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 <[email protected]>
--- 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;
};
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