shrink struct dentry
struct dentry is one of the most critical structures in the kernel. So it's sad to see it going neglected. With CONFIG_PROFILING turned on (which is probably the common case at least for distros and kernel developers), sizeof(struct dcache) == 208 here (64-bit). This gives 19 objects per slab. I packed d_mounted into a hole, and took another 4 bytes off the inline name length to take the padding out from the end of the structure. This shinks it to 200 bytes. I could have gone the other way and increased the length to 40, but I'm aiming for a magic number, read on... I then got rid of the d_cookie pointer. This shrinks it to 192 bytes. Rant: why was this ever a good idea? The cookie system should increase its hash size or use a tree or something if lookups are a problem. Also the "fast dcookie lookups" in oprofile should be moved into the dcookie code -- how can oprofile possibly care about the dcookie_mutex? It gets dropped after get_dcookie() returns so it can't be providing any sort of protection. At 192 bytes, 21 objects fit into a 4K page, saving about 3MB on my system with ~140 000 entries allocated. 192 is also a multiple of 64, so we get nice cacheline alignment on 64 and 32 byte line systems -- any given dentry will now require 3 cachelines to touch all fields wheras previously it would require 4. I know the inline name size was chosen quite carefully, however with the reduction in cacheline footprint, it should actually be just about as fast to do a name lookup for a 36 character name as it was before the patch (and faster for other sizes). The memory footprint savings for names which are <= 32 or > 36 bytes long should more than make up for the memory cost for 33-36 byte names. Performance is a feature... Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
parent
e2b689d82c
commit
c2452f3278
5 changed files with 35 additions and 22 deletions
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@ -297,7 +297,7 @@ static inline unsigned long fast_get_dcookie(struct path *path)
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{
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unsigned long cookie;
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if (path->dentry->d_cookie)
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if (path->dentry->d_flags & DCACHE_COOKIE)
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return (unsigned long)path->dentry;
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get_dcookie(path, &cookie);
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return cookie;
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@ -200,7 +200,7 @@ static inline unsigned long fast_get_dcookie(struct path *path)
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{
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unsigned long cookie;
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if (path->dentry->d_cookie)
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if (path->dentry->d_flags & DCACHE_COOKIE)
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return (unsigned long)path->dentry;
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get_dcookie(path, &cookie);
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return cookie;
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@ -34,7 +34,6 @@
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#include <linux/bootmem.h>
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#include "internal.h"
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int sysctl_vfs_cache_pressure __read_mostly = 100;
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EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
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@ -948,9 +947,6 @@ struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
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dentry->d_op = NULL;
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dentry->d_fsdata = NULL;
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dentry->d_mounted = 0;
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#ifdef CONFIG_PROFILING
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dentry->d_cookie = NULL;
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#endif
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INIT_HLIST_NODE(&dentry->d_hash);
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INIT_LIST_HEAD(&dentry->d_lru);
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INIT_LIST_HEAD(&dentry->d_subdirs);
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@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcookie(struct path *path)
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{
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struct dcookie_struct *dcs = kmem_cache_alloc(dcookie_cache,
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GFP_KERNEL);
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struct dentry *d;
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if (!dcs)
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return NULL;
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path->dentry->d_cookie = dcs;
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d = path->dentry;
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spin_lock(&d->d_lock);
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d->d_flags |= DCACHE_COOKIE;
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spin_unlock(&d->d_lock);
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dcs->path = *path;
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path_get(path);
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hash_dcookie(dcs);
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@ -119,14 +124,14 @@ int get_dcookie(struct path *path, unsigned long *cookie)
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goto out;
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}
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dcs = path->dentry->d_cookie;
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if (!dcs)
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if (path->dentry->d_flags & DCACHE_COOKIE) {
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dcs = find_dcookie((unsigned long)path->dentry);
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} else {
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dcs = alloc_dcookie(path);
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if (!dcs) {
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err = -ENOMEM;
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goto out;
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if (!dcs) {
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err = -ENOMEM;
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goto out;
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}
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}
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*cookie = dcookie_value(dcs);
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@ -251,7 +256,12 @@ static int dcookie_init(void)
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static void free_dcookie(struct dcookie_struct * dcs)
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{
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dcs->path.dentry->d_cookie = NULL;
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struct dentry *d = dcs->path.dentry;
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spin_lock(&d->d_lock);
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d->d_flags &= ~DCACHE_COOKIE;
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spin_unlock(&d->d_lock);
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path_put(&dcs->path);
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kmem_cache_free(dcookie_cache, dcs);
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}
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@ -75,14 +75,22 @@ full_name_hash(const unsigned char *name, unsigned int len)
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return end_name_hash(hash);
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}
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struct dcookie_struct;
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#define DNAME_INLINE_LEN_MIN 36
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/*
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* Try to keep struct dentry aligned on 64 byte cachelines (this will
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* give reasonable cacheline footprint with larger lines without the
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* large memory footprint increase).
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*/
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#ifdef CONFIG_64BIT
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#define DNAME_INLINE_LEN_MIN 32 /* 192 bytes */
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#else
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#define DNAME_INLINE_LEN_MIN 40 /* 128 bytes */
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#endif
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struct dentry {
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atomic_t d_count;
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unsigned int d_flags; /* protected by d_lock */
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spinlock_t d_lock; /* per dentry lock */
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int d_mounted;
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struct inode *d_inode; /* Where the name belongs to - NULL is
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* negative */
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/*
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@ -107,10 +115,7 @@ struct dentry {
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struct dentry_operations *d_op;
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struct super_block *d_sb; /* The root of the dentry tree */
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void *d_fsdata; /* fs-specific data */
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#ifdef CONFIG_PROFILING
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struct dcookie_struct *d_cookie; /* cookie, if any */
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#endif
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int d_mounted;
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unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
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};
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@ -177,6 +182,8 @@ d_iput: no no no yes
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#define DCACHE_INOTIFY_PARENT_WATCHED 0x0020 /* Parent inode is watched */
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#define DCACHE_COOKIE 0x0040 /* For use by dcookie subsystem */
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extern spinlock_t dcache_lock;
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extern seqlock_t rename_lock;
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