mm/damon: introduce DAMOS failed region quota charge ratio

TL; DR: Let users set different DAMOS quota charge ratios for DAMOS action failed regions, for deterministic and consistent DAMOS action progress.

Common Reports: Unexpectedly Slow DAMOS
=======================================

One common issue report that we get from DAMON users is that DAMOS action applying progress speed is sometimes much slower than expected. And one common root cause is that the DAMOS quota is exceeded by the action applying failed memory regions.

For example, a group of users tried to run DAMOS-based proactive memory reclamation (DAMON_RECLAIM) with 100 MiB per second DAMOS quota. They ran it on a system having no active workload which means all memory of the system is cold. The expectation was that the system will show 100 MiB per second reclamation until (nearly) all memory is reclaimed. But what they found is that the speed is quite inconsistent and sometimes it becomes very slower than the expectation, sometimes even no reclamation at all for about tens of seconds. The upper limit of the speed (100 MiB per second) was being kept as expected, though.

By monitoring the qt_exceeds (number of DAMOS quota exceed events) DAMOS stat, we found DAMOS quota is always exceeded when the speed is slow. By monitoring sz_tried and sz_applied (the total amount of DAMOS action tried memory and succeeded memory) DAMOS stats together, we found the reclamation attempts nearly always failed when the speed is slow.

DAMOS quota charges DAMOS action tried regions regardless of the successfulness of the try. Hence in the example reported case, there was unreclaimable memory spread around the system memory. Sometimes nearly 100 MiB of memory that DAMOS tried to reclaim in the given quota interval was reclaimable, and therefore showed nearly 100 MiB per second speed. Sometimes nearly 99 MiB of memory that DAMOS was trying to reclaim in the given quota interval was unreclaimable, and therefore showing only about 1 MiB per second reclaim speed.

We explained it is an expected behavior of the feature rather than a bug, as DAMOS quota is there for only the upper-limit of the speed. The users agreed and later reported a huge win from the adoption of DAMON_RECLAIM on their products.

It is Not a Bug but a Feature; But...
=====================================

So nothing is broken. DAMOS quota is working as intended, as the upper limit of the speed. It also provides its behavior observability via DAMOS stat. In the real world production environment that runs long term active workloads and matters stability, the speed sometimes being slow is not a real problem.

But, the non-deterministic behavior is sometimes annoying, especially in lab environments. Even in a realistic production environment, when there is a huge amount of DAMOS action unapplicable memory, the speed could be problematically slow. Let's suppose a virtual machines provider that setup 99% of the host memory as hugetlb pages that cannot be reclaimed, to give it to virtual machines. Also, when aim-oriented DAMOS auto-tuning is applied, this could also make the internal feedback loop confused.

The intention of the current behavior was that trying DAMOS action to regions would anyway impose some overhead, and therefore somehow be charged. But in the real world, the overhead for failed action is much lighter than successful action. Charging those at the same ratio may be unfair, or at least suboptimum in some environments.

DAMOS Action Failed Region Quota Charge Ratio
=============================================

Let users set the charge ratio for the action-failed memory, for more optimal and deterministic use of DAMOS. It allows users to specify the numerator and the denominator of the ratio for flexible setup. For example, let's suppose the numerator and the denominator are set to 1 and 4,096, respectively. The ratio is 1 / 4,096. A DAMOS scheme action is applied to 5 GiB memory. For 1 GiB of the memory, the action is succeeded. For the rest (4 GiB), the action is failed. Then, only 1 GiB and 1 MiB quota is charged.

The optimal charge ratio will depend on the use case and system/workload. I'd recommend starting from setting the nominator as 1 and the denominator as PAGE_SIZE and tune based on the results, because many DAMOS actions are applied at page level.

Tests
=====

I tested this feature in the steps below.

1. Allocate 50% of system memory and mlock() it using a test program.
2. Fill up the page cache to exhaust nearly all free memory.
3. Start DAMON-based proactive reclamation with 100 MiB/second DAMOS hard-quota. Auto-tune the DAMOS soft-quota under the hard-quota for achieving 40% free memory of the system with 'temporal' tuner.

For step 1, I run a simple C program that is written by Gemini. It is quite straightforward, so I'm not sharing the code here.

For step 2, I use dd command like below:

dd if=/dev/zero of=foo bs=1M count=$50_percent_of_system_memory

For step 3, I use the latest version of DAMON user-space tool (damo) like below.

    sudo damo start --damos_action pageout \
            ` # Do the pageout only up to 100 MiB per second ` \
            --damos_quota_space 100M --damos_quota_interval 1s \
            ` # Auto-tune the quota below the hard quota aiming` \
            ` # 40% free memory of the node 0 ` \
            ` # (entire node of the test system)` \
            --damos_quota_goal node_mem_free_bp 40% 0 \
            ` # use temporal tuner, which is easy to understnd ` \
            --damos_quota_goal_tuner temporal

As expected, the progress of the reclamation is not consistent, because the quota is exceeded for the failed reclamation of the unreclaimable memory.

I do this again, but with the failed region charge ratio feature. For this, the above 'damo' command is used, after appending command line option for setup of the charge ratio like below. Note that the option was added to 'damo' after v3.1.9.

    sudo ./damo start --damos_action pageout \
            [...]
            ` # quota-charge only 1/4096 for pageout-failed regions ` \
            --damos_quota_fail_charge_ratio 1 4096

The progress of the reclamation was nearly 100 MiB per second until the goal was achieved, meeting the expectation.

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

The first two patches make preparational changes. Patch 1 updates fully charged quota check to handle <min_region_sz remaining quota, which will be able to exist after this series is applied. Patch 2 merges regions after applying schemes is done as long as it is ok to do, since regions split operations for quota could happen much more frequently under a corner case that this series will make available.

Patch 3 implements the feature and exposes it via DAMON core API. Patch 4 implements DAMON sysfs ABI for the feature. Three following patches (5-7) document the feature and ABI on design, usage, and ABI documents, respectively. Four patches for testing of the new feature follow. Patch 8 implements a kunit test for the feature. Patches 9 and 10 extend DAMON selftest helpers for DAMON sysfs control and internal state dumping for adding a new selftest for the feature. Patch 11 extends existing DAMON sysfs interface selftest to test the new feature using the extended helper scripts.

Changelog
=========

Changes from RFC v5.2
(https://lore.kernel.org/[email protected])
- Rebase to latest mm-new.
Changes from RFC v5.1
(https://lore.kernel.org/[email protected])
- Add missed mergeback fixup.
Changes from RFC v5
(https://lore.kernel.org/[email protected])
- Merge back: merge whatever if it doesn't lose monitoring infomration and not violating min_nr_regions.
Changes from RFC v4
(https://lore.kernel.org/[email protected])
- Fix quota-sliced region merge-back issues.
- Use damon_for_each_region() instead of damon_for_each_region_safe().
- Avoid merging back of sliced but scheme unapplied regions, to keep the monitoring information.
Changes from RFC v3
(https://lore.kernel.org/[email protected])
- Make damos_quota_is_full() safe from overflow and easier to read.
- Avoid quota-based region split making too many new regions. Changes from RFC v2
(https://lore.kernel.org/[email protected])
- Handle <min_region_sz remaining quota.
- Document zero denum behavior.
- Fix typos: s/selftets/selftests/
Changes from RFC v1
(https://lore.kernel.org/[email protected])
- Avoid overflows in charge amount calculation.
- Fix/wordsmith documentation for grammar, typo, and wrong examples.
- Improve unit test for more consistent comparison source use.

SeongJae Park (11):
mm/damon/core: handle <min_region_sz remaining quota as empty mm/damon/core: merge regions after applying DAMOS schemes mm/damon/core: introduce failed region quota charge ratio mm/damon/sysfs-schemes: implement fail_charge_{num,denom} files Docs/mm/damon/design: document fail_charge_{num,denom} Docs/admin-guide/mm/damon/usage: document fail_charge_{num,denom}
files
Docs/ABI/damon: document fail_charge_{num,denom} mm/damon/tests/core-kunit: test fail_charge_{num,denom} committing selftests/damon/_damon_sysfs: support failed region quota charge ratio selftests/damon/drgn_dump_damon_status: support failed region quota
charge ratio
selftests/damon/sysfs.py: test failed region quota charge ratio

 .../ABI/testing/sysfs-kernel-mm-damon         |  12 ++
 Documentation/admin-guide/mm/damon/usage.rst  |  18 ++-
 Documentation/mm/damon/design.rst             |  22 ++++
 include/linux/damon.h                         |   9 ++
 mm/damon/core.c                               | 103 ++++++++++++++++--
 mm/damon/sysfs-schemes.c                      |  54 +++++++++
 mm/damon/tests/core-kunit.h                   |   6 +
 tools/testing/selftests/damon/_damon_sysfs.py |  21 +++-
 .../selftests/damon/drgn_dump_damon_status.py |   2 +
 tools/testing/selftests/damon/sysfs.py        |   6 +
 10 files changed, 236 insertions(+), 17 deletions(-)

base-commit: 5f9ac1ae1aa2393af99ee45c09c991b20087a9be

2.47.3

Less than min_region_sz remaining quota effectively means the quota is fully charged. In other words, no remaining quota. This is because DAMOS actions are applied in the region granularity, and each region should have min_region_sz or larger size. However the existing fully charged quota check, which is also used for setting charge_target_from and charge_addr_from of the quota, is not aware of the case. For the reason, charge_target_from and charge_addr_from of the quota will not be updated in the case. This can result in DAMOS action being applied more frequently to a specific area of the memory.

The case is unreal because quota charging is also made in the region granularity. It could be changed in future, though. Actually, the following commit will make the change, by allowing users to set arbitrary quota charging ratio for action-failed regions. To be prepared for the change, update the fully charged quota checks to treat having less than min_region_sz remaining quota as fully charged.

---
 mm/damon/core.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

index 278594847cf94..888b6ac29feff 100644
@@ -2047,6 +2047,20 @@ static void damos_walk_cancel(struct damon_ctx *ctx)
 	mutex_unlock(&ctx->walk_control_lock);
 }
 
+static bool damos_quota_is_full(struct damos_quota *quota,
+		unsigned long min_region_sz)
+{
+	if (!damos_quota_is_set(quota))
+		return false;
+	if (quota->charged_sz >= quota->esz)
+		return true;
+	/*
+	 * DAMOS action is applied per region, so <min_region_sz remaining
+	 * quota means the quota is effectively full.
+	 */
+	return quota->esz - quota->charged_sz < min_region_sz;
+}
+
 static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		struct damon_region *r, struct damos *s)
 {

@@ -2104,8 +2118,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		quota->total_charged_ns += timespec64_to_ns(&end) -
 			timespec64_to_ns(&begin);
 		quota->charged_sz += sz;
-		if (damos_quota_is_set(quota) &&
-				quota->charged_sz >= quota->esz) {
+		if (damos_quota_is_full(quota, c->min_region_sz)) {
 			quota->charge_target_from = t;
 			quota->charge_addr_from = r->ar.end + 1;
 		}

@@ -2133,8 +2146,7 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 			continue;
 
 		/* Check the quota */
-		if (damos_quota_is_set(quota) &&
-				quota->charged_sz >= quota->esz)
+		if (damos_quota_is_full(quota, c->min_region_sz))
 			continue;
 
 		if (damos_skip_charged_region(t, r, s, c->min_region_sz))

@@ -2460,8 +2472,7 @@ static void damos_adjust_quota(struct damon_ctx *c, struct damos *s)
 	if (!time_in_range_open(jiffies, quota->charged_from,
 				quota->charged_from +
 				msecs_to_jiffies(quota->reset_interval))) {
-		if (damos_quota_is_set(quota) &&
-				quota->charged_sz >= quota->esz)
+		if (damos_quota_is_full(quota, c->min_region_sz))
 			s->stat.qt_exceeds++;
 		quota->total_charged_sz += quota->charged_sz;
 		quota->charged_from = jiffies;

2.47.3

Not a blocker for this patch. Please read below for details.

Good finding. But the user impact is quite small, and anyway this bug is not introduced by this patch. I will separately work on fixing the bug. But this cannot be a blocker of this patch.

Thanks,
SJ

[...]

damos_apply_scheme() could split the given region if applying the scheme's action to the entire region can result in violating the quota-set upper limit. Keeping regions that are created by such split operations is unnecessary overhead.

The overhead would be negligible in the common case because such split operations could happen only up to the number of installed schemes per scheme apply interval. The following commit could make the impact larger, though. The following commit will allow the action-failed region to be charged in a different ratio. If both the ratio and the remaining quota is quite small while the region to apply the scheme is quite large and the action is nearly always failing, a high number of split operations could happen.

Remove the unnecessary overhead by merging regions after applying schemes is done for each region. The merge operation is made only if it will not lose monitoring information and keep min_nr_regions constraint. In the worst case, the max_nr_regions could still be violated until the next per-aggregation interval merge operation is made.

---
 mm/damon/core.c | 59 +++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 55 insertions(+), 4 deletions(-)

index 888b6ac29feff..8e1e19611c387 100644
@@ -2164,6 +2164,58 @@ static void damon_do_apply_schemes(struct damon_ctx *c,
 	}
 }
 
+/*
+ * damos_apply_target() - Apply DAMOS schemes to a given target.
+ * @c:			monitoring context to apply its DAMOS schemes to..
+ * @t:			monitoring target to apply the schemes to.
+ * @max_region_sz:	maximum region size for @c.
+ *
+ * This function could split regions for keeping the quota.  To minimize
+ * overhead from the split operations increased number of regions, this
+ * function will also merge regions after the schemes applying attempt is done,
+ * for each region.  The merge operation is made only when it doesn't lose the
+ * monitoring information and not violating @max_region_sz.
+ *
+ * Hence, after this function is called, the total number of regions could
+ * be increased or reduced.  The increase could make max_nr_regions temporarily
+ * be violated, until the next per-aggregation interval regions merge operation
+ * is executed.  The decrease will not violate min_nr_regions though, since it
+ * keeps @max_region_sz.
+ */
+static void damos_apply_target(struct damon_ctx *c, struct damon_target *t,
+		unsigned long max_region_sz)
+{
+	struct damon_region *r;
+
+	damon_for_each_region(r, t) {
+		struct damon_region *prev_r;
+
+		damon_do_apply_schemes(c, t, r);
+		/*
+		 * damon_do_apply_scheems() could split the region for the
+		 * quota.  Keeping the new slices is an overhead.  Merge back
+		 * the slices into the previous region if it doesn't lose any
+		 * information and not violating the max_region_sz.
+		 */
+		if (damon_first_region(t) == r)
+			continue;
+		prev_r = damon_prev_region(r);
+		if (prev_r->ar.end != r->ar.start)
+			continue;
+		if (prev_r->age != r->age)
+			continue;
+		if (prev_r->last_nr_accesses != r->last_nr_accesses)
+			continue;
+		if (prev_r->nr_accesses != r->nr_accesses)
+			continue;
+		if (r->ar.end - prev_r->ar.start > max_region_sz)
+			continue;
+		prev_r->ar.end = r->ar.end;
+		damon_destroy_region(r, t);
+		r = prev_r;
+	}
+}
+
 /*
  * damon_feed_loop_next_input() - get next input to achieve a target score.
  * @last_input	The last input.

@@ -2533,9 +2585,9 @@ static void damos_trace_stat(struct damon_ctx *c, struct damos *s)
 static void kdamond_apply_schemes(struct damon_ctx *c)
 {
 	struct damon_target *t;
-	struct damon_region *r;
 	struct damos *s;
 	bool has_schemes_to_apply = false;
+	unsigned long max_region_sz;
 
 	damon_for_each_scheme(s, c) {
 		if (time_before(c->passed_sample_intervals, s->next_apply_sis))

@@ -2552,13 +2604,12 @@ static void kdamond_apply_schemes(struct damon_ctx *c)
 	if (!has_schemes_to_apply)
 		return;
 
+	max_region_sz = damon_region_sz_limit(c);
 	mutex_lock(&c->walk_control_lock);
 	damon_for_each_target(t, c) {
 		if (c->ops.target_valid && c->ops.target_valid(t) == false)
 			continue;
-
-		damon_for_each_region(r, t)
-			damon_do_apply_schemes(c, t, r);
+		damos_apply_target(c, t, max_region_sz);
 	}
 
 	damon_for_each_scheme(s, c) {

2.47.3

DAMOS quota is charged to all DAMOS action application attempted memory, regardless of how much of the memory the action was successful and failed. This makes understanding quota behavior without DAMOS stat but only with end level metrics (e.g., increased amount of free memory for DAMOS_PAGEOUT action) difficult. Also, charging action-failed memory same as action-successful memory is somewhat unfair, as successful action application will induce more overhead in most cases.

Introduce DAMON core API for setting the charge ratio for such action-failed memory. It allows API callers to specify the ratio in a flexible way, by setting the numerator and the denominator.

---
 include/linux/damon.h |  9 +++++++++
 mm/damon/core.c       | 21 ++++++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

index d3054af1d964f..5fb1dc585658b 100644
@@ -235,6 +235,8 @@ enum damos_quota_goal_tuner {
  * @goals:		Head of quota tuning goals (&damos_quota_goal) list.
  * @goal_tuner:		Goal-based @esz tuning algorithm to use.
  * @esz:		Effective size quota in bytes.
+ * @fail_charge_num:	Failed regions charge rate numerator.
+ * @fail_charge_denom:	Failed regions charge rate denominator.
  *
  * @weight_sz:		Weight of the region's size for prioritization.
  * @weight_nr_accesses:	Weight of the region's nr_accesses for prioritization.

@@ -264,6 +266,10 @@ enum damos_quota_goal_tuner {
  *
  * The resulting effective size quota in bytes is set to @esz.
  *
+ * For DAMOS action applying failed amount of regions, charging those same to
+ * those that the action has successfully applied may be unfair.  For the
+ * reason, 'the size * @fail_charge_num / @fail_charge_denom' is charged.
+ *
  * For selecting regions within the quota, DAMON prioritizes current scheme's
  * target memory regions using the &struct damon_operations->get_scheme_score.
  * You could customize the prioritization logic by setting &weight_sz,

@@ -278,6 +284,9 @@ struct damos_quota {
 	enum damos_quota_goal_tuner goal_tuner;
 	unsigned long esz;
 
+	unsigned int fail_charge_num;
+	unsigned int fail_charge_denom;
+
 	unsigned int weight_sz;
 	unsigned int weight_nr_accesses;
 	unsigned int weight_age;

index 8e1e19611c387..b20544314ba55 100644
@@ -918,6 +918,8 @@ static int damos_commit_quota(struct damos_quota *dst, struct damos_quota *src)
 	if (err)
 		return err;
 	dst->goal_tuner = src->goal_tuner;
+	dst->fail_charge_num = src->fail_charge_num;
+	dst->fail_charge_denom = src->fail_charge_denom;
 	dst->weight_sz = src->weight_sz;
 	dst->weight_nr_accesses = src->weight_nr_accesses;
 	dst->weight_age = src->weight_age;

@@ -2047,6 +2049,23 @@ static void damos_walk_cancel(struct damon_ctx *ctx)
 	mutex_unlock(&ctx->walk_control_lock);
 }
 
+static void damos_charge_quota(struct damos_quota *quota,
+		unsigned long sz_region, unsigned long sz_applied)
+{
+	/*
+	 * sz_applied could be bigger than sz_region, depending on ops
+	 * implementation of the action, e.g., damos_pa_pageout().  Charge only
+	 * the region size in the case.
+	 */
+	if (!quota->fail_charge_denom || sz_applied > sz_region)
+		quota->charged_sz += sz_region;
+	else
+		quota->charged_sz += sz_applied + mult_frac(
+				(sz_region - sz_applied),
+				quota->fail_charge_num,
+				quota->fail_charge_denom);
+}
+
 static bool damos_quota_is_full(struct damos_quota *quota,
 		unsigned long min_region_sz)
 {

@@ -2117,7 +2136,7 @@ static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
 		ktime_get_coarse_ts64(&end);
 		quota->total_charged_ns += timespec64_to_ns(&end) -
 			timespec64_to_ns(&begin);
-		quota->charged_sz += sz;
+		damos_charge_quota(quota, sz, sz_applied);
 		if (damos_quota_is_full(quota, c->min_region_sz)) {
 			quota->charge_target_from = t;
 			quota->charge_addr_from = r->ar.end + 1;

2.47.3

No blocker in my opinion. Please read below for details.

Yes, the overflow could happen, but the user impact should be negligible. The user could also set addr_unit accordingly. Let's keep this as is for now. We can revisit if this becomes a real issue.

Sashiko's analysis is correct. But the user impact is expected to be minimal. That's because the region split is not expensive. It is anyway supposed to be much smaller than the successful DAMOS action. Also the second patch of this series prevented unconditionally increasing the number of regions in this scenario.

And after all, the user should set the ratio based on the recommendation (1/PAGE_SIZE), or after testing. I think the user impact of this scenario under the recommended configuration should be reasonable.

Thanks,
SJ

[...]

Implement the user-space ABI for the DAMOS action failed region quota-charge ratio setup. For this, add two new sysfs files under the DAMON sysfs interface for DAMOS quotas. Names of the files are fail_charge_num and fail_charge_denom, and work for reading and setting the numerator and denominator of the failed regions charge ratio.

---
 mm/damon/sysfs-schemes.c | 54 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

index c14a564dd8218..c3c1ff6108863 100644
@@ -1509,6 +1509,8 @@ struct damon_sysfs_quotas {
 	unsigned long reset_interval_ms;
 	unsigned long effective_sz;	/* Effective size quota in bytes */
 	enum damos_quota_goal_tuner goal_tuner;
+	unsigned int fail_charge_num;
+	unsigned int fail_charge_denom;
 };
 
 static struct damon_sysfs_quotas *damon_sysfs_quotas_alloc(void)

@@ -1683,6 +1685,48 @@ static ssize_t goal_tuner_store(struct kobject *kobj,
 	return -EINVAL;
 }
 
+static ssize_t fail_charge_num_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_quotas *quotas = container_of(kobj,
+			struct damon_sysfs_quotas, kobj);
+
+	return sysfs_emit(buf, "%u\n", quotas->fail_charge_num);
+}
+
+static ssize_t fail_charge_num_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_quotas *quotas = container_of(kobj,
+			struct damon_sysfs_quotas, kobj);
+	int err = kstrtouint(buf, 0, &quotas->fail_charge_num);
+
+	if (err)
+		return -EINVAL;
+	return count;
+}
+
+static ssize_t fail_charge_denom_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	struct damon_sysfs_quotas *quotas = container_of(kobj,
+			struct damon_sysfs_quotas, kobj);
+
+	return sysfs_emit(buf, "%u\n", quotas->fail_charge_denom);
+}
+
+static ssize_t fail_charge_denom_store(struct kobject *kobj,
+		struct kobj_attribute *attr, const char *buf, size_t count)
+{
+	struct damon_sysfs_quotas *quotas = container_of(kobj,
+			struct damon_sysfs_quotas, kobj);
+	int err = kstrtouint(buf, 0, &quotas->fail_charge_denom);
+
+	if (err)
+		return -EINVAL;
+	return count;
+}
+
 static void damon_sysfs_quotas_release(struct kobject *kobj)
 {
 	kfree(container_of(kobj, struct damon_sysfs_quotas, kobj));

@@ -1703,12 +1747,20 @@ static struct kobj_attribute damon_sysfs_quotas_effective_bytes_attr =
 static struct kobj_attribute damon_sysfs_quotas_goal_tuner_attr =
 		__ATTR_RW_MODE(goal_tuner, 0600);
 
+static struct kobj_attribute damon_sysfs_quotas_fail_charge_num_attr =
+		__ATTR_RW_MODE(fail_charge_num, 0600);
+
+static struct kobj_attribute damon_sysfs_quotas_fail_charge_denom_attr =
+		__ATTR_RW_MODE(fail_charge_denom, 0600);
+
 static struct attribute *damon_sysfs_quotas_attrs[] = {
 	&damon_sysfs_quotas_ms_attr.attr,
 	&damon_sysfs_quotas_sz_attr.attr,
 	&damon_sysfs_quotas_reset_interval_ms_attr.attr,
 	&damon_sysfs_quotas_effective_bytes_attr.attr,
 	&damon_sysfs_quotas_goal_tuner_attr.attr,
+	&damon_sysfs_quotas_fail_charge_num_attr.attr,
+	&damon_sysfs_quotas_fail_charge_denom_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(damon_sysfs_quotas);

@@ -2801,6 +2853,8 @@ static struct damos *damon_sysfs_mk_scheme(
 		.weight_nr_accesses = sysfs_weights->nr_accesses,
 		.weight_age = sysfs_weights->age,
 		.goal_tuner = sysfs_quotas->goal_tuner,
+		.fail_charge_num = sysfs_quotas->fail_charge_num,
+		.fail_charge_denom = sysfs_quotas->fail_charge_denom,
 	};
 	struct damos_watermarks wmarks = {
 		.metric = sysfs_wmarks->metric,

2.47.3

No blocker. Please read below for more details.

Yes, and the real world use case ain't set such extreme input. Even if the overflow happens, the impact is minor.

My opinion is same to the above one.

Thanks,
SJ

[...]

Update DAMON design document for the DAMOS action failed region quota charge ratio.

---
 Documentation/mm/damon/design.rst | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

index 622d24e35961e..fa7392b5a331d 100644
@@ -576,6 +576,28 @@ interface <sysfs_interface>`, refer to :ref:`weights <sysfs_quotas>` part of
 the documentation.
 
 
+.. _damon_design_damos_quotas_failed_memory_charging_ratio:
+
+Action-failed Memory Charging Ratio
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+DAMOS action to a given region can fail for some subsets of the memory of the
+region.  For example, if the action is ``pageout`` and the region has some
+unreclaimable pages, applying the action to the pages will fail.  The amount of
+system resource that is taken for such failed action applications is usually
+different from that for successful action applications.  For such cases, users
+can set different charging ratio for such failed memory.  The ratio can be
+specified using ``fail_charge_num`` and ``fail_charge_denom`` parameters.  The
+two parameters represent the numerator and denominator of the ratio.  The
+feature is enabled only if ``fail_charge_denom`` is not zero.
+
+For example, let's suppose a DAMOS action is applied to a region of 1,000 MiB
+size.  The action is successfully applied to only 700 MiB of the region.
+``fail_charge_num`` and ``fail_charge_denom`` are set to ``1`` and ``1024``,
+respectively.  Then only 700 MiB and 300 KiB of size (``700 MiB + 300 MiB * 1 /
+1024``) will be charged.
+
+
 .. _damon_design_damos_quotas_auto_tuning:
 
 Aim-oriented Feedback-driven Auto-tuning

2.47.3

Update DAMON usage document for the DAMOS action failed regions quota charge ratio control sysfs files.

---
 Documentation/admin-guide/mm/damon/usage.rst | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

index bfdb717441f05..d5548e460857c 100644
@@ -84,7 +84,9 @@ comma (",").
     │ │ │ │ │ │ │ │ sz/min,max
     │ │ │ │ │ │ │ │ nr_accesses/min,max
     │ │ │ │ │ │ │ │ age/min,max
-    │ │ │ │ │ │ │ :ref:`quotas <sysfs_quotas>`/ms,bytes,reset_interval_ms,effective_bytes,goal_tuner
+    │ │ │ │ │ │ │ :ref:`quotas <sysfs_quotas>`/ms,bytes,reset_interval_ms,
+    │ │ │ │ │ │ │     effective_bytes,goal_tuner,
+    │ │ │ │ │ │ │     fail_charge_num,fail_charge_denom
     │ │ │ │ │ │ │ │ weights/sz_permil,nr_accesses_permil,age_permil
     │ │ │ │ │ │ │ │ :ref:`goals <sysfs_schemes_quota_goals>`/nr_goals
     │ │ │ │ │ │ │ │ │ 0/target_metric,target_value,current_value,nid,path

@@ -381,9 +383,10 @@ schemes/<N>/quotas/
 The directory for the :ref:`quotas <damon_design_damos_quotas>` of the given
 DAMON-based operation scheme.
 
-Under ``quotas`` directory, five files (``ms``, ``bytes``,
-``reset_interval_ms``, ``effective_bytes`` and ``goal_tuner``) and two
-directories (``weights`` and ``goals``) exist.
+Under ``quotas`` directory, seven files (``ms``, ``bytes``,
+``reset_interval_ms``, ``effective_bytes``, ``goal_tuner``, ``fail_charge_num``
+and ``fail_charge_denom``) and two directories (``weights`` and ``goals``)
+exist.
 
 You can set the ``time quota`` in milliseconds, ``size quota`` in bytes, and
 ``reset interval`` in milliseconds by writing the values to the three files,

@@ -402,6 +405,13 @@ the background design of the feature and the name of the selectable algorithms.
 Refer to :ref:`goals directory <sysfs_schemes_quota_goals>` for the goals
 setup.
 
+You can set the action-failed memory quota charging ratio by writing the
+numerator and the denominator for the ratio to ``fail_charge_num`` and
+``fail_charge_denom`` files, respectively.  Reading those files will return the
+current set values.  Refer to :ref:`design
+<damon_design_damos_quotas_failed_memory_charging_ratio>` for more details of
+the ratio feature.
+
 The time quota is internally transformed to a size quota.  Between the
 transformed size quota and user-specified size quota, smaller one is applied.
 Based on the user-specified :ref:`goal <sysfs_schemes_quota_goals>`, the

2.47.3

Update DAMON ABI document for the DAMOS action failed regions quota charge ratio control sysfs files.

---
 Documentation/ABI/testing/sysfs-kernel-mm-damon | 12 ++++++++++++
 1 file changed, 12 insertions(+)

index 7059f540940f0..971c22e34e722 100644
@@ -329,6 +329,18 @@ Contact:	SeongJae Park <[email protected]>
 Description:	Writing to and reading from this file sets and gets the
 		goal-based effective quota auto-tuning algorithm to use.
 
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/quotas/fail_charge_num
+Date:		Mar 2026
+Contact:	SeongJae Park <[email protected]>
+Description:	Writing to and reading from this file sets and gets the
+		action-failed memory quota charging ratio numerator.
+
+What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/quotas/fail_charge_denom
+Date:		Mar 2026
+Contact:	SeongJae Park <[email protected]>
+Description:	Writing to and reading from this file sets and gets the
+		action-failed memory quota charging ratio denominator.
+
 What:		/sys/kernel/mm/damon/admin/kdamonds/<K>/contexts/<C>/schemes/<S>/quotas/weights/sz_permil
 Date:		Mar 2022
 Contact:	SeongJae Park <[email protected]>

2.47.3

Extend damos_test_commit_quotas() kunit test to ensure damos_commit_quota() handles fail_charge_{num,denom} parameters.

---
 mm/damon/tests/core-kunit.h | 6 ++++++
 1 file changed, 6 insertions(+)

index 0030f682b23b7..1b23a22ac04c4 100644
@@ -694,6 +694,8 @@ static void damos_test_commit_quota(struct kunit *test)
 		.ms = 2,
 		.sz = 3,
 		.goal_tuner = DAMOS_QUOTA_GOAL_TUNER_CONSIST,
+		.fail_charge_num = 2,
+		.fail_charge_denom = 3,
 		.weight_sz = 4,
 		.weight_nr_accesses = 5,
 		.weight_age = 6,

@@ -703,6 +705,8 @@ static void damos_test_commit_quota(struct kunit *test)
 		.ms = 8,
 		.sz = 9,
 		.goal_tuner = DAMOS_QUOTA_GOAL_TUNER_TEMPORAL,
+		.fail_charge_num = 1,
+		.fail_charge_denom = 1024,
 		.weight_sz = 10,
 		.weight_nr_accesses = 11,
 		.weight_age = 12,

@@ -717,6 +721,8 @@ static void damos_test_commit_quota(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, dst.ms, src.ms);
 	KUNIT_EXPECT_EQ(test, dst.sz, src.sz);
 	KUNIT_EXPECT_EQ(test, dst.goal_tuner, src.goal_tuner);
+	KUNIT_EXPECT_EQ(test, dst.fail_charge_num, src.fail_charge_num);
+	KUNIT_EXPECT_EQ(test, dst.fail_charge_denom, src.fail_charge_denom);
 	KUNIT_EXPECT_EQ(test, dst.weight_sz, src.weight_sz);
 	KUNIT_EXPECT_EQ(test, dst.weight_nr_accesses, src.weight_nr_accesses);
 	KUNIT_EXPECT_EQ(test, dst.weight_age, src.weight_age);

2.47.3

Extend _damon_sysfs.py for DAMOS action failed regions quota charge ratio setup, so that we can add kselftest for the new feature.

---
 tools/testing/selftests/damon/_damon_sysfs.py | 21 +++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

index 120b96ecbd741..8b12cc0484405 100644
@@ -132,14 +132,17 @@ class DamosQuota:
     goals = None                # quota goals
     goal_tuner = None           # quota goal tuner
     reset_interval_ms = None    # quota reset interval
+    fail_charge_num = None
+    fail_charge_denom = None
     weight_sz_permil = None
     weight_nr_accesses_permil = None
     weight_age_permil = None
     scheme = None               # owner scheme
 
     def __init__(self, sz=0, ms=0, goals=None, goal_tuner='consist',
-                 reset_interval_ms=0, weight_sz_permil=0,
-                 weight_nr_accesses_permil=0, weight_age_permil=0):
+                 reset_interval_ms=0, fail_charge_num=0, fail_charge_denom=0,
+                 weight_sz_permil=0, weight_nr_accesses_permil=0,
+                 weight_age_permil=0):
         self.sz = sz
         self.ms = ms
         self.reset_interval_ms = reset_interval_ms

@@ -151,6 +154,8 @@ class DamosQuota:
         for idx, goal in enumerate(self.goals):
             goal.idx = idx
             goal.quota = self
+        self.fail_charge_num = fail_charge_num
+        self.fail_charge_denom = fail_charge_denom
 
     def sysfs_dir(self):
         return os.path.join(self.scheme.sysfs_dir(), 'quotas')

@@ -197,6 +202,18 @@ class DamosQuota:
                 os.path.join(self.sysfs_dir(), 'goal_tuner'), self.goal_tuner)
         if err is not None:
             return err
+
+        err = write_file(
+                os.path.join(self.sysfs_dir(), 'fail_charge_num'),
+                self.fail_charge_num)
+        if err is not None:
+            return err
+        err = write_file(
+                os.path.join(self.sysfs_dir(), 'fail_charge_denom'),
+                self.fail_charge_denom)
+        if err is not None:
+            return err
+
         return None
 
 class DamosWatermarks:

2.47.3

Extend drgn_dump_damon_status.py to dump DAMON internal state for DAMOS action failed regions quota charge ratio, to be able to show if the internal state for the feature is working, with future DAMON selftests.

---
 tools/testing/selftests/damon/drgn_dump_damon_status.py | 2 ++
 1 file changed, 2 insertions(+)

index 5b90eb8e7ef88..972948e6215f1 100755
@@ -112,6 +112,8 @@ def damos_quota_to_dict(quota):
         ['goals', damos_quota_goals_to_list],
         ['goal_tuner', int],
         ['esz', int],
+        ['fail_charge_num', int],
+        ['fail_charge_denom', int],
         ['weight_sz', int],
         ['weight_nr_accesses', int],
         ['weight_age', int],

2.47.3

Extend sysfs.py DAMON selftest to setup DAMOS action failed region quota charge ratio and assert the setup is made into DAMON internal state.

---
 tools/testing/selftests/damon/sysfs.py | 6 ++++++
 1 file changed, 6 insertions(+)

index 387286dc449d5..cd4d82c852113 100755
@@ -73,6 +73,10 @@ def assert_quota_committed(quota, dump):
             }
     assert_true(dump['goal_tuner'] == tuner_val[quota.goal_tuner],
                 'goal_tuner', dump)
+    assert_true(dump['fail_charge_num'] == quota.fail_charge_num,
+                'fail_charge_num', dump)
+    assert_true(dump['fail_charge_denom'] == quota.fail_charge_denom,
+                'fail_charge_denom', dump)
     assert_true(dump['weight_sz'] == quota.weight_sz_permil, 'weight_sz', dump)
     assert_true(dump['weight_nr_accesses'] == quota.weight_nr_accesses_permil,
                 'weight_nr_accesses', dump)

@@ -278,6 +282,8 @@ def main():
                         nid=1)],
                     goal_tuner='temporal',
                     reset_interval_ms=1500,
+                    fail_charge_num=1,
+                    fail_charge_denom=4096,
                     weight_sz_permil=20,
                     weight_nr_accesses_permil=200,
                     weight_age_permil=1000),

2.47.3

Add, thanks.

As mentioned provately, Sashiko claims to have found things which it didn't see in the RFC.

	https://sashiko.dev/#/patchset/[email protected]

Hello Andrew,

TL; DR: I find no blocker for this patch series from the Sashiko reviews.

Now sashiko replies its reviews for DAMON patches to authors and [email protected]. So I replied [1,2,3] my review of the reviews to those on [email protected] mailing list. As I mentioned on the TL;DR, I find no blocker for this series.

And I think you didn't see those because those are sent to only authors and [email protected].

I nowadays reply-all to original recipients only if Sashiko found a blocker. I will also add short notice for non-RFC patches if Sashiko found zero issue. Let me know if you want a different approach.

If it helps, maybe I can also ask Sashiko to send the DAMON patch review mails to you together. Let me know if you want that.

[1] https://lore.kernel.org/[email protected]
[2] https://lore.kernel.org/[email protected]
[3] https://lore.kernel.org/[email protected]

Thanks,
SJ

... Now I think the short notice is only redundant and silly, as the full review is available on damon@ list and the one who primarily interested in (Andrew) understands that. I feel like the short-notice reply-all only increase unnecessary traffic and my redundant typing. I will not do the short notice broadcasting, unless someone makes a diffeernt voice.

Thanks,
SJ

[...]

For Damon Andrew should for now just trust your ACKs. If it has your ACK, it's good to go.

In the future, I expect you would pick up the patches yourself, which is where you as the component maintainer would look for any blockers.

So for Damon patches I don't think we need the AI review notices from Andrew.

Cheers,

David

On Fri, 1 May 2026 08:49:43 +0200 "David Hildenbrand (Arm)" <[email protected]> wrote:
[...]

FWIW I'm picking DAMON patches to damon/next tree on my own, for my testing and for a case that Andrew might miss those.

All make sense to me :)

Thanks,
SJ

[...]