ext4: improve extent cache shrink mechanism to avoid to burn CPU time

Now we maintain an proper in-order LRU list in ext4 to reclaim entries
from extent status tree when we are under heavy memory pressure.  For
keeping this order, a spin lock is used to protect this list.  But this
lock burns a lot of CPU time.  We can use the following steps to trigger
it.

  % cd /dev/shm
  % dd if=/dev/zero of=ext4-img bs=1M count=2k
  % mkfs.ext4 ext4-img
  % mount -t ext4 -o loop ext4-img /mnt
  % cd /mnt
  % for ((i=0;i<160;i++)); do truncate -s 64g $i; done
  % for ((i=0;i<160;i++)); do cp $i /dev/null &; done
  % perf record -a -g
  % perf report

This commit tries to fix this problem.  Now a new member called
i_touch_when is added into ext4_inode_info to record the last access
time for an inode.  Meanwhile we never need to keep a proper in-order
LRU list.  So this can avoid to burns some CPU time.  When we try to
reclaim some entries from extent status tree, we use list_sort() to get
a proper in-order list.  Then we traverse this list to discard some
entries.  In ext4_sb_info, we use s_es_last_sorted to record the last
time of sorting this list.  When we traverse the list, we skip the inode
that is newer than this time, and move this inode to the tail of LRU
list.  When the head of the list is newer than s_es_last_sorted, we will
sort the LRU list again.

In this commit, we break the loop if s_extent_cache_cnt == 0 because
that means that all extents in extent status tree have been reclaimed.

Meanwhile in this commit, ext4_es_{un}register_shrinker()'s prototype is
changed to save a local variable in these functions.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ext4.h

@@ -864,6 +864,7 @@ struct ext4_inode_info {
 	rwlock_t i_es_lock;
 	struct list_head i_es_lru;
 	unsigned int i_es_lru_nr;	/* protected by i_es_lock */
+	unsigned long i_touch_when;	/* jiffies of last accessing */
 
 	/* ialloc */
 	ext4_group_t	i_last_alloc_group;
@@ -1303,6 +1304,7 @@ struct ext4_sb_info {
 	/* Reclaim extents from extent status tree */
 	struct shrinker s_es_shrinker;
 	struct list_head s_es_lru;
+	unsigned long s_es_last_sorted;
 	struct percpu_counter s_extent_cache_cnt;
 	spinlock_t s_es_lru_lock ____cacheline_aligned_in_smp;
 };

fs/ext4/extents_status.c

@@ -10,6 +10,7 @@
  * Ext4 extents status tree core functions.
  */
 #include <linux/rbtree.h>
+#include <linux/list_sort.h>
 #include "ext4.h"
 #include "extents_status.h"
 #include "ext4_extents.h"
@@ -291,7 +292,6 @@ void ext4_es_find_delayed_extent_range(struct inode *inode,
 
 	read_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	trace_ext4_es_find_delayed_extent_range_exit(inode, es);
 }
 
@@ -672,7 +672,6 @@ int ext4_es_insert_extent(struct inode *inode, ext4_lblk_t lblk,
 error:
 	write_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	ext4_es_print_tree(inode);
 
 	return err;
@@ -734,7 +733,6 @@ int ext4_es_lookup_extent(struct inode *inode, ext4_lblk_t lblk,
 
 	read_unlock(&EXT4_I(inode)->i_es_lock);
 
-	ext4_es_lru_add(inode);
 	trace_ext4_es_lookup_extent_exit(inode, es, found);
 	return found;
 }
@@ -878,12 +876,28 @@ int ext4_es_zeroout(struct inode *inode, struct ext4_extent *ex)
 				     EXTENT_STATUS_WRITTEN);
 }
 
+static int ext4_inode_touch_time_cmp(void *priv, struct list_head *a,
+				     struct list_head *b)
+{
+	struct ext4_inode_info *eia, *eib;
+	eia = list_entry(a, struct ext4_inode_info, i_es_lru);
+	eib = list_entry(b, struct ext4_inode_info, i_es_lru);
+
+	if (eia->i_touch_when == eib->i_touch_when)
+		return 0;
+	if (time_after(eia->i_touch_when, eib->i_touch_when))
+		return 1;
+	else
+		return -1;
+}
+
 static int ext4_es_shrink(struct shrinker *shrink, struct shrink_control *sc)
 {
 	struct ext4_sb_info *sbi = container_of(shrink,
 					struct ext4_sb_info, s_es_shrinker);
 	struct ext4_inode_info *ei;
-	struct list_head *cur, *tmp, scanned;
+	struct list_head *cur, *tmp;
+	LIST_HEAD(skiped);
 	int nr_to_scan = sc->nr_to_scan;
 	int ret, nr_shrunk = 0;
 
@@ -893,65 +907,86 @@ static int ext4_es_shrink(struct shrinker *shrink, struct shrink_control *sc)
 	if (!nr_to_scan)
 		return ret;
 
-	INIT_LIST_HEAD(&scanned);
-
 	spin_lock(&sbi->s_es_lru_lock);
+
+	/*
+	 * If the inode that is at the head of LRU list is newer than
+	 * last_sorted time, that means that we need to sort this list.
+	 */
+	ei = list_first_entry(&sbi->s_es_lru, struct ext4_inode_info, i_es_lru);
+	if (sbi->s_es_last_sorted < ei->i_touch_when) {
+		list_sort(NULL, &sbi->s_es_lru, ext4_inode_touch_time_cmp);
+		sbi->s_es_last_sorted = jiffies;
+	}
+
 	list_for_each_safe(cur, tmp, &sbi->s_es_lru) {
-		list_move_tail(cur, &scanned);
+		/*
+		 * If we have already reclaimed all extents from extent
+		 * status tree, just stop the loop immediately.
+		 */
+		if (percpu_counter_read_positive(&sbi->s_extent_cache_cnt) == 0)
+			break;
 
 		ei = list_entry(cur, struct ext4_inode_info, i_es_lru);
 
-		read_lock(&ei->i_es_lock);
-		if (ei->i_es_lru_nr == 0) {
-			read_unlock(&ei->i_es_lock);
+		/* Skip the inode that is newer than the last_sorted time */
+		if (sbi->s_es_last_sorted < ei->i_touch_when) {
+			list_move_tail(cur, &skiped);
 			continue;
 		}
-		read_unlock(&ei->i_es_lock);
+
+		if (ei->i_es_lru_nr == 0)
+			continue;
 
 		write_lock(&ei->i_es_lock);
 		ret = __es_try_to_reclaim_extents(ei, nr_to_scan);
+		if (ei->i_es_lru_nr == 0)
+			list_del_init(&ei->i_es_lru);
 		write_unlock(&ei->i_es_lock);
 
 		nr_shrunk += ret;
 		nr_to_scan -= ret;
 		if (nr_to_scan == 0)
 			break;
 	}
-	list_splice_tail(&scanned, &sbi->s_es_lru);
+
+	/* Move the newer inodes into the tail of the LRU list. */
+	list_splice_tail(&skiped, &sbi->s_es_lru);
 	spin_unlock(&sbi->s_es_lru_lock);
 
 	ret = percpu_counter_read_positive(&sbi->s_extent_cache_cnt);
 	trace_ext4_es_shrink_exit(sbi->s_sb, nr_shrunk, ret);
 	return ret;
 }
 
-void ext4_es_register_shrinker(struct super_block *sb)
+void ext4_es_register_shrinker(struct ext4_sb_info *sbi)
 {
-	struct ext4_sb_info *sbi;
-
-	sbi = EXT4_SB(sb);
 	INIT_LIST_HEAD(&sbi->s_es_lru);
 	spin_lock_init(&sbi->s_es_lru_lock);
+	sbi->s_es_last_sorted = 0;
 	sbi->s_es_shrinker.shrink = ext4_es_shrink;
 	sbi->s_es_shrinker.seeks = DEFAULT_SEEKS;
 	register_shrinker(&sbi->s_es_shrinker);
 }
 
-void ext4_es_unregister_shrinker(struct super_block *sb)
+void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi)
 {
-	unregister_shrinker(&EXT4_SB(sb)->s_es_shrinker);
+	unregister_shrinker(&sbi->s_es_shrinker);
 }
 
 void ext4_es_lru_add(struct inode *inode)
 {
 	struct ext4_inode_info *ei = EXT4_I(inode);
 	struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 
+	ei->i_touch_when = jiffies;
+
+	if (!list_empty(&ei->i_es_lru))
+		return;
+
 	spin_lock(&sbi->s_es_lru_lock);
 	if (list_empty(&ei->i_es_lru))
 		list_add_tail(&ei->i_es_lru, &sbi->s_es_lru);
-	else
-		list_move_tail(&ei->i_es_lru, &sbi->s_es_lru);
 	spin_unlock(&sbi->s_es_lru_lock);
 }
 

fs/ext4/extents_status.h

@@ -39,6 +39,7 @@
 				 EXTENT_STATUS_DELAYED | \
 				 EXTENT_STATUS_HOLE)
 
+struct ext4_sb_info;
 struct ext4_extent;
 
 struct extent_status {
@@ -119,8 +120,8 @@ static inline void ext4_es_store_status(struct extent_status *es,
 	es->es_pblk = block;
 }
 
-extern void ext4_es_register_shrinker(struct super_block *sb);
-extern void ext4_es_unregister_shrinker(struct super_block *sb);
+extern void ext4_es_register_shrinker(struct ext4_sb_info *sbi);
+extern void ext4_es_unregister_shrinker(struct ext4_sb_info *sbi);
 extern void ext4_es_lru_add(struct inode *inode);
 extern void ext4_es_lru_del(struct inode *inode);
 

fs/ext4/inode.c

@@ -514,6 +514,8 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		  "logical block %lu\n", inode->i_ino, flags, map->m_len,
 		  (unsigned long) map->m_lblk);
 
+	ext4_es_lru_add(inode);
+
 	/* Lookup extent status tree firstly */
 	if (ext4_es_lookup_extent(inode, map->m_lblk, &es)) {
 		if (ext4_es_is_written(&es) || ext4_es_is_unwritten(&es)) {
@@ -1526,6 +1528,8 @@ static int ext4_da_map_blocks(struct inode *inode, sector_t iblock,
 		  "logical block %lu\n", inode->i_ino, map->m_len,
 		  (unsigned long) map->m_lblk);
 
+	ext4_es_lru_add(inode);
+
 	/* Lookup extent status tree firstly */
 	if (ext4_es_lookup_extent(inode, iblock, &es)) {
 

fs/ext4/super.c

@@ -773,7 +773,7 @@ static void ext4_put_super(struct super_block *sb)
 			ext4_abort(sb, "Couldn't clean up the journal");
 	}
 
-	ext4_es_unregister_shrinker(sb);
+	ext4_es_unregister_shrinker(sbi);
 	del_timer(&sbi->s_err_report);
 	ext4_release_system_zone(sb);
 	ext4_mb_release(sb);
@@ -862,6 +862,7 @@ static struct inode *ext4_alloc_inode(struct super_block *sb)
 	rwlock_init(&ei->i_es_lock);
 	INIT_LIST_HEAD(&ei->i_es_lru);
 	ei->i_es_lru_nr = 0;
+	ei->i_touch_when = 0;
 	ei->i_reserved_data_blocks = 0;
 	ei->i_reserved_meta_blocks = 0;
 	ei->i_allocated_meta_blocks = 0;
@@ -3799,7 +3800,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_err_report.data = (unsigned long) sb;
 
 	/* Register extent status tree shrinker */
-	ext4_es_register_shrinker(sb);
+	ext4_es_register_shrinker(sbi);
 
 	err = percpu_counter_init(&sbi->s_freeclusters_counter,
 			ext4_count_free_clusters(sb));
@@ -4127,7 +4128,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		sbi->s_journal = NULL;
 	}
 failed_mount3:
-	ext4_es_unregister_shrinker(sb);
+	ext4_es_unregister_shrinker(sbi);
 	del_timer(&sbi->s_err_report);
 	if (sbi->s_flex_groups)
 		ext4_kvfree(sbi->s_flex_groups);