tcp: cache result of earlier divides when mss-aligning things

The results is very unlikely change every so often so we
hardly need to divide again after doing that once for a
connection. Yet, if divide still becomes necessary we
detect that and do the right thing and again settle for
non-divide state. Takes the u16 space which was previously
taken by the plain xmit_size_goal.

This should take care part of the tso vs non-tso difference
we found earlier.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 2 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 886596ff0aae..0db9f3b984f7 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -665,7 +665,7 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
                                        int large_allowed)
 {
         struct tcp_sock *tp = tcp_sk(sk);
-        u32 xmit_size_goal;
+        u32 xmit_size_goal, old_size_goal;
 
         xmit_size_goal = mss_now;
 
@@ -676,7 +676,17 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
                                   tp->tcp_header_len);
 
                 xmit_size_goal = tcp_bound_to_half_wnd(tp, xmit_size_goal);
-                xmit_size_goal -= (xmit_size_goal % mss_now);
+
+                /* We try hard to avoid divides here */
+                old_size_goal = tp->xmit_size_goal_segs * mss_now;
+
+                if (likely(old_size_goal <= xmit_size_goal &&
+                           old_size_goal + mss_now > xmit_size_goal)) {
+                        xmit_size_goal = old_size_goal;
+                } else {
+                        tp->xmit_size_goal_segs = xmit_size_goal / mss_now;
+                        xmit_size_goal = tp->xmit_size_goal_segs * mss_now;
+                }
         }
 
         return xmit_size_goal;