[RFC PATCH 3/3] vfs: Enable list batching for the superblock's inode list

[Waiman Long]

The inode_sb_list_add() and inode_sb_list_del() functions in the vfs
layer just perform list addition and deletion under lock. So they can
use the new list batching facility to speed up the list operations
when many CPUs are trying to do it simultaneously.

In particular, the inode_sb_list_del() function can be a performance
bottleneck when large applications with many threads and associated
inodes exit. With an exit microbenchmark that creates a large number
of threads, attachs many inodes to them and then exits. The runtimes
of that microbenchmark with 1000 threads before and after the patch
on a 4-socket Intel E7-4820 v3 system (48 cores, 96 threads) were
as follows:

  Kernel        Elapsed Time    System Time
  ------        ------------    -----------
  Vanilla 4.4      65.29s         82m14s
  Patched 4.4      45.69s         49m44s

The elapsed time and the reported system time were reduced by 30%
and 40% respectively.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxxx>
---
 fs/inode.c         |   13 +++++--------
 fs/super.c         |    1 +
 include/linux/fs.h |    2 ++
 3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 9f62db3..456bd8a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -424,19 +424,16 @@ static void inode_lru_list_del(struct inode *inode)
  */
 void inode_sb_list_add(struct inode *inode)
 {
-	spin_lock(&inode->i_sb->s_inode_list_lock);
-	list_add(&inode->i_sb_list, &inode->i_sb->s_inodes);
-	spin_unlock(&inode->i_sb->s_inode_list_lock);
+	do_list_batch(lb_cmd_add, &inode->i_sb->s_inode_list_lock,
+			&inode->i_sb->s_list_batch, &inode->i_sb_list);
 }
 EXPORT_SYMBOL_GPL(inode_sb_list_add);
 
 static inline void inode_sb_list_del(struct inode *inode)
 {
-	if (!list_empty(&inode->i_sb_list)) {
-		spin_lock(&inode->i_sb->s_inode_list_lock);
-		list_del_init(&inode->i_sb_list);
-		spin_unlock(&inode->i_sb->s_inode_list_lock);
-	}
+	if (!list_empty(&inode->i_sb_list))
+		do_list_batch(lb_cmd_del_init, &inode->i_sb->s_inode_list_lock,
+				&inode->i_sb->s_list_batch, &inode->i_sb_list);
 }
 
 static unsigned long hash(struct super_block *sb, unsigned long hashval)
diff --git a/fs/super.c b/fs/super.c
index 1182af8..b0e8540 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -206,6 +206,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags)
 	mutex_init(&s->s_sync_lock);
 	INIT_LIST_HEAD(&s->s_inodes);
 	spin_lock_init(&s->s_inode_list_lock);
+	list_batch_init(&s->s_list_batch, &s->s_inodes);
 
 	if (list_lru_init_memcg(&s->s_dentry_lru))
 		goto fail;
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 1a20462..11d8b77 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -9,6 +9,7 @@
 #include <linux/stat.h>
 #include <linux/cache.h>
 #include <linux/list.h>
+#include <linux/list_batch.h>
 #include <linux/list_lru.h>
 #include <linux/llist.h>
 #include <linux/radix-tree.h>
@@ -1403,6 +1404,7 @@ struct super_block {
 	/* s_inode_list_lock protects s_inodes */
 	spinlock_t		s_inode_list_lock ____cacheline_aligned_in_smp;
 	struct list_head	s_inodes;	/* all inodes */
+	struct list_batch	s_list_batch;
 };
 
 extern struct timespec current_fs_time(struct super_block *sb);
-- 
1.7.1