tcp: change tcp_skb_pcount() location

Our goal is to access no more than one cache line access per skb in
a write or receive queue when doing the various walks.

After recent TCP_SKB_CB() reorganizations, it is almost done.

Last part is tcp_skb_pcount() which currently uses
skb_shinfo(skb)->gso_segs, which is a terrible choice, because it needs
3 cache lines in current kernel (skb->head, skb->end, and
shinfo->gso_segs are all in 3 different cache lines, far from skb->cb)

This very simple patch reuses space currently taken by tcp_tw_isn
only in input path, as tcp_skb_pcount is only needed for skb stored in
write queue.

This considerably speeds up tcp_ack(), granted we avoid shinfo->tx_flags
to get SKBTX_ACK_TSTAMP, which seems possible.

This also speeds up all sack processing in general.

This speeds up tcp_sendmsg() because it no longer has to access/dirty
shinfo.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 4 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index f3f016a15c5a..2c0af90231cf 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1295,9 +1295,9 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
         TCP_SKB_CB(prev)->end_seq += shifted;
         TCP_SKB_CB(skb)->seq += shifted;
 
-        skb_shinfo(prev)->gso_segs += pcount;
-        BUG_ON(skb_shinfo(skb)->gso_segs < pcount);
-        skb_shinfo(skb)->gso_segs -= pcount;
+        tcp_skb_pcount_add(prev, pcount);
+        BUG_ON(tcp_skb_pcount(skb) < pcount);
+        tcp_skb_pcount_add(skb, -pcount);
 
         /* When we're adding to gso_segs == 1, gso_size will be zero,
          * in theory this shouldn't be necessary but as long as DSACK
@@ -1310,7 +1310,7 @@ static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
         }
 
         /* CHECKME: To clear or not to clear? Mimics normal skb currently */
-        if (skb_shinfo(skb)->gso_segs <= 1) {
+        if (tcp_skb_pcount(skb) <= 1) {
                 skb_shinfo(skb)->gso_size = 0;
                 skb_shinfo(skb)->gso_type = 0;
         }