tcp: adjust window probe timers to safer values

With the advent of small rto timers in datacenter TCP,
(ip route ... rto_min x), the following can happen :

1) Qdisc is full, transmit fails.

   TCP sets a timer based on icsk_rto to retry the transmit, without
   exponential backoff.
   With low icsk_rto, and lot of sockets, all cpus are servicing timer
   interrupts like crazy.
   Intent of the code was to retry with a timer between 200 (TCP_RTO_MIN)
   and 500ms (TCP_RESOURCE_PROBE_INTERVAL)

2) Receivers can send zero windows if they don't drain their receive queue.

   TCP sends zero window probes, based on icsk_rto current value, with
   exponential backoff.
   With /proc/sys/net/ipv4/tcp_retries2 being 15 (or even smaller in
   some cases), sender can abort in less than one or two minutes !
   If receiver stops the sender, it obviously doesn't care of very tight
   rto. Probability of dropping the ACK reopening the window is not
   worth the risk.

Lets change the base timer to be at least 200ms (TCP_RTO_MIN) for these
events (but not normal RTO based retransmits)

A followup patch adds a new SNMP counter, as it would have helped a lot
diagnosing this issue.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 6d204f3f9df8cafb82d856db08769a7d24dfd79e..7a2248a35b137f8d82cc0a543fa7a9991f51aeaa 100644 (file)
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1043,14 +1043,31 @@ static inline bool tcp_is_cwnd_limited(const struct sock *sk)
        return tp->is_cwnd_limited;
 }
 
-static inline void tcp_check_probe_timer(struct sock *sk)
+/* Something is really bad, we could not queue an additional packet,
+ * because qdisc is full or receiver sent a 0 window.
+ * We do not want to add fuel to the fire, or abort too early,
+ * so make sure the timer we arm now is at least 200ms in the future,
+ * regardless of current icsk_rto value (as it could be ~2ms)
+ */
+static inline unsigned long tcp_probe0_base(const struct sock *sk)
 {
-       const struct tcp_sock *tp = tcp_sk(sk);
-       const struct inet_connection_sock *icsk = inet_csk(sk);
+       return max_t(unsigned long, inet_csk(sk)->icsk_rto, TCP_RTO_MIN);
+}
 
-       if (!tp->packets_out && !icsk->icsk_pending)
+/* Variant of inet_csk_rto_backoff() used for zero window probes */
+static inline unsigned long tcp_probe0_when(const struct sock *sk,
+                                           unsigned long max_when)
+{
+       u64 when = (u64)tcp_probe0_base(sk) << inet_csk(sk)->icsk_backoff;
+
+       return (unsigned long)min_t(u64, when, max_when);
+}
+
+static inline void tcp_check_probe_timer(struct sock *sk)
+{
+       if (!tcp_sk(sk)->packets_out && !inet_csk(sk)->icsk_pending)
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-                                         icsk->icsk_rto, TCP_RTO_MAX);
+                                         tcp_probe0_base(sk), TCP_RTO_MAX);
 }
 
 static inline void tcp_init_wl(struct tcp_sock *tp, u32 seq)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index df2ca615cd0c2221b439a921aeb28bdf2317b85c..cf8b20ff665852a370bfd5ed6adb51143546bf6b 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3233,7 +3233,7 @@ static void tcp_ack_probe(struct sock *sk)
                 * This function is not for random using!
                 */
        } else {
-               unsigned long when = inet_csk_rto_backoff(icsk, TCP_RTO_MAX);
+               unsigned long when = tcp_probe0_when(sk, TCP_RTO_MAX);
 
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
                                          when, TCP_RTO_MAX);

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index a369e8a70b2c775bfee94d7f329ee892c2cdc895..b76c719e1979288529f6d2f11288a7a6981a5eb6 100644 (file)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3490,7 +3490,7 @@ void tcp_send_probe0(struct sock *sk)
                probe_max = TCP_RESOURCE_PROBE_INTERVAL;
        }
        inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
-                                 inet_csk_rto_backoff(icsk, probe_max),
+                                 tcp_probe0_when(sk, probe_max),
                                  TCP_RTO_MAX);
 }