[NET]: Implement SKB fast cloning.

Protocols that make extensive use of SKB cloning,
for example TCP, eat at least 2 allocations per
packet sent as a result.

To cut the kmalloc() count in half, we implement
a pre-allocation scheme wherein we allocate
2 sk_buff objects in advance, then use a simple
reference count to free up the memory at the
correct time.

Based upon an initial patch by Thomas Graf and
suggestions from Herbert Xu.

Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 74 insertions, 12 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 39a161dbc16d..b853a9b29eb6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -69,6 +69,7 @@
 #include <asm/system.h>
 
 static kmem_cache_t *skbuff_head_cache;
+static kmem_cache_t *skbuff_fclone_cache;
 
 struct timeval __read_mostly skb_tv_base;
 
@@ -120,7 +121,7 @@ void skb_under_panic(struct sk_buff *skb, int sz, void *here)
  */
 
 /**
- *      alloc_skb       -       allocate a network buffer
+ *      __alloc_skb     -       allocate a network buffer
  *      @size: size to allocate
  *      @gfp_mask: allocation mask
  *
@@ -131,14 +132,20 @@ void skb_under_panic(struct sk_buff *skb, int sz, void *here)
  *      Buffers may only be allocated from interrupts using a @gfp_mask of
  *      %GFP_ATOMIC.
  */
-struct sk_buff *alloc_skb(unsigned int size, unsigned int __nocast gfp_mask)
+struct sk_buff *__alloc_skb(unsigned int size, unsigned int __nocast gfp_mask,
+                            int fclone)
 {
         struct sk_buff *skb;
         u8 *data;
 
         /* Get the HEAD */
-        skb = kmem_cache_alloc(skbuff_head_cache,
-                               gfp_mask & ~__GFP_DMA);
+        if (fclone)
+                skb = kmem_cache_alloc(skbuff_fclone_cache,
+                                       gfp_mask & ~__GFP_DMA);
+        else
+                skb = kmem_cache_alloc(skbuff_head_cache,
+                                       gfp_mask & ~__GFP_DMA);
+
         if (!skb)
                 goto out;
 
@@ -155,7 +162,15 @@ struct sk_buff *alloc_skb(unsigned int size, unsigned int __nocast gfp_mask)
         skb->data = data;
         skb->tail = data;
         skb->end  = data + size;
+        if (fclone) {
+                struct sk_buff *child = skb + 1;
+                atomic_t *fclone_ref = (atomic_t *) (child + 1);
 
+                skb->fclone = SKB_FCLONE_ORIG;
+                atomic_set(fclone_ref, 1);
+
+                child->fclone = SKB_FCLONE_UNAVAILABLE;
+        }
         atomic_set(&(skb_shinfo(skb)->dataref), 1);
         skb_shinfo(skb)->nr_frags  = 0;
         skb_shinfo(skb)->tso_size = 0;
@@ -268,8 +283,34 @@ void skb_release_data(struct sk_buff *skb)
  */
 void kfree_skbmem(struct sk_buff *skb)
 {
+        struct sk_buff *other;
+        atomic_t *fclone_ref;
+
         skb_release_data(skb);
-        kmem_cache_free(skbuff_head_cache, skb);
+        switch (skb->fclone) {
+        case SKB_FCLONE_UNAVAILABLE:
+                kmem_cache_free(skbuff_head_cache, skb);
+                break;
+
+        case SKB_FCLONE_ORIG:
+                fclone_ref = (atomic_t *) (skb + 2);
+                if (atomic_dec_and_test(fclone_ref))
+                        kmem_cache_free(skbuff_fclone_cache, skb);
+                break;
+
+        case SKB_FCLONE_CLONE:
+                fclone_ref = (atomic_t *) (skb + 1);
+                other = skb - 1;
+
+                /* The clone portion is available for
+                 * fast-cloning again.
+                 */
+                skb->fclone = SKB_FCLONE_UNAVAILABLE;
+
+                if (atomic_dec_and_test(fclone_ref))
+                        kmem_cache_free(skbuff_fclone_cache, other);
+                break;
+        };
 }
 
 /**
@@ -324,10 +365,20 @@ void __kfree_skb(struct sk_buff *skb)
 
 struct sk_buff *skb_clone(struct sk_buff *skb, unsigned int __nocast gfp_mask)
 {
-        struct sk_buff *n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
-
-        if (!n) 
-                return NULL;
+        struct sk_buff *n;
+
+        n = skb + 1;
+        if (skb->fclone == SKB_FCLONE_ORIG &&
+            n->fclone == SKB_FCLONE_UNAVAILABLE) {
+                atomic_t *fclone_ref = (atomic_t *) (n + 1);
+                n->fclone = SKB_FCLONE_CLONE;
+                atomic_inc(fclone_ref);
+        } else {
+                n = kmem_cache_alloc(skbuff_head_cache, gfp_mask);
+                if (!n)
+                        return NULL;
+                n->fclone = SKB_FCLONE_UNAVAILABLE;
+        }
 
 #define C(x) n->x = skb->x
 
@@ -409,6 +460,7 @@ static void copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
         new->mac.raw    = old->mac.raw + offset;
         memcpy(new->cb, old->cb, sizeof(old->cb));
         new->local_df   = old->local_df;
+        new->fclone     = SKB_FCLONE_UNAVAILABLE;
         new->pkt_type   = old->pkt_type;
         new->tstamp     = old->tstamp;
         new->destructor = NULL;
@@ -1647,13 +1699,23 @@ void __init skb_init(void)
                                               NULL, NULL);
         if (!skbuff_head_cache)
                 panic("cannot create skbuff cache");
+
+        skbuff_fclone_cache = kmem_cache_create("skbuff_fclone_cache",
+                                                (2*sizeof(struct sk_buff)) +
+                                                sizeof(atomic_t),
+                                                0,
+                                                SLAB_HWCACHE_ALIGN,
+                                                NULL, NULL);
+        if (!skbuff_fclone_cache)
+                panic("cannot create skbuff cache");
+
         do_gettimeofday(&skb_tv_base);
 }
 
 EXPORT_SYMBOL(___pskb_trim);
 EXPORT_SYMBOL(__kfree_skb);
 EXPORT_SYMBOL(__pskb_pull_tail);
-EXPORT_SYMBOL(alloc_skb);
+EXPORT_SYMBOL(__alloc_skb);
 EXPORT_SYMBOL(pskb_copy);
 EXPORT_SYMBOL(pskb_expand_head);
 EXPORT_SYMBOL(skb_checksum);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8d92ab562aed..75b68116682a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1582,7 +1582,7 @@ void tcp_send_fin(struct sock *sk)
         } else {
                 /* Socket is locked, keep trying until memory is available. */
                 for (;;) {
-                        skb = alloc_skb(MAX_TCP_HEADER, GFP_KERNEL);
+                        skb = alloc_skb_fclone(MAX_TCP_HEADER, GFP_KERNEL);
                         if (skb)
                                 break;
                         yield();
@@ -1804,7 +1804,7 @@ int tcp_connect(struct sock *sk)
 
         tcp_connect_init(sk);
 
-        buff = alloc_skb(MAX_TCP_HEADER + 15, sk->sk_allocation);
+        buff = alloc_skb_fclone(MAX_TCP_HEADER + 15, sk->sk_allocation);
         if (unlikely(buff == NULL))
                 return -ENOBUFS;