[PATCH v5 0/2] ksm: count allocated rmap_items and update documentation

[PATCH v5 0/2] ksm: count allocated rmap_items and update documentation

[xu xin]

KSM can save memory by merging identical pages, but also can consume
additional memory, because it needs to generate rmap_items to save
each scanned page's brief rmap information.

To determine how beneficial the ksm-policy (like madvise), they are using
brings, so we add a new interface /proc/<pid>/ksm_stat for each process
The value "ksm_rmap_items" in it indicates the total allocated ksm
rmap_items of this process.

The detailed description can be seen in the following patches' commit message.


-----------
v4->v5:
1. change ksm_rmp_items to ksm_rmap_items;
2. use /proc/*/ksm_stat as proc name rather than "ksm_rmap_items" and
   start filling it with the value "ksm_rmap_items" so that more values
   can be added in future.

v3->v4:
Fix the wrong writing format and some misspellings of the related documentaion.

v2->v3:
remake the patches based on the latest linux-next branch.

v1->v2:
Add documentation for the new item.

*** BLURB HERE ***

xu xin (2):
  ksm: count allocated ksm rmap_items for each process
  ksm: add profit monitoring documentation

 Documentation/admin-guide/mm/ksm.rst | 36 ++++++++++++++++++++++++++++
 fs/proc/base.c                       | 15 ++++++++++++
 include/linux/mm_types.h             |  5 ++++
 mm/ksm.c                             |  2 ++
 4 files changed, 58 insertions(+)

-- 
2.25.1

[PATCH v5 1/2] ksm: count allocated ksm rmap_items for each process

[xu xin]

KSM can save memory by merging identical pages, but also can consume
additional memory, because it needs to generate rmap_items to save
each scanned page's brief rmap information. Some of these pages may
be merged, but some may not be abled to be merged after being checked
several times, which are unprofitable memory consumed.

The information about whether KSM save memory or consume memory in
system-wide range can be determined by the comprehensive calculation
of pages_sharing, pages_shared, pages_unshared and pages_volatile.
A simple approximate calculation:

	profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
	         sizeof(rmap_item);

where all_rmap_items equals to the sum of pages_sharing, pages_shared,
pages_unshared and pages_volatile.

But we cannot calculate this kind of ksm profit inner single-process wide
because the information of ksm rmap_item's number of a process is lacked.
For user applications, if this kind of information could be obtained,
it helps upper users know how beneficial the ksm-policy (like madvise)
they are using brings, and then optimize their app code. For example,
one application madvise 1000 pages as MERGEABLE, while only a few pages
are really merged, then it's not cost-efficient.

So we add a new interface /proc/<pid>/ksm_stat for each process in which
the value of ksm_rmap_itmes is only shown now and so more values can be
added in future.

So similarly, we can calculate the ksm profit approximately for a single
process by:

	profit =~ ksm_merging_pages * sizeof(page) - ksm_rmap_items *
		 sizeof(rmap_item);

where ksm_merging_pages is shown at /proc/<pid>/ksm_merging_pages, and
ksm_rmap_items is shown in /proc/<pid>/ksm_stat.

Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Signed-off-by: CGEL ZTE <cgel.zte@gmail.com>
---
 fs/proc/base.c           | 15 +++++++++++++++
 include/linux/mm_types.h |  5 +++++
 mm/ksm.c                 |  2 ++
 3 files changed, 22 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 4ead8cf654e4..c66ac538eda4 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3199,6 +3199,19 @@ static int proc_pid_ksm_merging_pages(struct seq_file *m, struct pid_namespace *
 
 	return 0;
 }
+static int proc_pid_ksm_stat(struct seq_file *m, struct pid_namespace *ns,
+				struct pid *pid, struct task_struct *task)
+{
+	struct mm_struct *mm;
+
+	mm = get_task_mm(task);
+	if (mm) {
+		seq_printf(m, "ksm_rmap_items %lu\n", mm->ksm_rmap_items);
+		mmput(mm);
+	}
+
+	return 0;
+}
 #endif /* CONFIG_KSM */
 
 #ifdef CONFIG_STACKLEAK_METRICS
@@ -3334,6 +3347,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 #endif
 #ifdef CONFIG_KSM
 	ONE("ksm_merging_pages",  S_IRUSR, proc_pid_ksm_merging_pages),
+	ONE("ksm_stat",  S_IRUSR, proc_pid_ksm_stat),
 #endif
 };
 
@@ -3671,6 +3685,7 @@ static const struct pid_entry tid_base_stuff[] = {
 #endif
 #ifdef CONFIG_KSM
 	ONE("ksm_merging_pages",  S_IRUSR, proc_pid_ksm_merging_pages),
+	ONE("ksm_stat",  S_IRUSR, proc_pid_ksm_stat),
 #endif
 };
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index fb53717d571c..bd0993d20a20 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -656,6 +656,11 @@ struct mm_struct {
 		 * merging.
 		 */
 		unsigned long ksm_merging_pages;
+		/*
+		 * Represent how many pages are checked for ksm merging
+		 * including merged and not merged.
+		 */
+		unsigned long ksm_rmap_items;
 #endif
 #ifdef CONFIG_LRU_GEN
 		struct {
diff --git a/mm/ksm.c b/mm/ksm.c
index e34cc21d5556..0c76b3e004b7 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -387,6 +387,7 @@ static inline struct rmap_item *alloc_rmap_item(void)
 static inline void free_rmap_item(struct rmap_item *rmap_item)
 {
 	ksm_rmap_items--;
+	rmap_item->mm->ksm_rmap_items--;
 	rmap_item->mm = NULL;	/* debug safety */
 	kmem_cache_free(rmap_item_cache, rmap_item);
 }
@@ -2234,6 +2235,7 @@ static struct rmap_item *get_next_rmap_item(struct mm_slot *mm_slot,
 	if (rmap_item) {
 		/* It has already been zeroed */
 		rmap_item->mm = mm_slot->mm;
+		rmap_item->mm->ksm_rmap_items++;
 		rmap_item->address = addr;
 		rmap_item->rmap_list = *rmap_list;
 		*rmap_list = rmap_item;
-- 
2.25.1

[PATCH v5 2/2] ksm: add profit monitoring documentation

[xu xin]

Add the description of KSM profit and how to determine it separately
in system-wide range and inner a single process.

Signed-off-by: xu xin <xu.xin16@zte.com.cn>
Reviewed-by: Xiaokai Ran <ran.xiaokai@zte.com.cn>
Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
---
 Documentation/admin-guide/mm/ksm.rst | 36 ++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
index b244f0202a03..fb6ba2002a4b 100644
--- a/Documentation/admin-guide/mm/ksm.rst
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -184,6 +184,42 @@ The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
 ``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
 be increased accordingly.
 
+Monitoring KSM profit
+=====================
+
+KSM can save memory by merging identical pages, but also can consume
+additional memory, because it needs to generate a number of rmap_items to
+save each scanned page's brief rmap information. Some of these pages may
+be merged, but some may not be abled to be merged after being checked
+several times, which are unprofitable memory consumed.
+
+1) How to determine whether KSM save memory or consume memory in system-wide
+   range? Here is a simple approximate calculation for reference::
+
+	general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
+			  sizeof(rmap_item);
+
+   where all_rmap_items can be easily obtained by summing ``pages_sharing``,
+   ``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
+
+2) The KSM profit inner a single process can be similarly obtained by the
+   following approximate calculation::
+
+	process_profit =~ ksm_merging_pages * sizeof(page) -
+			  ksm_rmap_items * sizeof(rmap_item).
+
+   where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
+   and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``.
+
+From the perspective of application, a high ratio of ``ksm_rmap_items`` to
+``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
+administrators have to rethink how to change madvise policy. Giving an example
+for reference, a page's size is usually 4K, and the rmap_item's size is
+separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
+so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
+or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
+because the ksm profit is approximately zero or negative.
+
 Monitoring KSM events
 =====================
 
-- 
2.25.1

Re: [PATCH v5 0/2] ksm: count allocated rmap_items and update documentation

[Andrew Morton]

On Tue, 30 Aug 2022 14:37:31 +0000 xu xin <cgel.zte@gmail.com> wrote:

> KSM can save memory by merging identical pages, but also can consume
> additional memory, because it needs to generate rmap_items to save
> each scanned page's brief rmap information.
> 
> To determine how beneficial the ksm-policy (like madvise), they are using
> brings, so we add a new interface /proc/<pid>/ksm_stat for each process
> The value "ksm_rmap_items" in it indicates the total allocated ksm
> rmap_items of this process.

I can see the usefulness and the code change is very simple, so I'll
queue it for testing and shall see what other reviewers have to say.

It's useful that the per-process file is called "ksm_stat", because we
may with to add additional content to it in the future.  Because
concerns have been expressed (by Alexey) about the proliferation of
procfs files causing major memory use when something reads them all. 
Putting more things in the same procfs files will help avoid this.