vfs: optimize inode cache access patterns

The inode structure layout is largely random, and some of the vfs paths
really do care.  The path lookup in particular is already quite D$
intensive, and profiles show that accessing the 'inode->i_op->xyz'
fields is quite costly.

We already optimized the dcache to not unnecessarily load the d_op
structure for members that are often NULL using the DCACHE_OP_xyz bits
in dentry->d_flags, and this does something very similar for the inode
ops that are used during pathname lookup.

It also re-orders the fields so that the fields accessed by 'stat' are
together at the beginning of the inode structure, and roughly in the
order accessed.

The effect of this seems to be in the 1-2% range for an empty kernel
"make -j" run (which is fairly kernel-intensive, mostly in filename
lookup), so it's visible.  The numbers are fairly noisy, though, and
likely depend a lot on exact microarchitecture.  So there's more tuning
to be done.

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

1 files changed, 37 insertions, 22 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 786b3b1113cf..178cdb4f1d4a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -738,22 +738,54 @@ static inline int mapping_writably_mapped(struct address_space *mapping)
 struct posix_acl;
 #define ACL_NOT_CACHED ((void *)(-1))
 
+#define IOP_FASTPERM    0x0001
+#define IOP_LOOKUP      0x0002
+#define IOP_NOFOLLOW    0x0004
+
+/*
+ * Keep mostly read-only and often accessed (especially for
+ * the RCU path lookup and 'stat' data) fields at the beginning
+ * of the 'struct inode'
+ */
 struct inode {
-        /* RCU path lookup touches following: */
         umode_t                 i_mode;
+        unsigned short          i_opflags;
         uid_t                   i_uid;
         gid_t                   i_gid;
+        unsigned int            i_flags;
+
+#ifdef CONFIG_FS_POSIX_ACL
+        struct posix_acl        *i_acl;
+        struct posix_acl        *i_default_acl;
+#endif
+
         const struct inode_operations   *i_op;
         struct super_block      *i_sb;
+        struct address_space    *i_mapping;
 
-        spinlock_t              i_lock; /* i_blocks, i_bytes, maybe i_size */
-        unsigned int            i_flags;
-        unsigned long           i_state;
 #ifdef CONFIG_SECURITY
         void                    *i_security;
 #endif
-        struct mutex            i_mutex;
 
+        /* Stat data, not accessed from path walking */
+        unsigned long           i_ino;
+        unsigned int            i_nlink;
+        dev_t                   i_rdev;
+        loff_t                  i_size;
+        struct timespec         i_atime;
+        struct timespec         i_mtime;
+        struct timespec         i_ctime;
+        unsigned int            i_blkbits;
+        blkcnt_t                i_blocks;
+
+#ifdef __NEED_I_SIZE_ORDERED
+        seqcount_t              i_size_seqcount;
+#endif
+
+        /* Misc */
+        unsigned long           i_state;
+        spinlock_t              i_lock; /* i_blocks, i_bytes, maybe i_size */
+        struct mutex            i_mutex;
 
         unsigned long           dirtied_when;   /* jiffies of first dirtying */
 
@@ -765,25 +797,12 @@ struct inode {
                 struct list_head        i_dentry;
                 struct rcu_head         i_rcu;
         };
-        unsigned long           i_ino;
         atomic_t                i_count;
-        unsigned int            i_nlink;
-        dev_t                   i_rdev;
-        unsigned int            i_blkbits;
         u64                     i_version;
-        loff_t                  i_size;
-#ifdef __NEED_I_SIZE_ORDERED
-        seqcount_t              i_size_seqcount;
-#endif
-        struct timespec         i_atime;
-        struct timespec         i_mtime;
-        struct timespec         i_ctime;
-        blkcnt_t                i_blocks;
         unsigned short          i_bytes;
         atomic_t                i_dio_count;
         const struct file_operations    *i_fop; /* former ->i_op->default_file_ops */
         struct file_lock        *i_flock;
-        struct address_space    *i_mapping;
         struct address_space    i_data;
 #ifdef CONFIG_QUOTA
         struct dquot            *i_dquot[MAXQUOTAS];
@@ -806,10 +825,6 @@ struct inode {
         atomic_t                i_readcount; /* struct files open RO */
 #endif
         atomic_t                i_writecount;
-#ifdef CONFIG_FS_POSIX_ACL
-        struct posix_acl        *i_acl;
-        struct posix_acl        *i_default_acl;
-#endif
         void                    *i_private; /* fs or device private pointer */
 };