tcp: makes tcp_try_coalesce aware of skb->head_frag

TCP coalesce can check if skb to be merged has its skb->head mapped to a
page fragment, instead of a kmalloc() area.

We had to disable coalescing in this case, for performance reasons.

We 'upgrade' skb->head as a fragment in itself.

This reduces number of cache misses when user makes its copies, since a
less sk_buff are fetched.

This makes receive and ofo queues shorter and thus reduce cache line
misses in TCP stack.

This is a followup of patch "net: allow skb->head to be a page fragment"

Tested with tg3 nic, with GRO on or off. We can see "TCPRcvCoalesce"
counter being incremented.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Matt Carlson <mcarlson@broadcom.com>
Cc: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 43 insertions, 12 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index c93b0cbb7fc1..96a631deb4e6 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4464,10 +4464,12 @@ static inline int tcp_try_rmem_schedule(struct sock *sk, unsigned int size)
  */
 static bool tcp_try_coalesce(struct sock *sk,
                              struct sk_buff *to,
-                             struct sk_buff *from)
+                             struct sk_buff *from,
+                             bool *fragstolen)
 {
-        int len = from->len;
+        int delta, len = from->len;
 
+        *fragstolen = false;
         if (tcp_hdr(from)->fin)
                 return false;
         if (len <= skb_tailroom(to)) {
@@ -4478,15 +4480,19 @@ merge:
                 TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
                 return true;
         }
+
+        if (skb_has_frag_list(to) || skb_has_frag_list(from))
+                return false;
+
         if (skb_headlen(from) == 0 &&
-            !skb_has_frag_list(to) &&
-            !skb_has_frag_list(from) &&
             (skb_shinfo(to)->nr_frags +
              skb_shinfo(from)->nr_frags <= MAX_SKB_FRAGS)) {
-                int delta = from->truesize - ksize(from->head) -
-                            SKB_DATA_ALIGN(sizeof(struct sk_buff));
+                WARN_ON_ONCE(from->head_frag);
+                delta = from->truesize - ksize(from->head) -
+                        SKB_DATA_ALIGN(sizeof(struct sk_buff));
 
                 WARN_ON_ONCE(delta < len);
+copyfrags:
                 memcpy(skb_shinfo(to)->frags + skb_shinfo(to)->nr_frags,
                        skb_shinfo(from)->frags,
                        skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
@@ -4499,6 +4505,20 @@ merge:
                 to->data_len += len;
                 goto merge;
         }
+        if (from->head_frag) {
+                struct page *page;
+                unsigned int offset;
+
+                if (skb_shinfo(to)->nr_frags + skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
+                        return false;
+                page = virt_to_head_page(from->head);
+                offset = from->data - (unsigned char *)page_address(page);
+                skb_fill_page_desc(to, skb_shinfo(to)->nr_frags,
+                                   page, offset, skb_headlen(from));
+                *fragstolen = true;
+                delta = len; /* we dont know real truesize... */
+                goto copyfrags;
+        }
         return false;
 }
 
@@ -4540,10 +4560,15 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
         end_seq = TCP_SKB_CB(skb)->end_seq;
 
         if (seq == TCP_SKB_CB(skb1)->end_seq) {
-                if (!tcp_try_coalesce(sk, skb1, skb)) {
+                bool fragstolen;
+
+                if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
                         __skb_queue_after(&tp->out_of_order_queue, skb1, skb);
                 } else {
-                        __kfree_skb(skb);
+                        if (fragstolen)
+                                kmem_cache_free(skbuff_head_cache, skb);
+                        else
+                                __kfree_skb(skb);
                         skb = NULL;
                 }
 
@@ -4626,6 +4651,7 @@ static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
         const struct tcphdr *th = tcp_hdr(skb);
         struct tcp_sock *tp = tcp_sk(sk);
         int eaten = -1;
+        bool fragstolen = false;
 
         if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)
                 goto drop;
@@ -4672,7 +4698,9 @@ queue_and_out:
                                 goto drop;
 
                         tail = skb_peek_tail(&sk->sk_receive_queue);
-                        eaten = (tail && tcp_try_coalesce(sk, tail, skb)) ? 1 : 0;
+                        eaten = (tail &&
+                                 tcp_try_coalesce(sk, tail, skb,
+                                                  &fragstolen)) ? 1 : 0;
                         if (eaten <= 0) {
                                 skb_set_owner_r(skb, sk);
                                 __skb_queue_tail(&sk->sk_receive_queue, skb);
@@ -4699,9 +4727,12 @@ queue_and_out:
 
                 tcp_fast_path_check(sk);
 
-                if (eaten > 0)
-                        __kfree_skb(skb);
-                else if (!sock_flag(sk, SOCK_DEAD))
+                if (eaten > 0) {
+                        if (fragstolen)
+                                kmem_cache_free(skbuff_head_cache, skb);
+                        else
+                                __kfree_skb(skb);
+                } else if (!sock_flag(sk, SOCK_DEAD))
                         sk->sk_data_ready(sk, 0);
                 return;
         }