[RFC PATCH v1.1 0/7] mm/damon: update probe hits for runtime parameter commits

[RFC PATCH v1.1 0/7] mm/damon: update probe hits for runtime parameter commits

[SJ Park]

DAMON users can update DAMON parameters such as sampling and aggregation
intervals at runtime.  For such changes, monitoring results that depend
on the intervals should be properly updated for better accuracy.  For
example, the access frequency counter (nr_accesses) is updated.  The
data attribute monitoring counter (probe_hits) is not being updated,
though.  Do the updates for new parameters.

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

Patch 1 removes obsolete comments and test code on a function that this
series will touch.  Patches 2-5 rename functions that are being used for
nr_accesses update, to be able to be used for probe_hits without
confusion.  Patch 6 does the probe_hits update.  Patch 7 update
damon_probe_hits_mvsum() to cover a corner case from the update for
better accuracy.

Changelog
=========

Changes from RFC
- RFC: https://lore.kernel.org/20260707043828.97900-1-sj@kernel.org
- Remove obsolete comment and code on the test.

SJ Park (7):
  mm/damon/core: remove obsolete comment for nr_to_bp() divide-by-zero
  mm/damon/core: s/damon_max_nr_accesses()/damon_nr_samples_per_aggr()/
  mm/damon/core: s/accesses_bp_to_nr_accesses/sample_bp_to_count/
  mm/damon/core: s/nr_accesses_to_accesses_bp/sample_count_to_bp/
  mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/
  mm/damon/core: update probe hits for new parameter commit
  mm/damon/core: handle unreset probe_hits in probe_hits_mvsum()

 include/linux/damon.h       | 10 +++---
 mm/damon/core.c             | 64 ++++++++++++++++++++++++-------------
 mm/damon/lru_sort.c         |  2 +-
 mm/damon/ops-common.c       |  2 +-
 mm/damon/tests/core-kunit.h | 31 ++++++++----------
 5 files changed, 62 insertions(+), 47 deletions(-)


base-commit: 86f45ad9d6667f344421729eec5a9367e37bfc5b
-- 
2.47.3

[RFC PATCH v1.1 1/7] mm/damon/core: remove obsolete comment for nr_to_bp() divide-by-zero

[SJ Park]

damon_nr_accesses_to_accesses_bp() warns it can trigger division-by-zero
when the aggregation interval is zero.  Commit 35d4a3cf70a8
("mm/damon/ops-common: handle extreme intervals in damon_hot_score()")
modified damon_nr_samples_per_aggr() to always return non-zero.  Hence
no division-by-zero of the note can happen.  Remove the obsolete comment
on the function and its test code.  Also remove the unnecessary guard
code in the test.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c             |  8 +-------
 mm/damon/tests/core-kunit.h | 12 ------------
 2 files changed, 1 insertion(+), 19 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 390e00b3685ef..a15005db82899 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -883,13 +883,7 @@ static unsigned int damon_accesses_bp_to_nr_accesses(
 	return accesses_bp * damon_max_nr_accesses(attrs) / 10000;
 }
 
-/*
- * Convert nr_accesses to access ratio in bp (per 10,000).
- *
- * Callers should ensure attrs.aggr_interval is not zero, like
- * damon_update_monitoring_results() does .  Otherwise, divide-by-zero would
- * happen.
- */
+/* Convert nr_accesses to access ratio in bp (per 10,000) */
 static unsigned int damon_nr_accesses_to_accesses_bp(
 		unsigned int nr_accesses, struct damon_attrs *attrs)
 {
diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 0124f83b39b83..224a3ecaa0701 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -582,18 +582,6 @@ static void damon_test_nr_accesses_to_accesses_bp(struct kunit *test)
 		.aggr_interval = ((unsigned long)UINT_MAX + 1) * 10
 	};
 
-	/*
-	 * In some cases such as 32bit architectures where UINT_MAX is
-	 * ULONG_MAX, attrs.aggr_interval becomes zero.  Calling
-	 * damon_nr_accesses_to_accesses_bp() in the case will cause
-	 * divide-by-zero.  Such case is prohibited in normal execution since
-	 * the caution is documented on the comment for the function, and
-	 * damon_update_monitoring_results() does the check.  Skip the test in
-	 * the case.
-	 */
-	if (!attrs.aggr_interval)
-		kunit_skip(test, "aggr_interval is zero.");
-
 	KUNIT_EXPECT_EQ(test, damon_nr_accesses_to_accesses_bp(123, &attrs), 0);
 }
 
-- 
2.47.3

[RFC PATCH v1.1 2/7] mm/damon/core: s/damon_max_nr_accesses()/damon_nr_samples_per_aggr()/

[SJ Park]

damon_max_nr_accesses() actually returns the number of samples DAMON
checks for each region per each aggregation interval.  Rename it to
better describe what it really does and not confusing for more general
uses.

Signed-off-by: SJ Park <sj@kernel.org>
---
 include/linux/damon.h | 10 ++++++----
 mm/damon/core.c       |  4 ++--
 mm/damon/lru_sort.c   |  2 +-
 mm/damon/ops-common.c |  2 +-
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 19b7e839bde0b..63f596957c1f1 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -1056,15 +1056,17 @@ static inline bool damon_target_has_pid(const struct damon_ctx *ctx)
 	return ctx->ops.id == DAMON_OPS_VADDR || ctx->ops.id == DAMON_OPS_FVADDR;
 }
 
-static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs)
+/* Returns number of samples per aggregation interval */
+static inline unsigned int damon_nr_samples_per_aggr(
+		const struct damon_attrs *attrs)
 {
 	unsigned long sample_interval;
-	unsigned long max_nr_accesses;
+	unsigned long nr_samples;
 
 	sample_interval = attrs->sample_interval ? : 1;
-	max_nr_accesses = min(attrs->aggr_interval / sample_interval,
+	nr_samples = min(attrs->aggr_interval / sample_interval,
 			(unsigned long)UINT_MAX);
-	return max_nr_accesses ? : 1;
+	return nr_samples ? : 1;
 }
 
 
diff --git a/mm/damon/core.c b/mm/damon/core.c
index a15005db82899..177e5abc981ef 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -880,14 +880,14 @@ static unsigned int damon_age_for_new_attrs(unsigned int age,
 static unsigned int damon_accesses_bp_to_nr_accesses(
 		unsigned int accesses_bp, struct damon_attrs *attrs)
 {
-	return accesses_bp * damon_max_nr_accesses(attrs) / 10000;
+	return accesses_bp * damon_nr_samples_per_aggr(attrs) / 10000;
 }
 
 /* Convert nr_accesses to access ratio in bp (per 10,000) */
 static unsigned int damon_nr_accesses_to_accesses_bp(
 		unsigned int nr_accesses, struct damon_attrs *attrs)
 {
-	return mult_frac(nr_accesses, 10000, damon_max_nr_accesses(attrs));
+	return mult_frac(nr_accesses, 10000, damon_nr_samples_per_aggr(attrs));
 }
 
 static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,
diff --git a/mm/damon/lru_sort.c b/mm/damon/lru_sort.c
index e8c389ad3226f..7e077084cb03a 100644
--- a/mm/damon/lru_sort.c
+++ b/mm/damon/lru_sort.c
@@ -303,7 +303,7 @@ static int damon_lru_sort_apply_parameters(void)
 		goto out;
 
 	err = -ENOMEM;
-	hot_thres = damon_max_nr_accesses(&attrs) *
+	hot_thres = damon_nr_samples_per_aggr(&attrs) *
 		hot_thres_access_freq / 1000;
 	hot_scheme = damon_lru_sort_new_hot_scheme(hot_thres);
 	if (!hot_scheme)
diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c
index 6bdd1cfd3863a..89c694f6e4fa8 100644
--- a/mm/damon/ops-common.c
+++ b/mm/damon/ops-common.c
@@ -112,7 +112,7 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r,
 	int hotness;
 
 	freq_subscore = r->nr_accesses * DAMON_MAX_SUBSCORE /
-		damon_max_nr_accesses(&c->attrs);
+		damon_nr_samples_per_aggr(&c->attrs);
 
 	age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000;
 	if (age_in_sec)
-- 
2.47.3

[RFC PATCH v1.1 3/7] mm/damon/core: s/accesses_bp_to_nr_accesses/sample_bp_to_count/

[SJ Park]

accesses_bp_to_nr_accesses() actually converts a positive samples ratio
to the count.  Rename it to better describe what it really does and not
confusing for more general uses.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 177e5abc981ef..dea63083917b0 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -876,11 +876,11 @@ static unsigned int damon_age_for_new_attrs(unsigned int age,
 	return age * old_attrs->aggr_interval / new_attrs->aggr_interval;
 }
 
-/* convert access ratio in bp (per 10,000) to nr_accesses */
-static unsigned int damon_accesses_bp_to_nr_accesses(
-		unsigned int accesses_bp, struct damon_attrs *attrs)
+/* convert sample ratio in bp (per 10,000) to count */
+static unsigned int damon_sample_bp_to_count(
+		unsigned int bp, struct damon_attrs *attrs)
 {
-	return accesses_bp * damon_nr_samples_per_aggr(attrs) / 10000;
+	return bp * damon_nr_samples_per_aggr(attrs) / 10000;
 }
 
 /* Convert nr_accesses to access ratio in bp (per 10,000) */
@@ -893,7 +893,7 @@ static unsigned int damon_nr_accesses_to_accesses_bp(
 static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs)
 {
-	return damon_accesses_bp_to_nr_accesses(
+	return damon_sample_bp_to_count(
 			damon_nr_accesses_to_accesses_bp(
 				nr_accesses, old_attrs),
 			new_attrs);
-- 
2.47.3

[RFC PATCH v1.1 4/7] mm/damon/core: s/nr_accesses_to_accesses_bp/sample_count_to_bp/

[SJ Park]

damon_nr_accesses_to_accesses_bp() actually converts a positive sample
count to the ratio.  Rename it to better describe what it really does
and not confusing for more general uses.

Also remove the obsolete comment about division-by-zero.  Commit
35d4a3cf70a8 ("mm/damon/ops-common: handle extreme intervals in
damon_hot_score()") modified damon_nr_samples_per_aggr() to always
return non-zero.  Hence no division-by-zero of the note can happen.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c             | 11 +++++------
 mm/damon/tests/core-kunit.h |  6 +++---
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index dea63083917b0..718268df27fa7 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -883,19 +883,18 @@ static unsigned int damon_sample_bp_to_count(
 	return bp * damon_nr_samples_per_aggr(attrs) / 10000;
 }
 
-/* Convert nr_accesses to access ratio in bp (per 10,000) */
-static unsigned int damon_nr_accesses_to_accesses_bp(
-		unsigned int nr_accesses, struct damon_attrs *attrs)
+/* convert sample count to ratio in bp (per 10,000) */
+static unsigned int damon_sample_count_to_bp(
+		unsigned int count, struct damon_attrs *attrs)
 {
-	return mult_frac(nr_accesses, 10000, damon_nr_samples_per_aggr(attrs));
+	return mult_frac(count, 10000, damon_nr_samples_per_aggr(attrs));
 }
 
 static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs)
 {
 	return damon_sample_bp_to_count(
-			damon_nr_accesses_to_accesses_bp(
-				nr_accesses, old_attrs),
+			damon_sample_count_to_bp(nr_accesses, old_attrs),
 			new_attrs);
 }
 
diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 224a3ecaa0701..9e631b19b184a 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -575,14 +575,14 @@ static void damon_test_set_regions(struct kunit *test)
 			}, 3);
 }
 
-static void damon_test_nr_accesses_to_accesses_bp(struct kunit *test)
+static void damon_test_sample_count_to_bp(struct kunit *test)
 {
 	struct damon_attrs attrs = {
 		.sample_interval = 10,
 		.aggr_interval = ((unsigned long)UINT_MAX + 1) * 10
 	};
 
-	KUNIT_EXPECT_EQ(test, damon_nr_accesses_to_accesses_bp(123, &attrs), 0);
+	KUNIT_EXPECT_EQ(test, damon_sample_count_to_bp(123, &attrs), 0);
 }
 
 static void damon_test_update_monitoring_result(struct kunit *test)
@@ -1560,7 +1560,7 @@ static struct kunit_case damon_test_cases[] = {
 	KUNIT_CASE(damon_test_split_above_half_progresses),
 	KUNIT_CASE(damon_test_ops_registration),
 	KUNIT_CASE(damon_test_set_regions),
-	KUNIT_CASE(damon_test_nr_accesses_to_accesses_bp),
+	KUNIT_CASE(damon_test_sample_count_to_bp),
 	KUNIT_CASE(damon_test_update_monitoring_result),
 	KUNIT_CASE(damon_test_set_attrs),
 	KUNIT_CASE(damon_test_mvsum),
-- 
2.47.3

[RFC PATCH v1.1 5/7] mm/damon/core: s/nr_accesses_for_new_attrs/nr_samples_for_new_attrs/

[SJ Park]

damon_nr_accesses_for_new_attrs() can be used for not only nr_accesses
but also any positive sample count, like probe_hits.  Rename to be able
to be used for such general uses without confusion.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 718268df27fa7..cda318806b65b 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -890,7 +890,7 @@ static unsigned int damon_sample_count_to_bp(
 	return mult_frac(count, 10000, damon_nr_samples_per_aggr(attrs));
 }
 
-static unsigned int damon_nr_accesses_for_new_attrs(unsigned int nr_accesses,
+static unsigned int damon_nr_samples_for_new_attrs(unsigned int nr_accesses,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs)
 {
 	return damon_sample_bp_to_count(
@@ -902,10 +902,10 @@ static void damon_update_monitoring_result(struct damon_region *r,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs,
 		bool aggregating)
 {
-	r->last_nr_accesses = damon_nr_accesses_for_new_attrs(
+	r->last_nr_accesses = damon_nr_samples_for_new_attrs(
 			r->last_nr_accesses, old_attrs, new_attrs);
 	if (!aggregating)
-		r->nr_accesses = damon_nr_accesses_for_new_attrs(
+		r->nr_accesses = damon_nr_samples_for_new_attrs(
 				r->nr_accesses, old_attrs, new_attrs);
 	else
 		/*
-- 
2.47.3

[RFC PATCH v1.1 6/7] mm/damon/core: update probe hits for new parameter commit

[SJ Park]

Users can update DAMON parameters at runtime.  If the samples and/or
aggregation intervals are updated in this way, monitoring results
depending on the intervals should also be updated for a more accurate
snapshot.  The age and nr_accesses are properly updated, while
probe_hits are not updated in the way.  Do the update.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c             | 28 +++++++++++++++++++++++++---
 mm/damon/tests/core-kunit.h | 13 ++++++++++---
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index cda318806b65b..9a3ff0fe55a61 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -898,10 +898,32 @@ static unsigned int damon_nr_samples_for_new_attrs(unsigned int nr_accesses,
 			new_attrs);
 }
 
+static void damon_update_probe_hits(struct damon_region *r,
+		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs,
+		bool aggregating, struct damon_ctx *ctx)
+{
+	struct damon_probe *p;
+	int i = 0;
+
+	damon_for_each_probe(p, ctx) {
+		r->last_probe_hits[i] = damon_nr_samples_for_new_attrs(
+				r->last_probe_hits[i], old_attrs, new_attrs);
+		if (!aggregating)
+			r->probe_hits[i] = damon_nr_samples_for_new_attrs(
+					r->probe_hits[i], old_attrs,
+					new_attrs);
+		else
+			r->probe_hits[i] = 0;
+		i++;
+	}
+}
+
 static void damon_update_monitoring_result(struct damon_region *r,
 		struct damon_attrs *old_attrs, struct damon_attrs *new_attrs,
-		bool aggregating)
+		bool aggregating, struct damon_ctx *ctx)
 {
+	damon_update_probe_hits(r, old_attrs, new_attrs, aggregating, ctx);
+
 	r->last_nr_accesses = damon_nr_samples_for_new_attrs(
 			r->last_nr_accesses, old_attrs, new_attrs);
 	if (!aggregating)
@@ -941,8 +963,8 @@ static void damon_update_monitoring_results(struct damon_ctx *ctx,
 
 	damon_for_each_target(t, ctx)
 		damon_for_each_region(r, t)
-			damon_update_monitoring_result(
-					r, old_attrs, new_attrs, aggregating);
+			damon_update_monitoring_result(r, old_attrs, new_attrs,
+					aggregating, ctx);
 }
 
 /*
diff --git a/mm/damon/tests/core-kunit.h b/mm/damon/tests/core-kunit.h
index 9e631b19b184a..b4feef1a460f4 100644
--- a/mm/damon/tests/core-kunit.h
+++ b/mm/damon/tests/core-kunit.h
@@ -591,32 +591,39 @@ static void damon_test_update_monitoring_result(struct kunit *test)
 		.sample_interval = 10, .aggr_interval = 1000,};
 	struct damon_attrs new_attrs;
 	struct damon_region *r = damon_new_region(3, 7);
+	struct damon_ctx *ctx;
 
 	if (!r)
 		kunit_skip(test, "region alloc fail");
+	ctx = damon_new_ctx();
+	if (!ctx) {
+		damon_free_region(r);
+		kunit_skip(test, "ctx alloc fail");
+	}
 
 	r->nr_accesses = 15;
 	r->age = 20;
 
 	new_attrs = (struct damon_attrs){
 		.sample_interval = 100, .aggr_interval = 10000,};
-	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false);
+	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false, ctx);
 	KUNIT_EXPECT_EQ(test, r->nr_accesses, 15);
 	KUNIT_EXPECT_EQ(test, r->age, 2);
 
 	new_attrs = (struct damon_attrs){
 		.sample_interval = 1, .aggr_interval = 1000};
-	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false);
+	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false, ctx);
 	KUNIT_EXPECT_EQ(test, r->nr_accesses, 150);
 	KUNIT_EXPECT_EQ(test, r->age, 2);
 
 	new_attrs = (struct damon_attrs){
 		.sample_interval = 1, .aggr_interval = 100};
-	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false);
+	damon_update_monitoring_result(r, &old_attrs, &new_attrs, false, ctx);
 	KUNIT_EXPECT_EQ(test, r->nr_accesses, 150);
 	KUNIT_EXPECT_EQ(test, r->age, 20);
 
 	damon_free_region(r);
+	damon_destroy_ctx(ctx);
 }
 
 static void damon_test_set_attrs(struct kunit *test)
-- 
2.47.3

[RFC PATCH v1.1 7/7] mm/damon/core: handle unreset probe_hits in probe_hits_mvsum()

[SJ Park]

If damon_update_monitoring_result() is called at the end of the
aggregation interval, probe_hits is not reset.  That's because the value
will be exposed to the user via damon_region_aggregated trace event.

Meanwhile, damon_probe_hits_mvsum() can be called in this state.  Due to
its logic, it will return a value that is incorrectly high.  This could
happen  if the user requested DAMOS schemes applied regions sysfs files
update exactly in the time sequence.

The impact is minor, but better to avoid.  Check the timing and simply
return the fully aggregated last_probe_hits, like
damon_nr_accesses_mvsum() also does.  It is not 100% accurate since it
is the last interval's aggregation.  But better than the value that is
completely reset.

Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/core.c b/mm/damon/core.c
index 9a3ff0fe55a61..093da5c0b3693 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -290,6 +290,9 @@ unsigned char damon_probe_hits_mvsum(int probe_idx, struct damon_region *r,
 			ctx->passed_sample_intervals;
 	left_window_bp = mult_frac(left_window, 10000, window_len);
 
+	if (left_window_bp == 10000)
+		return r->last_probe_hits[probe_idx];
+
 	return damon_mvsum(r->probe_hits[probe_idx],
 			r->last_probe_hits[probe_idx], left_window_bp);
 }
-- 
2.47.3