[PATCH v3 3/7] fs: Introduce a per_cpu last_ino allocator

[Eric Dumazet <dada1@cosmosbay.com>]

new_inode() dirties a contended cache line to get increasing
inode numbers.

Solve this problem by providing to each cpu a per_cpu variable,
feeded by the shared last_ino, but once every 1024 allocations.

This reduce contention on the shared last_ino, and give same
spreading ino numbers than before.
(same wraparound after 2^32 allocations)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 fs/inode.c |   35 ++++++++++++++++++++++++++++++++---
 1 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index f94f889..dc8e72a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -556,6 +556,36 @@ repeat:
 	return node ? inode : NULL;
 }
 
+#ifdef CONFIG_SMP
+/*
+ * Each cpu owns a range of 1024 numbers.
+ * 'shared_last_ino' is dirtied only once out of 1024 allocations,
+ * to renew the exhausted range.
+ */
+static DEFINE_PER_CPU(int, last_ino);
+
+static int last_ino_get(void)
+{
+	static atomic_t shared_last_ino;
+	int *p = &get_cpu_var(last_ino);
+	int res = *p;
+
+	if (unlikely((res & 1023) == 0))
+		res = atomic_add_return(1024, &shared_last_ino) - 1024;
+
+	*p = ++res;
+	put_cpu_var(last_ino);
+	return res;
+}
+#else
+static int last_ino_get(void)
+{
+	static int last_ino;
+
+	return ++last_ino;
+}
+#endif
+
 /**
  *	new_inode 	- obtain an inode
  *	@sb: superblock
@@ -575,7 +605,6 @@ struct inode *new_inode(struct super_block *sb)
 	 * error if st_ino won't fit in target struct field. Use 32bit counter
 	 * here to attempt to avoid that.
 	 */
-	static unsigned int last_ino;
 	struct inode * inode;
 
 	spin_lock_prefetch(&inode_lock);
@@ -583,11 +612,11 @@ struct inode *new_inode(struct super_block *sb)
 	inode = alloc_inode(sb);
 	if (inode) {
 		percpu_counter_inc(&nr_inodes);
+		inode->i_state = 0;
+		inode->i_ino = last_ino_get();
 		spin_lock(&inode_lock);
 		list_add(&inode->i_list, &inode_in_use);
 		list_add(&inode->i_sb_list, &sb->s_inodes);
-		inode->i_ino = ++last_ino;
-		inode->i_state = 0;
 		spin_unlock(&inode_lock);
 	}
 	return inode;