[PATCH 3/6] fs: Introduce a per_cpu last_ino allocator

[Eric Dumazet <dada1@cosmosbay.com>]

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new_inode() dirties a contended cache line to get 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.

Note : last_ino_get() method must be called with preemption
disabled on SMP.


(socket8 bench result : no differences, but this is because inode_lock
cost is too heavy)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 fs/inode.c |   27 +++++++++++++++++++++++++--
 1 files changed, 25 insertions(+), 2 deletions(-)

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diff --git a/fs/inode.c b/fs/inode.c
index 0487ddb..d850050 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -534,6 +534,30 @@ repeat:
 	return node ? inode : NULL;
 }
 
+#ifdef CONFIG_SMP
+/*
+ * each cpu owns a block of 1024 numbers.
+ * The global 'last_ino' is dirtied once every 1024 allocations
+ */
+static DEFINE_PER_CPU(int, cpu_ino_alloc) = {0};
+static int last_ino_get(void)
+{
+	static atomic_t last_ino;
+	int *ptr = &__raw_get_cpu_var(cpu_ino_alloc);
+
+	if (unlikely((*ptr & 1023) == 0))
+		*ptr = atomic_add_return(1024, &last_ino);
+	return --(*ptr);
+}
+#else
+static int last_ino_get(void)
+{
+	static int last_ino;
+
+	return ++last_ino;
+}
+#endif
+
 /**
  *	new_inode 	- obtain an inode
  *	@sb: superblock
@@ -553,7 +577,6 @@ 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);
@@ -564,7 +587,7 @@ struct inode *new_inode(struct super_block *sb)
 		inodes_stat.nr_inodes++;
 		list_add(&inode->i_list, &inode_in_use);
 		list_add(&inode->i_sb_list, &sb->s_inodes);
-		inode->i_ino = ++last_ino;
+		inode->i_ino = last_ino_get();
 		inode->i_state = 0;
 		spin_unlock(&inode_lock);
 	}