linux-kernel - [PATCH] fs: inode per-cpu last

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Message-ID: <1285842136.2615.251.camel@edumazet-laptop>
Date:	Thu, 30 Sep 2010 12:22:16 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] fs: inode per-cpu last_ino allocator

Le jeudi 30 septembre 2010 à 01:14 -0700, Andrew Morton a écrit :

> Perhaps
> 
> 	WARN_ON_ONCE(preemptible());
> 
> if we had a developer-only version of WARN_ON_ONCE, which we don't.

Or just use a regular PER_CPU variable, even on !SMP, and get preempt
safe implementation.

What do you think of following patch, on top of current linux-2.6 tree ?

Thanks

[PATCH] fs: inode per-cpu last_ino allocator

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

Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
Signed-off-by: Nick Piggin <npiggin@...e.de>
Signed-off-by: Dave Chinner <dchinner@...hat.com>
---
 fs/inode.c |   45 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 38 insertions(+), 7 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 8646433..122914e 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -624,6 +624,43 @@ void inode_add_to_lists(struct super_block *sb, struct inode *inode)
 }
 EXPORT_SYMBOL_GPL(inode_add_to_lists);
 
+#define LAST_INO_BATCH 1024
+
+/*
+ * Each cpu owns a range of LAST_INO_BATCH numbers.
+ * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations,
+ * to renew the exhausted range.
+ *
+ * This does not significantly increase overflow rate because every CPU can
+ * consume at most LAST_INO_BATCH-1 unused inode numbers. So there is
+ * NR_CPUS*(LAST_INO_BATCH-1) wastage. At 4096 and 1024, this is ~0.1% of the
+ * 2^32 range, and is a worst-case. Even a 50% wastage would only increase
+ * overflow rate by 2x, which does not seem too significant.
+ *
+ * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
+ * error if st_ino won't fit in target struct field. Use 32bit counter
+ * here to attempt to avoid that.
+ */
+static DEFINE_PER_CPU(unsigned int, last_ino);
+
+static noinline unsigned int last_ino_get(void)
+{
+	unsigned int *p = &get_cpu_var(last_ino);
+	unsigned int res = *p;
+
+#ifdef CONFIG_SMP
+	if (unlikely((res & (LAST_INO_BATCH - 1)) == 0)) {
+		static atomic_t shared_last_ino;
+		int next = atomic_add_return(LAST_INO_BATCH, &shared_last_ino);
+
+		res = next - LAST_INO_BATCH;
+	}
+#endif
+	*p = ++res;
+	put_cpu_var(last_ino);
+	return res;
+}
+
 /**
  *	new_inode 	- obtain an inode
  *	@sb: superblock
@@ -638,12 +675,6 @@ EXPORT_SYMBOL_GPL(inode_add_to_lists);
  */
 struct inode *new_inode(struct super_block *sb)
 {
-	/*
-	 * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
-	 * 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);
@@ -652,7 +683,7 @@ struct inode *new_inode(struct super_block *sb)
 	if (inode) {
 		spin_lock(&inode_lock);
 		__inode_add_to_lists(sb, NULL, inode);
-		inode->i_ino = ++last_ino;
+		inode->i_ino = last_ino_get();
 		inode->i_state = 0;
 		spin_unlock(&inode_lock);
 	}


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