From c2452f32786159ed85f0e4b21fec09258f822fc8 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Mon, 1 Dec 2008 09:33:43 +0100 Subject: 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 --- fs/dcookies.c | 28 +++++++++++++++++++--------- 1 file changed, 19 insertions(+), 9 deletions(-) (limited to 'fs/dcookies.c') diff --git a/fs/dcookies.c b/fs/dcookies.c index 855d4b1d619a..180e9fec4ad8 100644 --- a/fs/dcookies.c +++ b/fs/dcookies.c @@ -93,10 +93,15 @@ static struct dcookie_struct *alloc_dcookie(struct path *path) { 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); @@ -119,14 +124,14 @@ int get_dcookie(struct path *path, unsigned long *cookie) goto out; } - dcs = path->dentry->d_cookie; - - if (!dcs) + if (path->dentry->d_flags & DCACHE_COOKIE) { + dcs = find_dcookie((unsigned long)path->dentry); + } else { dcs = alloc_dcookie(path); - - if (!dcs) { - err = -ENOMEM; - goto out; + if (!dcs) { + err = -ENOMEM; + goto out; + } } *cookie = dcookie_value(dcs); @@ -251,7 +256,12 @@ 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); } -- cgit v1.2.3-59-g8ed1b