Commit e114a710 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
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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
Signed-off-by: default avatarEric Dumazet <>
Signed-off-by: default avatarNeal Cardwell <>
Signed-off-by: default avatarYuchung Cheng <>
Signed-off-by: default avatarDavid S. Miller <>
parent 4e8bbb81
......@@ -230,6 +230,7 @@ struct tcp_sock {
u32 snd_cwnd_clamp; /* Do not allow snd_cwnd to grow above this */
u32 snd_cwnd_used;
u32 snd_cwnd_stamp;
u32 lsnd_pending; /* packets inflight or unsent since last xmit */
u32 prior_cwnd; /* Congestion window at start of Recovery. */
u32 prr_delivered; /* Number of newly delivered packets to
* receiver in Recovery. */
......@@ -974,7 +974,27 @@ static inline u32 tcp_wnd_end(const struct tcp_sock *tp)
return tp->snd_una + tp->snd_wnd;
bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight);
/* We follow the spirit of RFC2861 to validate cwnd but implement a more
* flexible approach. The RFC suggests cwnd should not be raised unless
* it was fully used previously. But we allow cwnd to grow as long as the
* application has used half the cwnd.
* Example :
* cwnd is 10 (IW10), but application sends 9 frames.
* We allow cwnd to reach 18 when all frames are ACKed.
* This check is safe because it's as aggressive as slow start which already
* risks 100% overshoot. The advantage is that we discourage application to
* either send more filler packets or data to artificially blow up the cwnd
* usage, and allow application-limited process to probe bw more aggressively.
* TODO: remove in_flight once we can fix all callers, and their callers...
static inline bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
const struct tcp_sock *tp = tcp_sk(sk);
return tp->snd_cwnd < 2 * tp->lsnd_pending;
static inline void tcp_check_probe_timer(struct sock *sk)
......@@ -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
......@@ -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))
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, unsent_segs);
return false;
return (push_one == 2) || (!tp->packets_out && tcp_send_head(sk));
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