OK, this is a new patch with updated changelog, and slightly tweaked
change to dcookie subsystem (now we'll *never* do any more hash lookups
on dcookie lookup, ignoring that "fast" dcookie lookups would effectively
mean my previous version wouldn't have done any more lookups anyway).
Andrew, please apply.
--
struct dentry is one of the most critical structures in the kernel.
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.
Get rid of the d_cookie pointer. This shrinks it to 192 bytes. Why was this
ever a good idea? With the observation that dcookie == dentry pointer, there is
nothing that d_cookie provided that can't be found by other means. The only
thing it provides actually is a flag as to whether a dentry has a dcookie or
not. Use a new d_flag bit for this. Saves 4/8 bytes with CONFIG_PROFILING.
Pack d_mounted into a hole, and take 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. We could have gone the other way and increased the length to 40, but I'm
aiming for a magic number, read on...
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.
32-bit systems can increase the inline name length to 40, giving them a nice
size of 128 bytes.
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.
Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
---
arch/powerpc/oprofile/cell/spu_task_sync.c | 2 -
drivers/oprofile/buffer_sync.c | 2 -
fs/dcache.c | 5 ----
fs/dcookies.c | 31 +++++++++++++++++------------
include/linux/dcache.h | 21 +++++++++++++------
5 files changed, 36 insertions(+), 25 deletions(-)
Index: linux-2.6/include/linux/dcache.h
===================================================================
--- linux-2.6.orig/include/linux/dcache.h
+++ linux-2.6/include/linux/dcache.h
@@ -75,14 +75,22 @@ full_name_hash(const unsigned char *name
return end_name_hash(hash);
}
-struct dcookie_struct;
-
-#define DNAME_INLINE_LEN_MIN 36
+/*
+ * Keep struct dentry aligned on 64 byte cachelines (this will give
+ * reasonable cacheline footprint on larger lines, without the large
+ * memory footprint increase of full cacheline alignment).
+ */
+#ifdef CONFIG_64BIT
+#define DNAME_INLINE_LEN_MIN 32 /* 192 bytes */
+#else
+#define DNAME_INLINE_LEN_MIN 40 /* 128 bytes */
+#endif
struct dentry {
atomic_t d_count;
unsigned int d_flags; /* protected by d_lock */
spinlock_t d_lock; /* per dentry lock */
+ int d_mounted;
struct inode *d_inode; /* Where the name belongs to - NULL is
* negative */
/*
@@ -107,10 +115,7 @@ struct dentry {
struct dentry_operations *d_op;
struct super_block *d_sb; /* The root of the dentry tree */
void *d_fsdata; /* fs-specific data */
-#ifdef CONFIG_PROFILING
- struct dcookie_struct *d_cookie; /* cookie, if any */
-#endif
- int d_mounted;
+
unsigned char d_iname[DNAME_INLINE_LEN_MIN]; /* small names */
};
@@ -177,6 +182,8 @@ d_iput: no no no yes
#define DCACHE_INOTIFY_PARENT_WATCHED 0x0020 /* Parent inode is watched */
+#define DCACHE_COOKIE 0x0040 /* For use by dcookie subsystem */
+
extern spinlock_t dcache_lock;
extern seqlock_t rename_lock;
Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c
+++ linux-2.6/fs/dcache.c
@@ -38,7 +38,7 @@
int sysctl_vfs_cache_pressure __read_mostly = 100;
EXPORT_SYMBOL_GPL(sysctl_vfs_cache_pressure);
- __cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lock);
+__cacheline_aligned_in_smp DEFINE_SPINLOCK(dcache_lock);
__cacheline_aligned_in_smp DEFINE_SEQLOCK(rename_lock);
EXPORT_SYMBOL(dcache_lock);
@@ -948,9 +948,6 @@ struct dentry *d_alloc(struct dentry * p
dentry->d_op = NULL;
dentry->d_fsdata = NULL;
dentry->d_mounted = 0;
-#ifdef CONFIG_PROFILING
- dentry->d_cookie = NULL;
-#endif
INIT_HLIST_NODE(&dentry->d_hash);
INIT_LIST_HEAD(&dentry->d_lru);
INIT_LIST_HEAD(&dentry->d_subdirs);
Index: linux-2.6/fs/dcookies.c
===================================================================
--- linux-2.6.orig/fs/dcookies.c
+++ linux-2.6/fs/dcookies.c
@@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcoo
{
struct dcookie_struct *dcs = kmem_cache_alloc(dcookie_cache,
GFP_KERNEL);
+ struct dentry *d;
if (!dcs)
return NULL;
- path->dentry->d_cookie = dcs;
+ d = path->dentry;
+ spin_lock(&d->d_lock);
+ d->d_flags |= DCACHE_COOKIE;
+ spin_unlock(&d->d_lock);
+
dcs->path = *path;
path_get(path);
hash_dcookie(dcs);
@@ -110,7 +115,6 @@ static struct dcookie_struct *alloc_dcoo
int get_dcookie(struct path *path, unsigned long *cookie)
{
int err = 0;
- struct dcookie_struct * dcs;
mutex_lock(&dcookie_mutex);
@@ -119,17 +123,15 @@ int get_dcookie(struct path *path, unsig
goto out;
}
- dcs = path->dentry->d_cookie;
-
- if (!dcs)
- dcs = alloc_dcookie(path);
-
- if (!dcs) {
- err = -ENOMEM;
- goto out;
+ if (!path->dentry->d_flags & DCACHE_COOKIE) {
+ if (!alloc_dcookie(path)) {
+ err = -ENOMEM;
+ goto out;
+ }
+ BUG_ON(!(path->dentry->d_flags & DCACHE_COOKIE));
}
- *cookie = dcookie_value(dcs);
+ *cookie = (unsigned long)path->dentry;
out:
mutex_unlock(&dcookie_mutex);
@@ -251,7 +253,12 @@ out_kmem:
static void free_dcookie(struct dcookie_struct * dcs)
{
- dcs->path.dentry->d_cookie = NULL;
+ struct dentry *d = dcs->path.dentry;
+
+ spin_lock(&d->d_lock);
+ d->d_flags &= ~DCACHE_COOKIE;
+ spin_unlock(&d->d_lock);
+
path_put(&dcs->path);
kmem_cache_free(dcookie_cache, dcs);
}
Index: linux-2.6/drivers/oprofile/buffer_sync.c
===================================================================
--- linux-2.6.orig/drivers/oprofile/buffer_sync.c
+++ linux-2.6/drivers/oprofile/buffer_sync.c
@@ -200,7 +200,7 @@ static inline unsigned long fast_get_dco
{
unsigned long cookie;
- if (path->dentry->d_cookie)
+ if (path->dentry->d_flags & DCACHE_COOKIE)
return (unsigned long)path->dentry;
get_dcookie(path, &cookie);
return cookie;
Index: linux-2.6/arch/powerpc/oprofile/cell/spu_task_sync.c
===================================================================
--- linux-2.6.orig/arch/powerpc/oprofile/cell/spu_task_sync.c
+++ linux-2.6/arch/powerpc/oprofile/cell/spu_task_sync.c
@@ -297,7 +297,7 @@ static inline unsigned long fast_get_dco
{
unsigned long cookie;
- if (path->dentry->d_cookie)
+ if (path->dentry->d_flags & DCACHE_COOKIE)
return (unsigned long)path->dentry;
get_dcookie(path, &cookie);
return cookie;
--
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