Commit 4ba4d4c0 authored Feb 10, 2007 by Eric Dumazet Committed by Linus Torvalds Feb 11, 2007

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



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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 0aa5de85

include/linux/mount.h

+8 −2

Original line number	Diff line number	Diff line
		@@ -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;
		};