path: root/system/easy-kernel/0252-rectify-ksm-inheritance.patch

From fe014f52184ec1a059184ef9e0262a3e0670a90d Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Fri, 22 Sep 2023 14:11:40 -0700
Subject: [PATCH 1/9] mm/ksm: support fork/exec for prctl

Today we have two ways to enable KSM:

1) madvise system call
   This allows to enable KSM for a memory region for a long time.

2) prctl system call
   This is a recent addition to enable KSM for the complete process.
   In addition when a process is forked, the KSM setting is inherited.

This change only affects the second case.

One of the use cases for (2) was to support the ability to enable
KSM for cgroups. This allows systemd to enable KSM for the seed
process. By enabling it in the seed process all child processes inherit
the setting.

This works correctly when the process is forked. However it doesn't
support fork/exec workflow.

From the previous cover letter:

....
Use case 3:
With the madvise call sharing opportunities are only enabled for the
current process: it is a workload-local decision. A considerable number
of sharing opportunities may exist across multiple workloads or jobs
(if they are part of the same security domain). Only a higler level
entity like a job scheduler or container can know for certain if its
running one or more instances of a job. That job scheduler however
doesn't have the necessary internal workload knowledge to make targeted
madvise calls.
....

In addition it can also be a bit surprising that fork keeps the KSM
setting and fork/exec does not.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Fixes: d7597f59d1d3 ("mm: add new api to enable ksm per process")
Reviewed-by: David Hildenbrand <david@redhat.com>
Reported-by: Carl Klemm <carl@uvos.xyz>
Tested-by: Carl Klemm <carl@uvos.xyz>
---
 include/linux/sched/coredump.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 1b37fa8fc..32414e891 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -87,10 +87,13 @@ static inline int get_dumpable(struct mm_struct *mm)
 
 #define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
 
+#define MMF_VM_MERGE_ANY	29
+#define MMF_VM_MERGE_ANY_MASK	(1 << MMF_VM_MERGE_ANY)
+
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
-				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)
+				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK |\
+				 MMF_VM_MERGE_ANY_MASK)
 
-#define MMF_VM_MERGE_ANY	29
 #define MMF_HAS_MDWE_NO_INHERIT	30
 
 static inline unsigned long mmf_init_flags(unsigned long flags)
-- 
2.43.0.rc2


From 41a07b06dc7b70da6afa1b8340fcf33a5f02121d Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Wed, 27 Sep 2023 09:22:19 -0700
Subject: [PATCH 2/9] mm/ksm: add "smart" page scanning mode

This change adds a "smart" page scanning mode for KSM. So far all the
candidate pages are continuously scanned to find candidates for
de-duplication. There are a considerably number of pages that cannot be
de-duplicated. This is costly in terms of CPU. By using smart scanning
considerable CPU savings can be achieved.

This change takes the history of scanning pages into account and skips
the page scanning of certain pages for a while if de-deduplication for
this page has not been successful in the past.

To do this it introduces two new fields in the ksm_rmap_item structure:
age and remaining_skips. age, is the KSM age and remaining_skips
determines how often scanning of this page is skipped. The age field is
incremented each time the page is scanned and the page cannot be de-
duplicated. age updated is capped at U8_MAX.

How often a page is skipped is dependent how often de-duplication has
been tried so far and the number of skips is currently limited to 8.
This value has shown to be effective with different workloads.

The feature is enabled by default and can be disabled with the new
smart_scan knob.

The feature has shown to be very effective: upt to 25% of the page scans
can be eliminated; the pages_to_scan rate can be reduced by 40 - 50% and
a similar de-duplication rate can be maintained.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 mm/ksm.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index 981af9c72..c0a2e7759 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -56,6 +56,8 @@
 #define DO_NUMA(x)	do { } while (0)
 #endif
 
+typedef u8 rmap_age_t;
+
 /**
  * DOC: Overview
  *
@@ -193,6 +195,8 @@ struct ksm_stable_node {
  * @node: rb node of this rmap_item in the unstable tree
  * @head: pointer to stable_node heading this list in the stable tree
  * @hlist: link into hlist of rmap_items hanging off that stable_node
+ * @age: number of scan iterations since creation
+ * @remaining_skips: how many scans to skip
  */
 struct ksm_rmap_item {
 	struct ksm_rmap_item *rmap_list;
@@ -205,6 +209,8 @@ struct ksm_rmap_item {
 	struct mm_struct *mm;
 	unsigned long address;		/* + low bits used for flags below */
 	unsigned int oldchecksum;	/* when unstable */
+	rmap_age_t age;
+	rmap_age_t remaining_skips;
 	union {
 		struct rb_node node;	/* when node of unstable tree */
 		struct {		/* when listed from stable tree */
@@ -281,6 +287,9 @@ static unsigned int zero_checksum __read_mostly;
 /* Whether to merge empty (zeroed) pages with actual zero pages */
 static bool ksm_use_zero_pages __read_mostly;
 
+/* Skip pages that couldn't be de-duplicated previously  */
+static bool ksm_smart_scan = 1;
+
 /* The number of zero pages which is placed by KSM */
 unsigned long ksm_zero_pages;
 
@@ -2305,6 +2314,73 @@ static struct ksm_rmap_item *get_next_rmap_item(struct ksm_mm_slot *mm_slot,
 	return rmap_item;
 }
 
+/*
+ * Calculate skip age for the ksm page age. The age determines how often
+ * de-duplicating has already been tried unsuccessfully. If the age is
+ * smaller, the scanning of this page is skipped for less scans.
+ *
+ * @age: rmap_item age of page
+ */
+static unsigned int skip_age(rmap_age_t age)
+{
+	if (age <= 3)
+		return 1;
+	if (age <= 5)
+		return 2;
+	if (age <= 8)
+		return 4;
+
+	return 8;
+}
+
+/*
+ * Determines if a page should be skipped for the current scan.
+ *
+ * @page: page to check
+ * @rmap_item: associated rmap_item of page
+ */
+static bool should_skip_rmap_item(struct page *page,
+				  struct ksm_rmap_item *rmap_item)
+{
+	rmap_age_t age;
+
+	if (!ksm_smart_scan)
+		return false;
+
+	/*
+	 * Never skip pages that are already KSM; pages cmp_and_merge_page()
+	 * will essentially ignore them, but we still have to process them
+	 * properly.
+	 */
+	if (PageKsm(page))
+		return false;
+
+	age = rmap_item->age;
+	if (age != U8_MAX)
+		rmap_item->age++;
+
+	/*
+	 * Smaller ages are not skipped, they need to get a chance to go
+	 * through the different phases of the KSM merging.
+	 */
+	if (age < 3)
+		return false;
+
+	/*
+	 * Are we still allowed to skip? If not, then don't skip it
+	 * and determine how much more often we are allowed to skip next.
+	 */
+	if (!rmap_item->remaining_skips) {
+		rmap_item->remaining_skips = skip_age(age);
+		return false;
+	}
+
+	/* Skip this page */
+	rmap_item->remaining_skips--;
+	remove_rmap_item_from_tree(rmap_item);
+	return true;
+}
+
 static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 {
 	struct mm_struct *mm;
@@ -2409,6 +2485,10 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 				if (rmap_item) {
 					ksm_scan.rmap_list =
 							&rmap_item->rmap_list;
+
+					if (should_skip_rmap_item(*page, rmap_item))
+						goto next_page;
+
 					ksm_scan.address += PAGE_SIZE;
 				} else
 					put_page(*page);
@@ -3449,6 +3529,28 @@ static ssize_t full_scans_show(struct kobject *kobj,
 }
 KSM_ATTR_RO(full_scans);
 
+static ssize_t smart_scan_show(struct kobject *kobj,
+			       struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%u\n", ksm_smart_scan);
+}
+
+static ssize_t smart_scan_store(struct kobject *kobj,
+				struct kobj_attribute *attr,
+				const char *buf, size_t count)
+{
+	int err;
+	bool value;
+
+	err = kstrtobool(buf, &value);
+	if (err)
+		return -EINVAL;
+
+	ksm_smart_scan = value;
+	return count;
+}
+KSM_ATTR(smart_scan);
+
 static struct attribute *ksm_attrs[] = {
 	&sleep_millisecs_attr.attr,
 	&pages_to_scan_attr.attr,
@@ -3469,6 +3571,7 @@ static struct attribute *ksm_attrs[] = {
 	&stable_node_chains_prune_millisecs_attr.attr,
 	&use_zero_pages_attr.attr,
 	&general_profit_attr.attr,
+	&smart_scan_attr.attr,
 	NULL,
 };
 
-- 
2.43.0.rc2


From ad6d220dab3aa4acc08457a201a72ec344076f00 Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Wed, 27 Sep 2023 09:22:20 -0700
Subject: [PATCH 3/9] mm/ksm: add pages_skipped metric

This change adds the "pages skipped" metric. To be able to evaluate how
successful smart page scanning is, the pages skipped metric can be
compared to the pages scanned metric.

The pages skipped metric is a cumulative counter. The counter is stored
under /sys/kernel/mm/ksm/pages_skipped.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 mm/ksm.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index c0a2e7759..1df25a66f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -293,6 +293,9 @@ static bool ksm_smart_scan = 1;
 /* The number of zero pages which is placed by KSM */
 unsigned long ksm_zero_pages;
 
+/* The number of pages that have been skipped due to "smart scanning" */
+static unsigned long ksm_pages_skipped;
+
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
@@ -2376,6 +2379,7 @@ static bool should_skip_rmap_item(struct page *page,
 	}
 
 	/* Skip this page */
+	ksm_pages_skipped++;
 	rmap_item->remaining_skips--;
 	remove_rmap_item_from_tree(rmap_item);
 	return true;
@@ -3463,6 +3467,13 @@ static ssize_t pages_volatile_show(struct kobject *kobj,
 }
 KSM_ATTR_RO(pages_volatile);
 
+static ssize_t pages_skipped_show(struct kobject *kobj,
+				  struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", ksm_pages_skipped);
+}
+KSM_ATTR_RO(pages_skipped);
+
 static ssize_t ksm_zero_pages_show(struct kobject *kobj,
 				struct kobj_attribute *attr, char *buf)
 {
@@ -3560,6 +3571,7 @@ static struct attribute *ksm_attrs[] = {
 	&pages_sharing_attr.attr,
 	&pages_unshared_attr.attr,
 	&pages_volatile_attr.attr,
+	&pages_skipped_attr.attr,
 	&ksm_zero_pages_attr.attr,
 	&full_scans_attr.attr,
 #ifdef CONFIG_NUMA
-- 
2.43.0.rc2


From 1c5c269d3fa05812a7da32bf5f83f6b9bdc8d6c4 Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Wed, 27 Sep 2023 09:22:21 -0700
Subject: [PATCH 4/9] mm/ksm: document smart scan mode

This adds documentation for the smart scan mode of KSM.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/ksm.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index 776f244bd..2b38a8bb0 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -155,6 +155,15 @@ stable_node_chains_prune_millisecs
         scan. It's a noop if not a single KSM page hit the
         ``max_page_sharing`` yet.
 
+smart_scan
+        Historically KSM checked every candidate page for each scan. It did
+        not take into account historic information.  When smart scan is
+        enabled, pages that have previously not been de-duplicated get
+        skipped. How often these pages are skipped depends on how often
+        de-duplication has already been tried and failed. By default this
+        optimization is enabled. The ``pages_skipped`` metric shows how
+        effective the setting is.
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit
-- 
2.43.0.rc2


From 218c97d1ef7ad28a79f1def130257c59b3a2a7a1 Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Wed, 27 Sep 2023 09:22:22 -0700
Subject: [PATCH 5/9] mm/ksm: document pages_skipped sysfs knob

This adds documentation for the new metric pages_skipped.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Reviewed-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/ksm.rst | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index 2b38a8bb0..0cadde17a 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -178,6 +178,8 @@ pages_unshared
         how many pages unique but repeatedly checked for merging
 pages_volatile
         how many pages changing too fast to be placed in a tree
+pages_skipped
+        how many pages did the "smart" page scanning algorithm skip
 full_scans
         how many times all mergeable areas have been scanned
 stable_node_chains
-- 
2.43.0.rc2


From 692c5e04efe16bc1f354376c156832d7fbd8a0c3 Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Mon, 18 Dec 2023 15:10:51 -0800
Subject: [PATCH 6/9] mm/ksm: add ksm advisor

This adds the ksm advisor. The ksm advisor automatically manages the
pages_to_scan setting to achieve a target scan time. The target scan
time defines how many seconds it should take to scan all the candidate
KSM pages. In other words the pages_to_scan rate is changed by the
advisor to achieve the target scan time. The algorithm has a max and min
value to:
- guarantee responsiveness to changes
- limit CPU resource consumption

The respective parameters are:
- ksm_advisor_target_scan_time (how many seconds a scan should take)
- ksm_advisor_max_cpu (maximum value for cpu percent usage)

- ksm_advisor_min_pages (minimum value for pages_to_scan per batch)
- ksm_advisor_max_pages (maximum value for pages_to_scan per batch)

The algorithm calculates the change value based on the target scan time
and the previous scan time. To avoid pertubations an exponentially
weighted moving average is applied.

The advisor is managed by two main parameters: target scan time,
cpu max time for the ksmd background thread. These parameters determine
how aggresive ksmd scans.

In addition there are min and max values for the pages_to_scan parameter
to make sure that its initial and max values are not set too low or too
high. This ensures that it is able to react to changes quickly enough.

The default values are:
- target scan time: 200 secs
- max cpu: 70%
- min pages: 500
- max pages: 30000

By default the advisor is disabled. Currently there are two advisors:
none and scan-time.

Tests with various workloads have shown considerable CPU savings. Most
of the workloads I have investigated have more candidate pages during
startup, once the workload is stable in terms of memory, the number of
candidate pages is reduced. Without the advisor, the pages_to_scan needs
to be sized for the maximum number of candidate pages. So having this
advisor definitely helps in reducing CPU consumption.

For the instagram workload, the advisor achieves a 25% CPU reduction.
Once the memory is stable, the pages_to_scan parameter gets reduced to
about 40% of its max value.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/ksm.c | 158 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 157 insertions(+), 1 deletion(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 1df25a66f..aef991e20 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -21,6 +21,7 @@
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/coredump.h>
+#include <linux/sched/cputime.h>
 #include <linux/rwsem.h>
 #include <linux/pagemap.h>
 #include <linux/rmap.h>
@@ -248,6 +249,9 @@ static struct kmem_cache *rmap_item_cache;
 static struct kmem_cache *stable_node_cache;
 static struct kmem_cache *mm_slot_cache;
 
+/* Default number of pages to scan per batch */
+#define DEFAULT_PAGES_TO_SCAN 100
+
 /* The number of pages scanned */
 static unsigned long ksm_pages_scanned;
 
@@ -276,7 +280,7 @@ static unsigned int ksm_stable_node_chains_prune_millisecs = 2000;
 static int ksm_max_page_sharing = 256;
 
 /* Number of pages ksmd should scan in one batch */
-static unsigned int ksm_thread_pages_to_scan = 100;
+static unsigned int ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
 
 /* Milliseconds ksmd should sleep between batches */
 static unsigned int ksm_thread_sleep_millisecs = 20;
@@ -296,6 +300,152 @@ unsigned long ksm_zero_pages;
 /* The number of pages that have been skipped due to "smart scanning" */
 static unsigned long ksm_pages_skipped;
 
+/* Don't scan more than max pages per batch. */
+static unsigned long ksm_advisor_max_pages_to_scan = 30000;
+
+/* Min CPU for scanning pages per scan */
+#define KSM_ADVISOR_MIN_CPU 10
+
+/* Max CPU for scanning pages per scan */
+static unsigned int ksm_advisor_max_cpu =  70;
+
+/* Target scan time in seconds to analyze all KSM candidate pages. */
+static unsigned long ksm_advisor_target_scan_time = 200;
+
+/* Exponentially weighted moving average. */
+#define EWMA_WEIGHT 30
+
+/**
+ * struct advisor_ctx - metadata for KSM advisor
+ * @start_scan: start time of the current scan
+ * @scan_time: scan time of previous scan
+ * @change: change in percent to pages_to_scan parameter
+ * @cpu_time: cpu time consumed by the ksmd thread in the previous scan
+ */
+struct advisor_ctx {
+	ktime_t start_scan;
+	unsigned long scan_time;
+	unsigned long change;
+	unsigned long long cpu_time;
+};
+static struct advisor_ctx advisor_ctx;
+
+/* Define different advisor's */
+enum ksm_advisor_type {
+	KSM_ADVISOR_NONE,
+	KSM_ADVISOR_SCAN_TIME,
+};
+static enum ksm_advisor_type ksm_advisor;
+
+static inline void advisor_start_scan(void)
+{
+	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+		advisor_ctx.start_scan = ktime_get();
+}
+
+/*
+ * Use previous scan time if available, otherwise use current scan time as an
+ * approximation for the previous scan time.
+ */
+static inline unsigned long prev_scan_time(struct advisor_ctx *ctx,
+					   unsigned long scan_time)
+{
+	return ctx->scan_time ? ctx->scan_time : scan_time;
+}
+
+/* Calculate exponential weighted moving average */
+static unsigned long ewma(unsigned long prev, unsigned long curr)
+{
+	return ((100 - EWMA_WEIGHT) * prev + EWMA_WEIGHT * curr) / 100;
+}
+
+/*
+ * The scan time advisor is based on the current scan rate and the target
+ * scan rate.
+ *
+ *      new_pages_to_scan = pages_to_scan * (scan_time / target_scan_time)
+ *
+ * To avoid perturbations it calculates a change factor of previous changes.
+ * A new change factor is calculated for each iteration and it uses an
+ * exponentially weighted moving average. The new pages_to_scan value is
+ * multiplied with that change factor:
+ *
+ *      new_pages_to_scan *= change facor
+ *
+ * The new_pages_to_scan value is limited by the cpu min and max values. It
+ * calculates the cpu percent for the last scan and calculates the new
+ * estimated cpu percent cost for the next scan. That value is capped by the
+ * cpu min and max setting.
+ *
+ * In addition the new pages_to_scan value is capped by the max and min
+ * limits.
+ */
+static void scan_time_advisor(void)
+{
+	unsigned int cpu_percent;
+	unsigned long cpu_time;
+	unsigned long cpu_time_diff;
+	unsigned long cpu_time_diff_ms;
+	unsigned long pages;
+	unsigned long per_page_cost;
+	unsigned long factor;
+	unsigned long change;
+	unsigned long last_scan_time;
+	unsigned long scan_time;
+
+	/* Convert scan time to seconds */
+	scan_time = div_s64(ktime_ms_delta(ktime_get(), advisor_ctx.start_scan),
+			    MSEC_PER_SEC);
+	scan_time = scan_time ? scan_time : 1;
+
+	/* Calculate CPU consumption of ksmd background thread */
+	cpu_time = task_sched_runtime(current);
+	cpu_time_diff = cpu_time - advisor_ctx.cpu_time;
+	cpu_time_diff_ms = cpu_time_diff / 1000 / 1000;
+
+	cpu_percent = (cpu_time_diff_ms * 100) / (scan_time * 1000);
+	cpu_percent = cpu_percent ? cpu_percent : 1;
+	last_scan_time = prev_scan_time(&advisor_ctx, scan_time);
+
+	/* Calculate scan time as percentage of target scan time */
+	factor = ksm_advisor_target_scan_time * 100 / scan_time;
+	factor = factor ? factor : 1;
+
+	/*
+	 * Calculate scan time as percentage of last scan time and use
+	 * exponentially weighted average to smooth it
+	 */
+	change = scan_time * 100 / last_scan_time;
+	change = change ? change : 1;
+	change = ewma(advisor_ctx.change, change);
+
+	/* Calculate new scan rate based on target scan rate. */
+	pages = ksm_thread_pages_to_scan * 100 / factor;
+	/* Update pages_to_scan by weighted change percentage. */
+	pages = pages * change / 100;
+
+	/* Cap new pages_to_scan value */
+	per_page_cost = ksm_thread_pages_to_scan / cpu_percent;
+	per_page_cost = per_page_cost ? per_page_cost : 1;
+
+	pages = min(pages, per_page_cost * ksm_advisor_max_cpu);
+	pages = max(pages, per_page_cost * KSM_ADVISOR_MIN_CPU);
+	pages = min(pages, ksm_advisor_max_pages_to_scan);
+
+	/* Update advisor context */
+	advisor_ctx.change = change;
+	advisor_ctx.scan_time = scan_time;
+	advisor_ctx.cpu_time = cpu_time;
+
+	ksm_thread_pages_to_scan = pages;
+}
+
+static void advisor_stop_scan(void)
+{
+	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+		scan_time_advisor();
+}
+
 #ifdef CONFIG_NUMA
 /* Zeroed when merging across nodes is not allowed */
 static unsigned int ksm_merge_across_nodes = 1;
@@ -2400,6 +2550,7 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 
 	mm_slot = ksm_scan.mm_slot;
 	if (mm_slot == &ksm_mm_head) {
+		advisor_start_scan();
 		trace_ksm_start_scan(ksm_scan.seqnr, ksm_rmap_items);
 
 		/*
@@ -2557,6 +2708,8 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
 	if (mm_slot != &ksm_mm_head)
 		goto next_mm;
 
+	advisor_stop_scan();
+
 	trace_ksm_stop_scan(ksm_scan.seqnr, ksm_rmap_items);
 	ksm_scan.seqnr++;
 	return NULL;
@@ -3243,6 +3396,9 @@ static ssize_t pages_to_scan_store(struct kobject *kobj,
 	unsigned int nr_pages;
 	int err;
 
+	if (ksm_advisor != KSM_ADVISOR_NONE)
+		return -EINVAL;
+
 	err = kstrtouint(buf, 10, &nr_pages);
 	if (err)
 		return -EINVAL;
-- 
2.43.0.rc2


From 3b9c233ed130557f396326ff74be98afd0819775 Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Mon, 18 Dec 2023 15:10:52 -0800
Subject: [PATCH 7/9] mm/ksm: add sysfs knobs for advisor

This adds four new knobs for the KSM advisor to influence its behaviour.

The knobs are:
- advisor_mode:
    none:      no advisor (default)
    scan-time: scan time advisor
- advisor_max_cpu: 70 (default, cpu usage percent)
- advisor_min_pages_to_scan: 500 (default)
- advisor_max_pages_to_scan: 30000 (default)
- advisor_target_scan_time: 200 (default in seconds)

The new values will take effect on the next scan round.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
 mm/ksm.c | 148 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 148 insertions(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index aef991e20..c3bc292b1 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -337,6 +337,25 @@ enum ksm_advisor_type {
 };
 static enum ksm_advisor_type ksm_advisor;
 
+#ifdef CONFIG_SYSFS
+/*
+ * Only called through the sysfs control interface:
+ */
+
+/* At least scan this many pages per batch. */
+static unsigned long ksm_advisor_min_pages_to_scan = 500;
+
+static void set_advisor_defaults(void)
+{
+	if (ksm_advisor == KSM_ADVISOR_NONE) {
+		ksm_thread_pages_to_scan = DEFAULT_PAGES_TO_SCAN;
+	} else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME) {
+		advisor_ctx = (const struct advisor_ctx){ 0 };
+		ksm_thread_pages_to_scan = ksm_advisor_min_pages_to_scan;
+	}
+}
+#endif /* CONFIG_SYSFS */
+
 static inline void advisor_start_scan(void)
 {
 	if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
@@ -3718,6 +3737,130 @@ static ssize_t smart_scan_store(struct kobject *kobj,
 }
 KSM_ATTR(smart_scan);
 
+static ssize_t advisor_mode_show(struct kobject *kobj,
+				 struct kobj_attribute *attr, char *buf)
+{
+	const char *output;
+
+	if (ksm_advisor == KSM_ADVISOR_NONE)
+		output = "[none] scan-time";
+	else if (ksm_advisor == KSM_ADVISOR_SCAN_TIME)
+		output = "none [scan-time]";
+
+	return sysfs_emit(buf, "%s\n", output);
+}
+
+static ssize_t advisor_mode_store(struct kobject *kobj,
+				  struct kobj_attribute *attr, const char *buf,
+				  size_t count)
+{
+	enum ksm_advisor_type curr_advisor = ksm_advisor;
+
+	if (sysfs_streq("scan-time", buf))
+		ksm_advisor = KSM_ADVISOR_SCAN_TIME;
+	else if (sysfs_streq("none", buf))
+		ksm_advisor = KSM_ADVISOR_NONE;
+	else
+		return -EINVAL;
+
+	/* Set advisor default values */
+	if (curr_advisor != ksm_advisor)
+		set_advisor_defaults();
+
+	return count;
+}
+KSM_ATTR(advisor_mode);
+
+static ssize_t advisor_max_cpu_show(struct kobject *kobj,
+				    struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%u\n", ksm_advisor_max_cpu);
+}
+
+static ssize_t advisor_max_cpu_store(struct kobject *kobj,
+				     struct kobj_attribute *attr,
+				     const char *buf, size_t count)
+{
+	int err;
+	unsigned long value;
+
+	err = kstrtoul(buf, 10, &value);
+	if (err)
+		return -EINVAL;
+
+	ksm_advisor_max_cpu = value;
+	return count;
+}
+KSM_ATTR(advisor_max_cpu);
+
+static ssize_t advisor_min_pages_to_scan_show(struct kobject *kobj,
+					struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", ksm_advisor_min_pages_to_scan);
+}
+
+static ssize_t advisor_min_pages_to_scan_store(struct kobject *kobj,
+					struct kobj_attribute *attr,
+					const char *buf, size_t count)
+{
+	int err;
+	unsigned long value;
+
+	err = kstrtoul(buf, 10, &value);
+	if (err)
+		return -EINVAL;
+
+	ksm_advisor_min_pages_to_scan = value;
+	return count;
+}
+KSM_ATTR(advisor_min_pages_to_scan);
+
+static ssize_t advisor_max_pages_to_scan_show(struct kobject *kobj,
+					struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", ksm_advisor_max_pages_to_scan);
+}
+
+static ssize_t advisor_max_pages_to_scan_store(struct kobject *kobj,
+					struct kobj_attribute *attr,
+					const char *buf, size_t count)
+{
+	int err;
+	unsigned long value;
+
+	err = kstrtoul(buf, 10, &value);
+	if (err)
+		return -EINVAL;
+
+	ksm_advisor_max_pages_to_scan = value;
+	return count;
+}
+KSM_ATTR(advisor_max_pages_to_scan);
+
+static ssize_t advisor_target_scan_time_show(struct kobject *kobj,
+					     struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", ksm_advisor_target_scan_time);
+}
+
+static ssize_t advisor_target_scan_time_store(struct kobject *kobj,
+					      struct kobj_attribute *attr,
+					      const char *buf, size_t count)
+{
+	int err;
+	unsigned long value;
+
+	err = kstrtoul(buf, 10, &value);
+	if (err)
+		return -EINVAL;
+	if (value < 1)
+		return -EINVAL;
+
+	ksm_advisor_target_scan_time = value;
+	return count;
+}
+KSM_ATTR(advisor_target_scan_time);
+
 static struct attribute *ksm_attrs[] = {
 	&sleep_millisecs_attr.attr,
 	&pages_to_scan_attr.attr,
@@ -3740,6 +3883,11 @@ static struct attribute *ksm_attrs[] = {
 	&use_zero_pages_attr.attr,
 	&general_profit_attr.attr,
 	&smart_scan_attr.attr,
+	&advisor_mode_attr.attr,
+	&advisor_max_cpu_attr.attr,
+	&advisor_min_pages_to_scan_attr.attr,
+	&advisor_max_pages_to_scan_attr.attr,
+	&advisor_target_scan_time_attr.attr,
 	NULL,
 };
 
-- 
2.43.0.rc2


From bd5a62b2620729cbe4c6625341e5dcac471fc21c Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Mon, 18 Dec 2023 15:10:53 -0800
Subject: [PATCH 8/9] mm/ksm: add tracepoint for ksm advisor

This adds a new tracepoint for the ksm advisor. It reports the last scan
time, the new setting of the pages_to_scan parameter and the average cpu
percent usage of the ksmd background thread for the last scan.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
 include/trace/events/ksm.h | 33 +++++++++++++++++++++++++++++++++
 mm/ksm.c                   |  1 +
 2 files changed, 34 insertions(+)

diff --git a/include/trace/events/ksm.h b/include/trace/events/ksm.h
index b5ac35c1d..e728647b5 100644
--- a/include/trace/events/ksm.h
+++ b/include/trace/events/ksm.h
@@ -245,6 +245,39 @@ TRACE_EVENT(ksm_remove_rmap_item,
 			__entry->pfn, __entry->rmap_item, __entry->mm)
 );
 
+/**
+ * ksm_advisor - called after the advisor has run
+ *
+ * @scan_time:		scan time in seconds
+ * @pages_to_scan:	new pages_to_scan value
+ * @cpu_percent:	cpu usage in percent
+ *
+ * Allows to trace the ksm advisor.
+ */
+TRACE_EVENT(ksm_advisor,
+
+	TP_PROTO(s64 scan_time, unsigned long pages_to_scan,
+		 unsigned int cpu_percent),
+
+	TP_ARGS(scan_time, pages_to_scan, cpu_percent),
+
+	TP_STRUCT__entry(
+		__field(s64,		scan_time)
+		__field(unsigned long,	pages_to_scan)
+		__field(unsigned int,	cpu_percent)
+	),
+
+	TP_fast_assign(
+		__entry->scan_time	= scan_time;
+		__entry->pages_to_scan	= pages_to_scan;
+		__entry->cpu_percent	= cpu_percent;
+	),
+
+	TP_printk("ksm scan time %lld pages_to_scan %lu cpu percent %u",
+			__entry->scan_time, __entry->pages_to_scan,
+			__entry->cpu_percent)
+);
+
 #endif /* _TRACE_KSM_H */
 
 /* This part must be outside protection */
diff --git a/mm/ksm.c b/mm/ksm.c
index c3bc292b1..a1b5aa12a 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -457,6 +457,7 @@ static void scan_time_advisor(void)
 	advisor_ctx.cpu_time = cpu_time;
 
 	ksm_thread_pages_to_scan = pages;
+	trace_ksm_advisor(scan_time, pages, cpu_percent);
 }
 
 static void advisor_stop_scan(void)
-- 
2.43.0.rc2


From 7a0a7aa00db82570f827e5caadbafa5874e047db Mon Sep 17 00:00:00 2001
From: Stefan Roesch <shr@devkernel.io>
Date: Mon, 18 Dec 2023 15:10:54 -0800
Subject: [PATCH 9/9] mm/ksm: document ksm advisor and its sysfs knobs

This documents the KSM advisor and its new knobs in /sys/fs/kernel/mm.

Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
---
 Documentation/admin-guide/mm/ksm.rst | 55 ++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index 0cadde17a..ad2bb8771 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -80,6 +80,9 @@ pages_to_scan
         how many pages to scan before ksmd goes to sleep
         e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
 
+        The pages_to_scan value cannot be changed if ``advisor_mode`` has
+        been set to scan-time.
+
         Default: 100 (chosen for demonstration purposes)
 
 sleep_millisecs
@@ -164,6 +167,29 @@ smart_scan
         optimization is enabled. The ``pages_skipped`` metric shows how
         effective the setting is.
 
+advisor_mode
+        The ``advisor_mode`` selects the current advisor. Two modes are
+        supported: none and scan-time. The default is none. By setting
+        ``advisor_mode`` to scan-time, the scan time advisor is enabled.
+        The section about ``advisor`` explains in detail how the scan time
+        advisor works.
+
+adivsor_max_cpu
+        specifies the upper limit of the cpu percent usage of the ksmd
+        background thread. The default is 70.
+
+advisor_target_scan_time
+        specifies the target scan time in seconds to scan all the candidate
+        pages. The default value is 200 seconds.
+
+advisor_min_pages_to_scan
+        specifies the lower limit of the ``pages_to_scan`` parameter of the
+        scan time advisor. The default is 500.
+
+adivsor_max_pages_to_scan
+        specifies the upper limit of the ``pages_to_scan`` parameter of the
+        scan time advisor. The default is 30000.
+
 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
 
 general_profit
@@ -263,6 +289,35 @@ ksm_swpin_copy
 	note that KSM page might be copied when swapping in because do_swap_page()
 	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
 
+Advisor
+=======
+
+The number of candidate pages for KSM is dynamic. It can be often observed
+that during the startup of an application more candidate pages need to be
+processed. Without an advisor the ``pages_to_scan`` parameter needs to be
+sized for the maximum number of candidate pages. The scan time advisor can
+changes the ``pages_to_scan`` parameter based on demand.
+
+The advisor can be enabled, so KSM can automatically adapt to changes in the
+number of candidate pages to scan. Two advisors are implemented: none and
+scan-time. With none, no advisor is enabled. The default is none.
+
+The scan time advisor changes the ``pages_to_scan`` parameter based on the
+observed scan times. The possible values for the ``pages_to_scan`` parameter is
+limited by the ``advisor_max_cpu`` parameter. In addition there is also the
+``advisor_target_scan_time`` parameter. This parameter sets the target time to
+scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
+decides how aggressive the scan time advisor scans candidate pages. Lower
+values make the scan time advisor to scan more aggresively. This is the most
+important parameter for the configuration of the scan time advisor.
+
+The initial value and the maximum value can be changed with
+``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default
+values are sufficient for most workloads and use cases.
+
+The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
+
+
 --
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
-- 
2.43.0.rc2