[PATCH] struct vfsmount : keep mnt_count & mnt_expiry_mark away from mnt_flags

[Eric Dumazet <dada1@cosmosbay.com>]

I noticed cache misses in touch_atime() that can be avoided if we keep 
mnt_count & mnt_expiry_mark in a different cache line than mnt_flags (mostly 
read)

mnt_count & mnt_expiry_mark are modified each time a file is opened/closed in 
a file system.

touch_atime() is called each time a file is read, and generally needs to read 
mnt_flags.

Other fields of struct vfsmount are mostly read so I chose to move mnt_count & 
mnt_expiry_mark at the end of struct vfsmount. And adding a comment so that 
nobody tries to re-arrange fields to fill the holes :)

On 64bits platforms, the new offsetof(mnt_count) is 0xC0
On 32bits platforms, it is 0x60, so I didnot add a 
____cacheline_aligned_in_smp because it would have a too big impact on the 
size of this object (in particular if CONFIG_X86_L1_CACHE_SHIFT=7)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

--- linux-2.6.20-rc1-mm1/include/linux/mount.h	2006-12-14 02:14:23.000000000 +0100
+++ linux-2.6.20-rc1-mm1-ed/include/linux/mount.h	2006-12-15 19:30:54.000000000 +0100
@@ -43,9 +43,8 @@ struct vfsmount {
 	struct super_block *mnt_sb;	/* pointer to superblock */
 	struct list_head mnt_mounts;	/* list of children, anchored here */
 	struct list_head mnt_child;	/* and going through their mnt_child */
-	atomic_t mnt_count;
 	int mnt_flags;
-	int mnt_expiry_mark;		/* true if marked for expiry */
+	/* 4 bytes hole on 64bits arches */
 	char *mnt_devname;		/* Name of device e.g. /dev/dsk/hda1 */
 	struct list_head mnt_list;
 	struct list_head mnt_expire;	/* link in fs-specific expiry list */
@@ -54,6 +53,13 @@ struct vfsmount {
 	struct list_head mnt_slave;	/* slave list entry */
 	struct vfsmount *mnt_master;	/* slave is on master->mnt_slave_list */
 	struct mnt_namespace *mnt_ns;	/* containing namespace */
+	/*
+	 * We put mnt_count & mnt_expiry_mark at the end of struct vfsmount
+	 * to let these frequently modified fields in a separate cache line
+	 * (so that reads of mnt_flags wont ping-pong on SMP machines)
+	 */
+	atomic_t mnt_count;
+	int mnt_expiry_mark;		/* true if marked for expiry */
 	int mnt_pinned;
 };