path: root/net
diff options
authorEric Dumazet <edumazet@google.com>2014-04-30 11:58:13 -0700
committerDavid S. Miller <davem@davemloft.net>2014-05-02 17:54:35 -0400
commite114a710aa5058c0ba4aa1dfb105132aefeb5e04 (patch)
tree3d7c656358bbc5cd37f7c2a973923e6be6ced1d9 /net
parentnet: Allow tc changes in user namespaces (diff)
tcp: fix cwnd limited checking to improve congestion control
Yuchung discovered tcp_is_cwnd_limited() was returning false in slow start phase even if the application filled the socket write queue. All congestion modules take into account tcp_is_cwnd_limited() before increasing cwnd, so this behavior limits slow start from probing the bandwidth at full speed. The problem is that even if write queue is full (aka we are _not_ application limited), cwnd can be under utilized if TSO should auto defer or TCP Small queues decided to hold packets. So the in_flight can be kept to smaller value, and we can get to the point tcp_is_cwnd_limited() returns false. With TCP Small Queues and FQ/pacing, this issue is more visible. We fix this by having tcp_cwnd_validate(), which is supposed to track such things, take into account unsent_segs, the number of segs that we are not sending at the moment due to TSO or TSQ, but intend to send real soon. Then when we are cwnd-limited, remember this fact while we are processing the window of ACKs that comes back. For example, suppose we have a brand new connection with cwnd=10; we are in slow start, and we send a flight of 9 packets. By the time we have received ACKs for all 9 packets we want our cwnd to be 18. We implement this by setting tp->lsnd_pending to 9, and considering ourselves to be cwnd-limited while cwnd is less than twice tp->lsnd_pending (2*9 -> 18). This makes tcp_is_cwnd_limited() more understandable, by removing the GSO/TSO kludge, that tried to work around the issue. Note the in_flight parameter can be removed in a followup cleanup patch. Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
2 files changed, 14 insertions, 27 deletions
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 2b9464c93b88..a93b41ba05ff 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -276,26 +276,6 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
return err;
-/* RFC2861 Check whether we are limited by application or congestion window
- * This is the inverse of cwnd check in tcp_tso_should_defer
- */
-bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
- const struct tcp_sock *tp = tcp_sk(sk);
- u32 left;
- if (in_flight >= tp->snd_cwnd)
- return true;
- left = tp->snd_cwnd - in_flight;
- if (sk_can_gso(sk) &&
- left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
- left < tp->xmit_size_goal_segs)
- return true;
- return left <= tcp_max_tso_deferred_mss(tp);
/* Slow start is used when congestion window is no greater than the slow start
* threshold. We base on RFC2581 and also handle stretch ACKs properly.
* We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 20847de991ea..f9181a133462 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1402,12 +1402,13 @@ static void tcp_cwnd_application_limited(struct sock *sk)
tp->snd_cwnd_stamp = tcp_time_stamp;
-/* Congestion window validation. (RFC2861) */
-static void tcp_cwnd_validate(struct sock *sk)
+static void tcp_cwnd_validate(struct sock *sk, u32 unsent_segs)
struct tcp_sock *tp = tcp_sk(sk);
- if (tp->packets_out >= tp->snd_cwnd) {
+ tp->lsnd_pending = tp->packets_out + unsent_segs;
+ if (tcp_is_cwnd_limited(sk, 0)) {
/* Network is feed fully. */
tp->snd_cwnd_used = 0;
tp->snd_cwnd_stamp = tcp_time_stamp;
@@ -1880,7 +1881,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
struct tcp_sock *tp = tcp_sk(sk);
struct sk_buff *skb;
- unsigned int tso_segs, sent_pkts;
+ unsigned int tso_segs, sent_pkts, unsent_segs = 0;
int cwnd_quota;
int result;
@@ -1924,7 +1925,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
} else {
if (!push_one && tcp_tso_should_defer(sk, skb))
- break;
+ goto compute_unsent_segs;
/* TCP Small Queues :
@@ -1949,8 +1950,14 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
* there is no smp_mb__after_set_bit() yet
- if (atomic_read(&sk->sk_wmem_alloc) > limit)
+ if (atomic_read(&sk->sk_wmem_alloc) > limit) {
+ u32 unsent_bytes;
+ unsent_bytes = tp->write_seq - tp->snd_nxt;
+ unsent_segs = DIV_ROUND_UP(unsent_bytes, mss_now);
+ }
limit = mss_now;
@@ -1990,7 +1997,7 @@ repair:
/* Send one loss probe per tail loss episode. */
if (push_one != 2)
- tcp_cwnd_validate(sk);
+ tcp_cwnd_validate(sk, unsent_segs);
return false;
return (push_one == 2) || (!tp->packets_out && tcp_send_head(sk));