Re: [v2 PATCH -mm] mm: account deferred split THPs into MemAvailable

[Kirill A. Shutemov]

On Tue, Aug 27, 2019 at 03:17:39PM +0300, Kirill A. Shutemov wrote:
> On Tue, Aug 27, 2019 at 02:09:23PM +0200, Michal Hocko wrote:
> > On Tue 27-08-19 14:01:56, Vlastimil Babka wrote:
> > > On 8/27/19 1:02 PM, Kirill A. Shutemov wrote:
> > > > On Tue, Aug 27, 2019 at 08:01:39AM +0200, Michal Hocko wrote:
> > > >> On Mon 26-08-19 16:15:38, Kirill A. Shutemov wrote:
> > > >>>
> > > >>> Unmapped completely pages will be freed with current code. Deferred split
> > > >>> only applies to partly mapped THPs: at least on 4k of the THP is still
> > > >>> mapped somewhere.
> > > >>
> > > >> Hmm, I am probably misreading the code but at least current Linus' tree
> > > >> reads page_remove_rmap -> [page_remove_anon_compound_rmap ->\ deferred_split_huge_page even
> > > >> for fully mapped THP.
> > > > 
> > > > Well, you read correctly, but it was not intended. I screwed it up at some
> > > > point.
> > > > 
> > > > See the patch below. It should make it work as intened.
> > > > 
> > > > It's not bug as such, but inefficientcy. We add page to the queue where
> > > > it's not needed.
> > > 
> > > But that adding to queue doesn't affect whether the page will be freed
> > > immediately if there are no more partial mappings, right? I don't see
> > > deferred_split_huge_page() pinning the page.
> > > So your patch wouldn't make THPs freed immediately in cases where they
> > > haven't been freed before immediately, it just fixes a minor
> > > inefficiency with queue manipulation?
> > 
> > Ohh, right. I can see that in free_transhuge_page now. So fully mapped
> > THPs really do not matter and what I have considered an odd case is
> > really happening more often.
> > 
> > That being said this will not help at all for what Yang Shi is seeing
> > and we need a more proactive deferred splitting as I've mentioned
> > earlier.
> 
> It was not intended to fix the issue. It's fix for current logic. I'm
> playing with the work approach now.

Below is what I've come up with. It appears to be functional.

Any comments?

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index d77d717c620c..c576e9d772b7 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -758,6 +758,8 @@ typedef struct pglist_data {
 	spinlock_t split_queue_lock;
 	struct list_head split_queue;
 	unsigned long split_queue_len;
+	unsigned int deferred_split_calls;
+	struct work_struct deferred_split_work;
 #endif
 
 	/* Fields commonly accessed by the page reclaim scanner */
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index de1f15969e27..12d109bbe8ac 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2820,22 +2820,6 @@ void free_transhuge_page(struct page *page)
 	free_compound_page(page);
 }
 
-void deferred_split_huge_page(struct page *page)
-{
-	struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
-	unsigned long flags;
-
-	VM_BUG_ON_PAGE(!PageTransHuge(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++;
-	}
-	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
-}
-
 static unsigned long deferred_split_count(struct shrinker *shrink,
 		struct shrink_control *sc)
 {
@@ -2901,6 +2885,44 @@ static struct shrinker deferred_split_shrinker = {
 	.flags = SHRINKER_NUMA_AWARE,
 };
 
+void flush_deferred_split_queue(struct work_struct *work)
+{
+	struct pglist_data *pgdata;
+	struct shrink_control sc;
+
+	pgdata = container_of(work, struct pglist_data, deferred_split_work);
+	sc.nid = pgdata->node_id;
+	sc.nr_to_scan = 0; /* Unlimited */
+
+	deferred_split_scan(NULL, &sc);
+}
+
+#define NR_CALLS_TO_SPLIT 32
+#define NR_PAGES_ON_QUEUE_TO_SPLIT 16
+
+void deferred_split_huge_page(struct page *page)
+{
+	struct pglist_data *pgdata = NODE_DATA(page_to_nid(page));
+	unsigned long flags;
+
+	VM_BUG_ON_PAGE(!PageTransHuge(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++;
+		pgdata->deferred_split_calls++;
+	}
+
+	if (pgdata->deferred_split_calls > NR_CALLS_TO_SPLIT &&
+			pgdata->split_queue_len > NR_PAGES_ON_QUEUE_TO_SPLIT) {
+		pgdata->deferred_split_calls = 0;
+		schedule_work(&pgdata->deferred_split_work);
+	}
+	spin_unlock_irqrestore(&pgdata->split_queue_lock, flags);
+}
+
 #ifdef CONFIG_DEBUG_FS
 static int split_huge_pages_set(void *data, u64 val)
 {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9c9194959271..86af66d463e9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6636,11 +6636,14 @@ static unsigned long __init calc_memmap_size(unsigned long spanned_pages,
 }
 
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
+void flush_deferred_split_queue(struct work_struct *work);
 static void pgdat_init_split_queue(struct pglist_data *pgdat)
 {
 	spin_lock_init(&pgdat->split_queue_lock);
 	INIT_LIST_HEAD(&pgdat->split_queue);
 	pgdat->split_queue_len = 0;
+	pgdat->deferred_split_calls = 0;
+	INIT_WORK(&pgdat->deferred_split_work, flush_deferred_split_queue);
 }
 #else
 static void pgdat_init_split_queue(struct pglist_data *pgdat) {}
-- 
 Kirill A. Shutemov