tcp: improve latencies of timer triggered events

Modern TCP stack highly depends on tcp_write_timer() having a small
latency, but current implementation doesn't exactly meet the
expectations.

When a timer fires but finds the socket is owned by the user, it rearms
itself for an additional delay hoping next run will be more
successful.

tcp_write_timer() for example uses a 50ms delay for next try, and it
defeats many attempts to get predictable TCP behavior in term of
latencies.

Use the recently introduced tcp_release_cb(), so that the user owning
the socket will call various handlers right before socket release.

This will permit us to post a followup patch to address the
tcp_tso_should_defer() syndrome (some deferred packets have to wait
RTO timer to be transmitted, while cwnd should allow us to send them
sooner)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: H.K. Jerry Chu <hkchu@google.com>
Cc: John Heffner <johnwheffner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 37 insertions, 33 deletions

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index e911e6c523ec..6df36ad55a38 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -32,17 +32,6 @@ int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
 int sysctl_tcp_orphan_retries __read_mostly;
 int sysctl_tcp_thin_linear_timeouts __read_mostly;
 
-static void tcp_write_timer(unsigned long);
-static void tcp_delack_timer(unsigned long);
-static void tcp_keepalive_timer (unsigned long data);
-
-void tcp_init_xmit_timers(struct sock *sk)
-{
-        inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
-                                  &tcp_keepalive_timer);
-}
-EXPORT_SYMBOL(tcp_init_xmit_timers);
-
 static void tcp_write_err(struct sock *sk)
 {
         sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
@@ -205,21 +194,11 @@ static int tcp_write_timeout(struct sock *sk)
         return 0;
 }
 
-static void tcp_delack_timer(unsigned long data)
+void tcp_delack_timer_handler(struct sock *sk)
 {
-        struct sock *sk = (struct sock *)data;
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);
 
-        bh_lock_sock(sk);
-        if (sock_owned_by_user(sk)) {
-                /* Try again later. */
-                icsk->icsk_ack.blocked = 1;
-                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
-                sk_reset_timer(sk, &icsk->icsk_delack_timer, jiffies + TCP_DELACK_MIN);
-                goto out_unlock;
-        }
-
         sk_mem_reclaim_partial(sk);
 
         if (sk->sk_state == TCP_CLOSE || !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
@@ -260,7 +239,21 @@ static void tcp_delack_timer(unsigned long data)
 out:
         if (sk_under_memory_pressure(sk))
                 sk_mem_reclaim(sk);
-out_unlock:
+}
+
+static void tcp_delack_timer(unsigned long data)
+{
+        struct sock *sk = (struct sock *)data;
+
+        bh_lock_sock(sk);
+        if (!sock_owned_by_user(sk)) {
+                tcp_delack_timer_handler(sk);
+        } else {
+                inet_csk(sk)->icsk_ack.blocked = 1;
+                NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
+                /* deleguate our work to tcp_release_cb() */
+                set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags);
+        }
         bh_unlock_sock(sk);
         sock_put(sk);
 }
@@ -450,19 +443,11 @@ out_reset_timer:
 out:;
 }
 
-static void tcp_write_timer(unsigned long data)
+void tcp_write_timer_handler(struct sock *sk)
 {
-        struct sock *sk = (struct sock *)data;
         struct inet_connection_sock *icsk = inet_csk(sk);
         int event;
 
-        bh_lock_sock(sk);
-        if (sock_owned_by_user(sk)) {
-                /* Try again later */
-                sk_reset_timer(sk, &icsk->icsk_retransmit_timer, jiffies + (HZ / 20));
-                goto out_unlock;
-        }
-
         if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)
                 goto out;
 
@@ -485,7 +470,19 @@ static void tcp_write_timer(unsigned long data)
 
 out:
         sk_mem_reclaim(sk);
-out_unlock:
+}
+
+static void tcp_write_timer(unsigned long data)
+{
+        struct sock *sk = (struct sock *)data;
+
+        bh_lock_sock(sk);
+        if (!sock_owned_by_user(sk)) {
+                tcp_write_timer_handler(sk);
+        } else {
+                /* deleguate our work to tcp_release_cb() */
+                set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags);
+        }
         bh_unlock_sock(sk);
         sock_put(sk);
 }
@@ -602,3 +599,10 @@ out:
         bh_unlock_sock(sk);
         sock_put(sk);
 }
+
+void tcp_init_xmit_timers(struct sock *sk)
+{
+        inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
+                                  &tcp_keepalive_timer);
+}
+EXPORT_SYMBOL(tcp_init_xmit_timers);