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>

4 files changed, 106 insertions, 34 deletions

diff --git a/fs/inode.c b/fs/inode.c
index 5aab80dc008c..73920d555c88 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -143,6 +143,7 @@ int inode_init_always(struct super_block *sb, struct inode *inode)
         inode->i_op = &empty_iops;
         inode->i_fop = &empty_fops;
         inode->i_nlink = 1;
+        inode->i_opflags = 0;
         inode->i_uid = 0;
         inode->i_gid = 0;
         atomic_set(&inode->i_writecount, 0);
diff --git a/fs/namei.c b/fs/namei.c
index 3d607bd80e09..4a98bf154d88 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -308,6 +308,26 @@ int generic_permission(struct inode *inode, int mask)
         return -EACCES;
 }
 
+/*
+ * We _really_ want to just do "generic_permission()" without
+ * even looking at the inode->i_op values. So we keep a cache
+ * flag in inode->i_opflags, that says "this has not special
+ * permission function, use the fast case".
+ */
+static inline int do_inode_permission(struct inode *inode, int mask)
+{
+        if (unlikely(!(inode->i_opflags & IOP_FASTPERM))) {
+                if (likely(inode->i_op->permission))
+                        return inode->i_op->permission(inode, mask);
+
+                /* This gets set once for the inode lifetime */
+                spin_lock(&inode->i_lock);
+                inode->i_opflags |= IOP_FASTPERM;
+                spin_unlock(&inode->i_lock);
+        }
+        return generic_permission(inode, mask);
+}
+
 /**
  * inode_permission  -  check for access rights to a given inode
  * @inode:      inode to check permission on
@@ -322,7 +342,7 @@ int inode_permission(struct inode *inode, int mask)
 {
         int retval;
 
-        if (mask & MAY_WRITE) {
+        if (unlikely(mask & MAY_WRITE)) {
                 umode_t mode = inode->i_mode;
 
                 /*
@@ -339,11 +359,7 @@ int inode_permission(struct inode *inode, int mask)
                         return -EACCES;
         }
 
-        if (inode->i_op->permission)
-                retval = inode->i_op->permission(inode, mask);
-        else
-                retval = generic_permission(inode, mask);
-
+        retval = do_inode_permission(inode, mask);
         if (retval)
                 return retval;
 
@@ -1245,6 +1261,26 @@ static void terminate_walk(struct nameidata *nd)
         }
 }
 
+/*
+ * Do we need to follow links? We _really_ want to be able
+ * to do this check without having to look at inode->i_op,
+ * so we keep a cache of "no, this doesn't need follow_link"
+ * for the common case.
+ */
+static inline int do_follow_link(struct inode *inode, int follow)
+{
+        if (unlikely(!(inode->i_opflags & IOP_NOFOLLOW))) {
+                if (likely(inode->i_op->follow_link))
+                        return follow;
+
+                /* This gets set once for the inode lifetime */
+                spin_lock(&inode->i_lock);
+                inode->i_opflags |= IOP_NOFOLLOW;
+                spin_unlock(&inode->i_lock);
+        }
+        return 0;
+}
+
 static inline int walk_component(struct nameidata *nd, struct path *path,
                 struct qstr *name, int type, int follow)
 {
@@ -1267,7 +1303,7 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
                 terminate_walk(nd);
                 return -ENOENT;
         }
-        if (unlikely(inode->i_op->follow_link) && follow) {
+        if (do_follow_link(inode, follow)) {
                 if (nd->flags & LOOKUP_RCU) {
                         if (unlikely(unlazy_walk(nd, path->dentry))) {
                                 terminate_walk(nd);
@@ -1320,6 +1356,26 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
 }
 
 /*
+ * We really don't want to look at inode->i_op->lookup
+ * when we don't have to. So we keep a cache bit in
+ * the inode ->i_opflags field that says "yes, we can
+ * do lookup on this inode".
+ */
+static inline int can_lookup(struct inode *inode)
+{
+        if (likely(inode->i_opflags & IOP_LOOKUP))
+                return 1;
+        if (likely(!inode->i_op->lookup))
+                return 0;
+
+        /* We do this once for the lifetime of the inode */
+        spin_lock(&inode->i_lock);
+        inode->i_opflags |= IOP_LOOKUP;
+        spin_unlock(&inode->i_lock);
+        return 1;
+}
+
+/*
  * Name resolution.
  * This is the basic name resolution function, turning a pathname into
  * the final dentry. We expect 'base' to be positive and a directory.
@@ -1398,10 +1454,10 @@ static int link_path_walk(const char *name, struct nameidata *nd)
                         if (err)
                                 return err;
                 }
+                if (can_lookup(nd->inode))
+                        continue;
                 err = -ENOTDIR; 
-                if (!nd->inode->i_op->lookup)
-                        break;
-                continue;
+                break;
                 /* here ends the main loop */
 
 last_component:
diff --git a/fs/stat.c b/fs/stat.c
index 961039121cb8..ba5316ffac61 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -27,12 +27,12 @@ void generic_fillattr(struct inode *inode, struct kstat *stat)
         stat->uid = inode->i_uid;
         stat->gid = inode->i_gid;
         stat->rdev = inode->i_rdev;
+        stat->size = i_size_read(inode);
         stat->atime = inode->i_atime;
         stat->mtime = inode->i_mtime;
         stat->ctime = inode->i_ctime;
-        stat->size = i_size_read(inode);
-        stat->blocks = inode->i_blocks;
         stat->blksize = (1 << inode->i_blkbits);
+        stat->blocks = inode->i_blocks;
 }
 
 EXPORT_SYMBOL(generic_fillattr);
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 */
 };