[RFC v4 PATCH 3/5] mm/rmqueue_bulk: alloc without touching individual page structure

[Aaron Lu]

Profile on Intel Skylake server shows the most time consuming part
under zone->lock on allocation path is accessing those to-be-returned
page's "struct page" on the free_list inside zone->lock. One explanation
is, different CPUs are releasing pages to the head of free_list and
those page's 'struct page' may very well be cache cold for the allocating
CPU when it grabs these pages from free_list' head. The purpose here
is to avoid touching these pages one by one inside zone->lock.

One idea is, we just take the requested number of pages off free_list
with something like list_cut_position() and then adjust nr_free of
free_area accordingly inside zone->lock and other operations like
clearing PageBuddy flag for these pages are done outside of zone->lock.

list_cut_position() needs to know where to cut, that's what the new
'struct cluster' meant to provide. All pages on order 0's free_list
belongs to a cluster so when a number of pages is needed, the cluster
to which head page of free_list belongs is checked and then tail page
of the cluster could be found. With tail page, list_cut_position() can
be used to drop the cluster off free_list. The 'struct cluster' also has
'nr' to tell how many pages this cluster has so nr_free of free_area can
be adjusted inside the lock too.

This caused a race window though: from the moment zone->lock is dropped
till these pages' PageBuddy flags get cleared, these pages are not in
buddy but still have PageBuddy flag set.

This doesn't cause problems for users that access buddy pages through
free_list. But there are other users, like move_freepages() which is
used to move a pageblock pages from one migratetype to another in
fallback allocation path, will test PageBuddy flag of a page derived
from PFN. The end result could be that for pages in the race window,
they are moved back to free_list of another migratetype. For this
reason, a synchronization function zone_wait_cluster_alloc() is
introduced to wait till all pages are in correct state. This function
is meant to be called with zone->lock held, so after this function
returns, we do not need to worry about new pages becoming racy state.

Another user is compaction, where it will scan a pageblock for
migratable candidates. In this process, pages derived from PFN will
be checked for PageBuddy flag to decide if it is a merge skipped page.
To avoid a racy page getting merged back into buddy, the
zone_wait_and_disable_cluster_alloc() function is introduced to:
1 disable clustered allocation by increasing zone->cluster.disable_depth;
2 wait till the race window pass by calling zone_wait_cluster_alloc().
This function is also meant to be called with zone->lock held so after
it returns, all pages are in correct state and no more cluster alloc
will be attempted till zone_enable_cluster_alloc() is called to decrease
zone->cluster.disable_depth.

The two patches could eliminate zone->lock contention entirely but at
the same time, pgdat->lru_lock contention rose to 82%. Final performance
increased about 8.3%.

Suggested-by: Ying Huang <ying.huang@xxxxxxxxx>
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>
---
 include/linux/mm_types.h |  19 +--
 include/linux/mmzone.h   |  35 +++++
 mm/compaction.c          |   4 +
 mm/internal.h            |  34 +++++
 mm/page_alloc.c          | 288 +++++++++++++++++++++++++++++++++++++--
 5 files changed, 363 insertions(+), 17 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index aed93053ef6e..3abe1515502e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -85,8 +85,14 @@ struct page {
 			 */
 			struct list_head lru;
 			/* See page-flags.h for PAGE_MAPPING_FLAGS */
-			struct address_space *mapping;
-			pgoff_t index;		/* Our offset within mapping. */
+			union {
+				struct address_space *mapping;
+				struct cluster *cluster;
+			};
+			union {
+				pgoff_t index;		/* Our offset within mapping. */
+				bool buddy_merge_skipped;
+			};
 			/**
 			 * @private: Mapping-private opaque data.
 			 * Usually used for buffer_heads if PagePrivate.
@@ -179,13 +185,8 @@ struct page {
 		int units;			/* SLOB */
 	};
 
-	union {
-		/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
-		atomic_t _refcount;
-
-		/* For pages in Buddy: if skipped merging when added to Buddy */
-		bool buddy_merge_skipped;
-	};
+	/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
+	atomic_t _refcount;
 
 #ifdef CONFIG_MEMCG
 	struct mem_cgroup *mem_cgroup;
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1e22d96734e0..765567366ddb 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -356,6 +356,40 @@ enum zone_type {
 
 #ifndef __GENERATING_BOUNDS_H
 
+struct cluster {
+	struct page     *tail;  /* tail page of the cluster */
+	int             nr;     /* how many pages are in this cluster */
+};
+
+struct order0_cluster {
+	/* order 0 cluster array, dynamically allocated */
+	struct cluster *array;
+	/*
+	 * order 0 cluster array length, also used to indicate if cluster
+	 * allocation is enabled for this zone(cluster allocation is disabled
+	 * for small zones whose batch size is smaller than 1, like DMA zone)
+	 */
+	int             len;
+	/*
+	 * smallest position from where we search for an
+	 * empty cluster from the cluster array
+	 */
+	int		zero_bit;
+	/* bitmap used to quickly locate an empty cluster from cluster array */
+	unsigned long   *bitmap;
+
+	/* disable cluster allocation to avoid new pages becoming racy state. */
+	unsigned long	disable_depth;
+
+	/*
+	 * used to indicate if there are pages allocated in cluster mode
+	 * still in racy state. Caller with zone->lock held could use helper
+	 * function zone_wait_cluster_alloc() to wait all such pages to exit
+	 * the race window.
+	 */
+	atomic_t        in_progress;
+};
+
 struct zone {
 	/* Read-mostly fields */
 
@@ -460,6 +494,7 @@ struct zone {
 
 	/* free areas of different sizes */
 	struct free_area	free_area[MAX_ORDER];
+	struct order0_cluster	cluster;
 
 	/* zone flags, see below */
 	unsigned long		flags;
diff --git a/mm/compaction.c b/mm/compaction.c
index 0c9c7a30dde3..b732136dfc4c 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1601,6 +1601,8 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
 
 	migrate_prep_local();
 
+	zone_wait_and_disable_cluster_alloc(zone);
+
 	while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) {
 		int err;
 
@@ -1699,6 +1701,8 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
 			zone->compact_cached_free_pfn = free_pfn;
 	}
 
+	zone_enable_cluster_alloc(zone);
+
 	count_compact_events(COMPACTMIGRATE_SCANNED, cc->total_migrate_scanned);
 	count_compact_events(COMPACTFREE_SCANNED, cc->total_free_scanned);
 
diff --git a/mm/internal.h b/mm/internal.h
index c166735a559e..fb4e8f7976e5 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -546,12 +546,46 @@ static inline bool can_skip_merge(struct zone *zone, int order)
 	if (order)
 		return false;
 
+	/*
+	 * Clustered allocation is only disabled when high-order pages
+	 * are needed, e.g. in compaction and CMA alloc, so we should
+	 * also skip merging in that case.
+	 */
+	if (zone->cluster.disable_depth)
+		return false;
+
 	return true;
 }
+
+static inline void zone_wait_cluster_alloc(struct zone *zone)
+{
+	while (atomic_read(&zone->cluster.in_progress))
+		cpu_relax();
+}
+
+static inline void zone_wait_and_disable_cluster_alloc(struct zone *zone)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&zone->lock, flags);
+	zone->cluster.disable_depth++;
+	zone_wait_cluster_alloc(zone);
+	spin_unlock_irqrestore(&zone->lock, flags);
+}
+
+static inline void zone_enable_cluster_alloc(struct zone *zone)
+{
+	unsigned long flags;
+	spin_lock_irqsave(&zone->lock, flags);
+	zone->cluster.disable_depth--;
+	spin_unlock_irqrestore(&zone->lock, flags);
+}
 #else /* CONFIG_COMPACTION */
 static inline bool can_skip_merge(struct zone *zone, int order)
 {
 	return false;
 }
+static inline void zone_wait_cluster_alloc(struct zone *zone) {}
+static inline void zone_wait_and_disable_cluster_alloc(struct zone *zone) {}
+static inline void zone_enable_cluster_alloc(struct zone *zone) {}
 #endif  /* CONFIG_COMPACTION */
 #endif	/* __MM_INTERNAL_H */
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 76d471e0ab24..e60a248030dc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -707,6 +707,82 @@ static inline void set_page_order(struct page *page, unsigned int order)
 	__SetPageBuddy(page);
 }
 
+static inline struct cluster *new_cluster(struct zone *zone, int nr,
+						struct page *tail)
+{
+	struct order0_cluster *cluster = &zone->cluster;
+	int n = find_next_zero_bit(cluster->bitmap, cluster->len, cluster->zero_bit);
+	if (n == cluster->len) {
+		printk_ratelimited("node%d zone %s cluster used up\n",
+				zone->zone_pgdat->node_id, zone->name);
+		return NULL;
+	}
+	cluster->zero_bit = n;
+	set_bit(n, cluster->bitmap);
+	cluster->array[n].nr = nr;
+	cluster->array[n].tail = tail;
+	return &cluster->array[n];
+}
+
+static inline struct cluster *add_to_cluster_common(struct page *page,
+			struct zone *zone, struct page *neighbor)
+{
+	struct cluster *c;
+
+	if (neighbor) {
+		int batch = this_cpu_ptr(zone->pageset)->pcp.batch;
+		c = neighbor->cluster;
+		if (c && c->nr < batch) {
+			page->cluster = c;
+			c->nr++;
+			return c;
+		}
+	}
+
+	c = new_cluster(zone, 1, page);
+	if (unlikely(!c))
+		return NULL;
+
+	page->cluster = c;
+	return c;
+}
+
+/*
+ * Add this page to the cluster where the previous head page belongs.
+ * Called after page is added to free_list(and becoming the new head).
+ */
+static inline void add_to_cluster_head(struct page *page, struct zone *zone,
+				       int order, int mt)
+{
+	struct page *neighbor;
+
+	if (order || !zone->cluster.len)
+		return;
+
+	neighbor = page->lru.next == &zone->free_area[0].free_list[mt] ?
+		   NULL : list_entry(page->lru.next, struct page, lru);
+	add_to_cluster_common(page, zone, neighbor);
+}
+
+/*
+ * Add this page to the cluster where the previous tail page belongs.
+ * Called after page is added to free_list(and becoming the new tail).
+ */
+static inline void add_to_cluster_tail(struct page *page, struct zone *zone,
+				       int order, int mt)
+{
+	struct page *neighbor;
+	struct cluster *c;
+
+	if (order || !zone->cluster.len)
+		return;
+
+	neighbor = page->lru.prev == &zone->free_area[0].free_list[mt] ?
+		   NULL : list_entry(page->lru.prev, struct page, lru);
+	c = add_to_cluster_common(page, zone, neighbor);
+	c->tail = page;
+}
+
 static inline void add_to_buddy_common(struct page *page, struct zone *zone,
 					unsigned int order)
 {
@@ -720,6 +796,7 @@ static inline void add_to_buddy_head(struct page *page, struct zone *zone,
 {
 	add_to_buddy_common(page, zone, order);
 	list_add(&page->lru, &zone->free_area[order].free_list[mt]);
+	add_to_cluster_head(page, zone, order, mt);
 }
 
 static inline void add_to_buddy_tail(struct page *page, struct zone *zone,
@@ -727,6 +804,7 @@ static inline void add_to_buddy_tail(struct page *page, struct zone *zone,
 {
 	add_to_buddy_common(page, zone, order);
 	list_add_tail(&page->lru, &zone->free_area[order].free_list[mt]);
+	add_to_cluster_tail(page, zone, order, mt);
 }
 
 static inline void rmv_page_order(struct page *page)
@@ -735,9 +813,29 @@ static inline void rmv_page_order(struct page *page)
 	set_page_private(page, 0);
 }
 
+/* called before removed from free_list */
+static inline void remove_from_cluster(struct page *page, struct zone *zone)
+{
+	struct cluster *c = page->cluster;
+	if (!c)
+		return;
+
+	page->cluster = NULL;
+	c->nr--;
+	if (!c->nr) {
+		int bit = c - zone->cluster.array;
+		c->tail = NULL;
+		clear_bit(bit, zone->cluster.bitmap);
+		if (bit < zone->cluster.zero_bit)
+			zone->cluster.zero_bit = bit;
+	} else if (page == c->tail)
+		c->tail = list_entry(page->lru.prev, struct page, lru);
+}
+
 static inline void remove_from_buddy(struct page *page, struct zone *zone,
 					unsigned int order)
 {
+	remove_from_cluster(page, zone);
 	list_del(&page->lru);
 	zone->free_area[order].nr_free--;
 	rmv_page_order(page);
@@ -2098,6 +2196,17 @@ static int move_freepages(struct zone *zone,
 	if (num_movable)
 		*num_movable = 0;
 
+	/*
+	 * Cluster alloced pages may have their PageBuddy flag unclear yet
+	 * after dropping zone->lock in rmqueue_bulk() and steal here could
+	 * move them back to free_list. So it's necessary to wait till all
+	 * those pages have their flags properly cleared.
+	 *
+	 * We do not need to disable cluster alloc though since we already
+	 * held zone->lock and no allocation could happen.
+	 */
+	zone_wait_cluster_alloc(zone);
+
 	for (page = start_page; page <= end_page;) {
 		if (!pfn_valid_within(page_to_pfn(page))) {
 			page++;
@@ -2122,8 +2231,10 @@ static int move_freepages(struct zone *zone,
 		}
 
 		order = page_order(page);
+		remove_from_cluster(page, zone);
 		list_move(&page->lru,
 			  &zone->free_area[order].free_list[migratetype]);
+		add_to_cluster_head(page, zone, order, migratetype);
 		page += 1 << order;
 		pages_moved += 1 << order;
 	}
@@ -2272,7 +2383,9 @@ static void steal_suitable_fallback(struct zone *zone, struct page *page,
 
 single_page:
 	area = &zone->free_area[current_order];
+	remove_from_cluster(page, zone);
 	list_move(&page->lru, &area->free_list[start_type]);
+	add_to_cluster_head(page, zone, current_order, start_type);
 }
 
 /*
@@ -2533,6 +2646,145 @@ __rmqueue(struct zone *zone, unsigned int order, int migratetype)
 	return page;
 }
 
+static int __init zone_order0_cluster_init(void)
+{
+	struct zone *zone;
+
+	for_each_zone(zone) {
+		int len, mt, batch;
+		unsigned long flags;
+		struct order0_cluster *cluster;
+
+		if (!managed_zone(zone))
+			continue;
+
+		/* no need to enable cluster allocation for batch<=1 zone */
+		preempt_disable();
+		batch = this_cpu_ptr(zone->pageset)->pcp.batch;
+		preempt_enable();
+		if (batch <= 1)
+			continue;
+
+		cluster = &zone->cluster;
+		/* FIXME: possible overflow of int type */
+		len = DIV_ROUND_UP(zone->managed_pages, batch);
+		cluster->array = vzalloc(len * sizeof(struct cluster));
+		if (!cluster->array)
+			return -ENOMEM;
+		cluster->bitmap = vzalloc(DIV_ROUND_UP(len, BITS_PER_LONG) *
+				sizeof(unsigned long));
+		if (!cluster->bitmap)
+			return -ENOMEM;
+
+		spin_lock_irqsave(&zone->lock, flags);
+		cluster->len = len;
+		for (mt = 0; mt < MIGRATE_PCPTYPES; mt++) {
+			struct page *page;
+			list_for_each_entry_reverse(page,
+					&zone->free_area[0].free_list[mt], lru)
+				add_to_cluster_head(page, zone, 0, mt);
+		}
+		spin_unlock_irqrestore(&zone->lock, flags);
+	}
+
+	return 0;
+}
+subsys_initcall(zone_order0_cluster_init);
+
+static inline int __rmqueue_bulk_cluster(struct zone *zone, unsigned long count,
+						struct list_head *list, int mt)
+{
+	struct list_head *head = &zone->free_area[0].free_list[mt];
+	int nr = 0;
+
+	while (nr < count) {
+		struct page *head_page;
+		struct list_head *tail, tmp_list;
+		struct cluster *c;
+		int bit;
+
+		head_page = list_first_entry_or_null(head, struct page, lru);
+		if (!head_page || !head_page->cluster)
+			break;
+
+		c = head_page->cluster;
+		tail = &c->tail->lru;
+
+		/* drop the cluster off free_list and attach to list */
+		list_cut_position(&tmp_list, head, tail);
+		list_splice_tail(&tmp_list, list);
+
+		nr += c->nr;
+		zone->free_area[0].nr_free -= c->nr;
+
+		/* this cluster is empty now */
+		c->tail = NULL;
+		c->nr = 0;
+		bit = c - zone->cluster.array;
+		clear_bit(bit, zone->cluster.bitmap);
+		if (bit < zone->cluster.zero_bit)
+			zone->cluster.zero_bit = bit;
+	}
+
+	return nr;
+}
+
+static inline int rmqueue_bulk_cluster(struct zone *zone, unsigned int order,
+				unsigned long count, struct list_head *list,
+				int migratetype)
+{
+	int alloced;
+	struct page *page;
+
+	/*
+	 * Cluster alloc races with merging so don't try cluster alloc when we
+	 * can't skip merging. Note that can_skip_merge() keeps the same return
+	 * value from here till all pages have their flags properly processed,
+	 * i.e. the end of the function where in_progress is incremented, even
+	 * we have dropped the lock in the middle because the only place that
+	 * can change can_skip_merge()'s return value is compaction code and
+	 * compaction needs to wait on in_progress.
+	 */
+	if (!can_skip_merge(zone, 0))
+		return 0;
+
+	/* Cluster alloc is disabled, mostly compaction is already in progress */
+	if (zone->cluster.disable_depth)
+		return 0;
+
+	/* Cluster alloc is disabled for this zone */
+	if (unlikely(!zone->cluster.len))
+		return 0;
+
+	alloced = __rmqueue_bulk_cluster(zone, count, list, migratetype);
+	if (!alloced)
+		return 0;
+
+	/*
+	 * Cache miss on page structure could slow things down
+	 * dramatically so accessing these alloced pages without
+	 * holding lock for better performance.
+	 *
+	 * Since these pages still have PageBuddy set, there is a race
+	 * window between now and when PageBuddy is cleared for them
+	 * below. Any operation that would scan a pageblock and check
+	 * PageBuddy(page), e.g. compaction, will need to wait till all
+	 * such pages are properly processed. in_progress is used for
+	 * such purpose so increase it now before dropping the lock.
+	 */
+	atomic_inc(&zone->cluster.in_progress);
+	spin_unlock(&zone->lock);
+
+	list_for_each_entry(page, list, lru) {
+		rmv_page_order(page);
+		page->cluster = NULL;
+		set_pcppage_migratetype(page, migratetype);
+	}
+	atomic_dec(&zone->cluster.in_progress);
+
+	return alloced;
+}
+
 /*
  * Obtain a specified number of elements from the buddy allocator, all under
  * a single hold of the lock, for efficiency.  Add them to the supplied list.
@@ -2542,17 +2794,23 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 			unsigned long count, struct list_head *list,
 			int migratetype)
 {
-	int i, alloced = 0;
+	int i, alloced;
+	struct page *page, *tmp;
 
 	spin_lock(&zone->lock);
-	for (i = 0; i < count; ++i) {
-		struct page *page = __rmqueue(zone, order, migratetype);
+	alloced = rmqueue_bulk_cluster(zone, order, count, list, migratetype);
+	if (alloced > 0) {
+		if (alloced >= count)
+			goto out;
+		else
+			spin_lock(&zone->lock);
+	}
+
+	for (; alloced < count; alloced++) {
+		page = __rmqueue(zone, order, migratetype);
 		if (unlikely(page == NULL))
 			break;
 
-		if (unlikely(check_pcp_refill(page)))
-			continue;
-
 		/*
 		 * Split buddy pages returned by expand() are received here in
 		 * physical page order. The page is added to the tail of
@@ -2564,7 +2822,18 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 		 * pages are ordered properly.
 		 */
 		list_add_tail(&page->lru, list);
-		alloced++;
+	}
+	spin_unlock(&zone->lock);
+
+out:
+	i = alloced;
+	list_for_each_entry_safe(page, tmp, list, lru) {
+		if (unlikely(check_pcp_refill(page))) {
+			list_del(&page->lru);
+			alloced--;
+			continue;
+		}
+
 		if (is_migrate_cma(get_pcppage_migratetype(page)))
 			__mod_zone_page_state(zone, NR_FREE_CMA_PAGES,
 					      -(1 << order));
@@ -2577,7 +2846,6 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 	 * pages added to the pcp list.
 	 */
 	__mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));
-	spin_unlock(&zone->lock);
 	return alloced;
 }
 
@@ -7925,6 +8193,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	unsigned long outer_start, outer_end;
 	unsigned int order;
 	int ret = 0;
+	struct zone *zone = page_zone(pfn_to_page(start));
 
 	struct compact_control cc = {
 		.nr_migratepages = 0,
@@ -7967,6 +8236,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 	if (ret)
 		return ret;
 
+	zone_wait_and_disable_cluster_alloc(zone);
 	/*
 	 * In case of -EBUSY, we'd like to know which page causes problem.
 	 * So, just fall through. test_pages_isolated() has a tracepoint
@@ -8049,6 +8319,8 @@ int alloc_contig_range(unsigned long start, unsigned long end,
 done:
 	undo_isolate_page_range(pfn_max_align_down(start),
 				pfn_max_align_up(end), migratetype);
+
+	zone_enable_cluster_alloc(zone);
 	return ret;
 }
 
-- 
2.17.2