thp, vmstats: count deferred split events

Count how many times we put a THP in split queue.  Currently, it happens
on partial unmap of a THP.

Rapidly growing value can indicate that an application behaves
unfriendly wrt THP: often fault in huge page and then unmap part of it.
This leads to unnecessary memory fragmentation and the application may
require tuning.

The event also can help with debugging kernel [mis-]behaviour.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
index 21cf34f3ddb2..0dc8632aa01e 100644
--- a/Documentation/vm/transhuge.txt
+++ b/Documentation/vm/transhuge.txt
@@ -229,6 +229,11 @@ thp_split_page is incremented every time a huge page is split into base
 thp_split_page_failed is is incremented if kernel fails to split huge
         page. This can happen if the page was pinned by somebody.
 
+thp_deferred_split_page is incremented when a huge page is put onto split
+        queue. This happens when a huge page is partially unmapped and
+        splitting it would free up some memory. Pages on split queue are
+        going to be split under memory pressure.
+
 thp_split_pmd is incremented every time a PMD split into table of PTEs.
         This can happen, for instance, when application calls mprotect() or
         munmap() on part of huge page. It doesn't split huge page, only
diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
index 58ecc056ee45..ec084321fe09 100644
--- a/include/linux/vm_event_item.h
+++ b/include/linux/vm_event_item.h
@@ -72,6 +72,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
                 THP_COLLAPSE_ALLOC_FAILED,
                 THP_SPLIT_PAGE,
                 THP_SPLIT_PAGE_FAILED,
+                THP_DEFERRED_SPLIT_PAGE,
                 THP_SPLIT_PMD,
                 THP_ZERO_PAGE_ALLOC,
                 THP_ZERO_PAGE_ALLOC_FAILED,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1ea21e203a70..1dddfb21fc22 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3455,6 +3455,7 @@ void deferred_split_huge_page(struct page *page)
 
         spin_lock_irqsave(&pgdata->split_queue_lock, flags);
         if (list_empty(page_deferred_list(page))) {
+                count_vm_event(THP_DEFERRED_SPLIT_PAGE);
                 list_add_tail(page_deferred_list(page), &pgdata->split_queue);
                 pgdata->split_queue_len++;
         }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index f80066248c94..5e4300482897 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -848,6 +848,7 @@ const char * const vmstat_text[] = {
         "thp_collapse_alloc_failed",
         "thp_split_page",
         "thp_split_page_failed",
+        "thp_deferred_split_page",
         "thp_split_pmd",
         "thp_zero_page_alloc",
         "thp_zero_page_alloc_failed",