723ffe8b51be 100644
--- a/Documentation/mm/damon/design.rst
+++ b/Documentation/mm/damon/design.rst
@@ -19,6 +19,13 @@ types of monitoring.
 To know how user-space can do the configurations and start/stop DAMON, refer to
 :ref:`DAMON sysfs interface <sysfs_interface>` documentation.
 
+Users can also request each context execution to be paused and resumed.  When
+it is paused, the kdamond does nothing but only online parameters updates
+including resume request handling.
+
+To know how user-space can pause/resume each context, refer to :ref:`DAMON
+sysfs context <sysfs_context>` usage documentation.
+
 
 Overall Architecture
 ====================
-- 
2.47.3

[RFC PATCH 04/10] Docs/admin-guide/mm/damon/usage: update for pause file

[SeongJae Park]

Update DAMON usage document for the DAMON context execution pause/resume
feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 Documentation/admin-guide/mm/damon/usage.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/mm/damon/usage.rst b/Documentation/admin-guide/mm/damon/usage.rst
index 534e1199cf091..bfdb717441f05 100644
--- a/Documentation/admin-guide/mm/damon/usage.rst
+++ b/Documentation/admin-guide/mm/damon/usage.rst
@@ -66,7 +66,8 @@ comma (",").
     │ :ref:`kdamonds <sysfs_kdamonds>`/nr_kdamonds
     │ │ :ref:`0 <sysfs_kdamond>`/state,pid,refresh_ms
     │ │ │ :ref:`contexts <sysfs_contexts>`/nr_contexts
-    │ │ │ │ :ref:`0 <sysfs_context>`/avail_operations,operations,addr_unit
+    │ │ │ │ :ref:`0 <sysfs_context>`/avail_operations,operations,addr_unit,
+    │ │ │ │   pause
     │ │ │ │ │ :ref:`monitoring_attrs <sysfs_monitoring_attrs>`/
     │ │ │ │ │ │ intervals/sample_us,aggr_us,update_us
     │ │ │ │ │ │ │ intervals_goal/access_bp,aggrs,min_sample_us,max_sample_us
@@ -194,9 +195,9 @@ details).  At the moment, only one context per kdamond is supported, so only
 contexts/<N>/
 -------------
 
-In each context directory, three files (``avail_operations``, ``operations``
-and ``addr_unit``) and three directories (``monitoring_attrs``, ``targets``,
-and ``schemes``) exist.
+In each context directory, four files (``avail_operations``, ``operations``,
+``addr_unit`` and ``pause``) and three directories (``monitoring_attrs``,
+``targets``, and ``schemes``) exist.
 
 DAMON supports multiple types of :ref:`monitoring operations
 <damon_design_configurable_operations_set>`, including those for virtual address
@@ -214,6 +215,9 @@ reading from the ``operations`` file.
 ``addr_unit`` file is for setting and getting the :ref:`address unit
 <damon_design_addr_unit>` parameter of the operations set.
 
+``pause`` file is for setting and getting the :ref:`pause request
+<damon_design_execution_model_and_data_structures>` parameter of the context.
+
 .. _sysfs_monitoring_attrs:
 
 contexts/<N>/monitoring_attrs/
-- 
2.47.3

[RFC PATCH 05/10] Docs/ABI/damon: update for pause sysfs file

[SeongJae Park]

Update DAMON ABI document for the DAMON context execution pause/resume
feature.

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 2424237ebb105..7059f540940f0 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-damon
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-damon
@@ -84,6 +84,13 @@ Description:	Writing an integer to this file sets the 'address unit'
 		parameter of the given operations set of the context.  Reading
 		the file returns the last-written 'address unit' value.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/pause
+Date:		Mar 2026
+Contact:	SeongJae Park <sj@kernel.org>
+Description:	Writing a boolean keyword to this file sets the 'pause' request
+		parameter for the context.  Reading the file returns the
+		last-written 'pause' value.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/monitoring_attrs/intervals/sample_us
 Date:		Mar 2022
 Contact:	SeongJae Park <sj@kernel.org>
-- 
2.47.3

[RFC PATCH 06/10] mm/damon/tests/core-kunit: test pause commitment

[SeongJae Park]

Add a kunit test for commitment of damon_ctx->pause parameter that can
be done using damon_commit_ctx().

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/tests/core-kunit.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 9e5904c2beeb2..0030f682b23b7 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -1077,6 +1077,10 @@ static void damon_test_commit_ctx(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), 0);
 	src->min_region_sz = 4095;
 	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), -EINVAL);
+	src->min_region_sz = 4096;
+	src->pause = true;
+	KUNIT_EXPECT_EQ(test, damon_commit_ctx(dst, src), 0);
+	KUNIT_EXPECT_TRUE(test, dst->pause);
 	damon_destroy_ctx(src);
 	damon_destroy_ctx(dst);
 }
-- 
2.47.3

[RFC PATCH 07/10] selftests/damon/_damon_sysfs: support pause file staging

[SeongJae Park]

DAMON test-purpose sysfs interface control Python module, _damon_sysfs,
is not supporting the newly added pause file.  Add the support of the
file, for future test and use of the feature.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/_damon_sysfs.py | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/damon/_damon_sysfs.py b/tools/testing/selftests/damon/_damon_sysfs.py
index 2b4df655d9fd0..120b96ecbd741 100644
--- a/tools/testing/selftests/damon/_damon_sysfs.py
+++ b/tools/testing/selftests/damon/_damon_sysfs.py
@@ -604,10 +604,11 @@ class DamonCtx:
     targets = None
     schemes = None
     kdamond = None
+    pause = None
     idx = None
 
     def __init__(self, ops='paddr', monitoring_attrs=DamonAttrs(), targets=[],
-            schemes=[]):
+            schemes=[], pause=False):
         self.ops = ops
         self.monitoring_attrs = monitoring_attrs
         self.monitoring_attrs.context = self
@@ -622,6 +623,8 @@ class DamonCtx:
             scheme.idx = idx
             scheme.context = self
 
+        self.pause=pause
+
     def sysfs_dir(self):
         return os.path.join(self.kdamond.sysfs_dir(), 'contexts',
                 '%d' % self.idx)
@@ -662,6 +665,11 @@ class DamonCtx:
             err = scheme.stage()
             if err is not None:
                 return err
+
+        err = write_file(os.path.join(self.sysfs_dir(), 'pause'), self.pause)
+        if err is not None:
+            return err
+
         return None
 
 class Kdamond:
-- 
2.47.3

[RFC PATCH 08/10] selftests/damon/drgn_dump_damon_status: dump pause

[SeongJae Park]

drgn_dump_damon_status is not dumping the damon_ctx->pause parameter
value, so it cannot be tested.  Dump it for future tests.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/drgn_dump_damon_status.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/damon/drgn_dump_damon_status.py b/tools/testing/selftests/damon/drgn_dump_damon_status.py
index af99b07a4f565..5b90eb8e7ef88 100755
--- a/tools/testing/selftests/damon/drgn_dump_damon_status.py
+++ b/tools/testing/selftests/damon/drgn_dump_damon_status.py
@@ -200,6 +200,7 @@ def damon_ctx_to_dict(ctx):
         ['attrs', attrs_to_dict],
         ['adaptive_targets', targets_to_list],
         ['schemes', schemes_to_list],
+        ['pause', bool],
         ])
 
 def main():
-- 
2.47.3

[RFC PATCH 09/10] selftests/damon/sysfs.py: check pause on assert_ctx_committed()

[SeongJae Park]

Extend sysfs.py tests to confirm damon_ctx->pause can be set using the
pause sysfs file.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/sysfs.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index 3aa5c91548a53..e6d34ba05893f 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -190,6 +190,7 @@ def assert_ctx_committed(ctx, dump):
     assert_monitoring_attrs_committed(ctx.monitoring_attrs, dump['attrs'])
     assert_monitoring_targets_committed(ctx.targets, dump['adaptive_targets'])
     assert_schemes_committed(ctx.schemes, dump['schemes'])
+    assert_true(dump['pause'] == ctx.pause, 'pause', dump)
 
 def assert_ctxs_committed(kdamonds):
     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
-- 
2.47.3

[RFC PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

The sysfs.py test commits DAMON parameters, dump the internal DAMON
state, and show if the parameters are committed as expected using the
dumped state.  While the dumping is ongoing, DAMON is alive.  It can
make internal changes including addition and removal of regions.  It can
therefore make a race that can result in false test results.  Pause
DAMON execution during the state dumping to avoid such races.

Signed-off-by: SeongJae Park <sj@kernel.org>
---
 tools/testing/selftests/damon/sysfs.py | 27 ++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
index e6d34ba05893f..a1a29f1a7c27b 100755
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -193,18 +193,44 @@ def assert_ctx_committed(ctx, dump):
     assert_true(dump['pause'] == ctx.pause, 'pause', dump)
 
 def assert_ctxs_committed(kdamonds):
+    paused_for_dump = False
+    if kdamonds.kdamonds[0].contexts[0].pause is False:
+        kdamonds.kdamonds[0].contexts[0].pause = True
+        err = kdamonds.kdamonds[0].commit()
+        if err is not None:
+            print('pause fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)
+        paused_for_dump = True
+
     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
     if err is not None:
         print(err)
         kdamonds.stop()
         exit(1)
 
+    if paused_for_dump:
+        # resume
+        kdamonds.kdamonds[0].contexts[0].pause = False
+        err = kdamonds.kdamonds[0].commit()
+        if err is not None:
+            print('resume fail (%s)' % err)
+            kdamonds.stop()
+            exit(1)
+
+        # restore for comparison
+        kdamonds.kdamonds[0].contexts[0].pause = True
+
     ctxs = kdamonds.kdamonds[0].contexts
     dump = status['contexts']
     assert_true(len(ctxs) == len(dump), 'ctxs length', dump)
     for idx, ctx in enumerate(ctxs):
         assert_ctx_committed(ctx, dump[idx])
 
+    if paused_for_dump:
+        # restore for the caller
+        kdamonds.kdamonds[0].contexts[0].pause = False
+
 def main():
     kdamonds = _damon_sysfs.Kdamonds(
             [_damon_sysfs.Kdamond(
@@ -302,6 +328,7 @@ def main():
         print('kdamond start failed: %s' % err)
         exit(1)
     kdamonds.kdamonds[0].contexts[0].targets[1].obsolete = True
+    kdamonds.kdamonds[0].contexts[0].pause = True
     kdamonds.kdamonds[0].commit()
     del kdamonds.kdamonds[0].contexts[0].targets[1]
     assert_ctxs_committed(kdamonds)
-- 
2.47.3

Re: [RFC PATCH 01/10] mm/damon/core: introduce damon_ctx->paused

[SeongJae Park]

On Sun, 15 Mar 2026 14:00:00 -0700 SeongJae Park <sj@kernel.org> wrote:

> DAMON supports only start and stop of the execution.  When it is
> stopped, its internal data that it self-trained goes away.  It will be
> useful if the execution can be paused and resumed with the previous
> self-trained data.
> 
> Introduce per-context API parameter, 'paused', for the purpose.  The
> parameter can be set and unset while DAMON is running and paused, using
> the online parameters commit helper functions (damon_commit_ctx() and
> damon_call()).  Once 'paused' is set, the kdamond_fn() main loop does
> only limited works with sampling interval sleep during the works.  The
> limited works include the handling of the online parameters update, so
> that users can unset the 'pause' and resume the execution when they
> want.  It also keep checking DAMON stop conditions and handling of it,
> so that DAMON can be stopped while paused if needed.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  include/linux/damon.h | 2 ++
>  mm/damon/core.c       | 8 ++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index 3a441fbca170d..421e51eff3bd2 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -811,6 +811,8 @@ struct damon_ctx {
>  	 * intervals tuning
>  	 */
>  	unsigned long next_intervals_tune_sis;
> +	/* pause kdamond main loop */
> +	bool pause;
>  	/* for waiting until the execution of the kdamond_fn is started */
>  	struct completion kdamond_started;
>  	/* for scheme quotas prioritization */
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index f9854aedc42d1..1e9f6aa569fd2 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -1331,6 +1331,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
>  		if (err)
>  			return err;
>  	}
> +	dst->pause = src->pause;
>  	dst->ops = src->ops;
>  	dst->addr_unit = src->addr_unit;
>  	dst->min_region_sz = src->min_region_sz;
> @@ -2978,6 +2979,13 @@ static int kdamond_fn(void *data)
>  		 * kdamond_merge_regions() if possible, to reduce overhead
>  		 */
>  		kdamond_call(ctx, false);
> +		while (ctx->pause) {
> +			if (kdamond_need_stop(ctx))
> +				goto done;
> +			kdamond_usleep(ctx->attrs.sample_interval);
> +			/* allow caller unset pause via damon_call() */
> +			kdamond_call(ctx, false);
> +		}

sashiko.dev comments [1] below.  Let me reply in line.

: Could this cause a deadlock if a walk request is initiated while DAMON is
: paused?
: 
: Looking at kdamond_fn(), when ctx->pause is true, the thread stays in the
: while (ctx->pause) loop and skips kdamond_apply_schemes(). If a user
: triggers a sysfs command that initiates a walk (like writing
: update_schemes_tried_regions), the sysfs thread calls damos_walk() and waits
: for walk_control->completion.
: 
: Since the kdamond thread is paused, the completion is never signaled, and the
: blocked sysfs writer will hold the global damon_sysfs_lock permanently. This
: prevents further interactions with the DAMON sysfs interface, making it
: impossible to even unpause the context.

Correct.  I was able to trigger the deadlock on my tet setup.

: 
: Should we call damos_walk_cancel(ctx) inside the pause loop to abort pending
: walk requests, similar to what is done in kdamond_wait_activation()?

Good suggestion.  I will add below attaching fixup change on the next spin.  I
confirmed the deadlock cannot be triggered after applying the fixup.

[1] https://sashiko.dev/#/patchset/20260315210012.94846-2-sj@kernel.org


Thanks,
SJ

[...]
=== >8 ===

--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -3405,6 +3405,7 @@ static int kdamond_fn(void *data)
                        kdamond_usleep(ctx->attrs.sample_interval);
                        /* allow caller unset pause via damon_call() */
                        kdamond_call(ctx, false);
+                       damos_walk_cancel(ctx);
                }
                if (!list_empty(&ctx->schemes))
                        kdamond_apply_schemes(ctx);

Re: [RFC PATCH 02/10] mm/damon/sysfs: add pause file under context dir

[SeongJae Park]

On Sun, 15 Mar 2026 14:00:01 -0700 SeongJae Park <sj@kernel.org> wrote:

> Add pause DAMON sysfs file under the context directory.  It exposes the
> damon_ctx->pause API parameter to the users so that they can use the
> pause/resume feature.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  mm/damon/sysfs.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
> index 576d1ddd736bf..4cbb8b9aaba3c 100644
> --- a/mm/damon/sysfs.c
> +++ b/mm/damon/sysfs.c
> @@ -866,6 +866,7 @@ struct damon_sysfs_context {
>  	struct damon_sysfs_attrs *attrs;
>  	struct damon_sysfs_targets *targets;
>  	struct damon_sysfs_schemes *schemes;
> +	bool pause;
>  };

sashiko.dev comments [1] below.

: Is the new pause field left uninitialized when a context is allocated?
: 
: Looking at damon_sysfs_context_alloc(), memory is allocated via kmalloc_obj()
: which does not zero-fill by default, and the new field is not explicitly
: initialized:
: 
: static struct damon_sysfs_context *damon_sysfs_context_alloc(
:                 enum damon_ops_id ops_id)
: {
:         struct damon_sysfs_context *context = kmalloc_obj(*context);
: 
:         if (!context)
:                 return NULL;
:         context->kobj = (struct kobject){};
:         context->ops_id = ops_id;
:         context->addr_unit = 1;
:         return context;
: }
: 
: If a user reads the pause sysfs file before writing to it, could this return
: uninitialized kernel heap memory?

Good catch.  I will add below fixup to the next spin.

'''
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1432,6 +1432,7 @@ static struct damon_sysfs_context *damon_sysfs_context_alloc(
        context->kobj = (struct kobject){};
        context->ops_id = ops_id;
        context->addr_unit = 1;
+       context->pause = false;
        return context;
 }
'''

Btw, somehow sashiko.dev added the comment to not this patch but the sixth
patch of this series.

[1] https://sashiko.dev/#/patchset/20260315210012.94846-7-sj@kernel.org


Thanks,
SJ

[...]

Re: [RFC PATCH 10/10] selftets/damon/sysfs.py: pause DAMON before dumping status

[SeongJae Park]

On Sun, 15 Mar 2026 14:00:09 -0700 SeongJae Park <sj@kernel.org> wrote:

> The sysfs.py test commits DAMON parameters, dump the internal DAMON
> state, and show if the parameters are committed as expected using the
> dumped state.  While the dumping is ongoing, DAMON is alive.  It can
> make internal changes including addition and removal of regions.  It can
> therefore make a race that can result in false test results.  Pause
> DAMON execution during the state dumping to avoid such races.
> 
> Signed-off-by: SeongJae Park <sj@kernel.org>
> ---
>  tools/testing/selftests/damon/sysfs.py | 27 ++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/tools/testing/selftests/damon/sysfs.py b/tools/testing/selftests/damon/sysfs.py
> index e6d34ba05893f..a1a29f1a7c27b 100755
> --- a/tools/testing/selftests/damon/sysfs.py
> +++ b/tools/testing/selftests/damon/sysfs.py
> @@ -193,18 +193,44 @@ def assert_ctx_committed(ctx, dump):
>      assert_true(dump['pause'] == ctx.pause, 'pause', dump)
>  
>  def assert_ctxs_committed(kdamonds):
> +    paused_for_dump = False
> +    if kdamonds.kdamonds[0].contexts[0].pause is False:
> +        kdamonds.kdamonds[0].contexts[0].pause = True

Quoting sashiko.dev comments [1] with ': ' prefix below.

: Does this code only pause the first context? The validation loop below
: iterates over all contexts in the kdamond, so if there are multiple
: contexts, will the others remain unpaused and vulnerable to the race
: condition during the state dump?

There is no real caller of this function that uses multiple contexts.  So there
is no real problem.  That said, I think this code will be better to take care
of such case that might happen in the future.

I will therefore add below change to the next version of this patch.

'''
--- a/tools/testing/selftests/damon/sysfs.py
+++ b/tools/testing/selftests/damon/sysfs.py
@@ -196,15 +196,17 @@ def assert_ctx_committed(ctx, dump):
     assert_true(dump['pause'] == ctx.pause, 'pause', dump)

 def assert_ctxs_committed(kdamonds):
-    paused_for_dump = False
-    if kdamonds.kdamonds[0].contexts[0].pause is False:
-        kdamonds.kdamonds[0].contexts[0].pause = True
-        err = kdamonds.kdamonds[0].commit()
-        if err is not None:
-            print('pause fail (%s)' % err)
-            kdamonds.stop()
-            exit(1)
-        paused_for_dump = True
+    ctxs_paused_for_dump = []
+    for kd in kdamonds.kdamonds:
+        for ctx in kd.contexts:
+            if ctx.pause is False:
+                ctx.pause = True
+                err = kd.commit()
+                if err is not None:
+                    print('pause fail (%s)' % err)
+                    kdamonds.stop()
+                    exit(1)
+                ctxs_paused_for_dump.append(ctx)

     status, err = dump_damon_status_dict(kdamonds.kdamonds[0].pid)
     if err is not None:
@@ -212,17 +214,17 @@ def assert_ctxs_committed(kdamonds):
         kdamonds.stop()
         exit(1)

-    if paused_for_dump:
-        # resume
-        kdamonds.kdamonds[0].contexts[0].pause = False
-        err = kdamonds.kdamonds[0].commit()
-        if err is not None:
-            print('resume fail (%s)' % err)
-            kdamonds.stop()
-            exit(1)
-
-        # restore for comparison
-        kdamonds.kdamonds[0].contexts[0].pause = True
+    for kd in kdamonds.kdamonds:
+        for ctx in kd.contexts:
+            if ctx in ctxs_paused_for_dump:
+                ctx.pause = False
+                err = kd.commit()
+                if err is not None:
+                    print('resume fail (%s)' % err)
+                    kdamonds.stop()
+                    exit(1)
+                # restore for comparison
+                ctx.pause = True

     ctxs = kdamonds.kdamonds[0].contexts
     dump = status['contexts']
@@ -230,9 +232,11 @@ def assert_ctxs_committed(kdamonds):
     for idx, ctx in enumerate(ctxs):
         assert_ctx_committed(ctx, dump[idx])

-    if paused_for_dump:
-        # restore for the caller
-        kdamonds.kdamonds[0].contexts[0].pause = False
+    # restore for the caller
+    for kd in kdamonds.kdamonds:
+        for ctx in kd.contexts:
+            if ctx in ctxs_paused_for_dump:
+                ctx.pause = False

 def main():
     global kdamonds
'''

> +        err = kdamonds.kdamonds[0].commit()

: Does calling commit() here inadvertently mask test failures by forcing the
: entire Python object state to the kernel right before reading the status?
: 
: For example, if a test marks a target obsolete, commits it, and deletes
: the target from the Python list to verify if the kernel autonomously
: removed it, this commit() would explicitly push the target's deletion to
: the kernel, potentially bypassing the test's purpose.

No.  Callers that could have such problem should call this function after
pausing the context on their own.  That's what the target obsolete test case is
doing.

[1] https://sashiko.dev/#/patchset/20260315210012.94846-11-sj@kernel.org


Thanks,
SJ

[...]