Messages in this thread Patch in this message | | | From | Pengcheng Yang <> | Subject | [PATCH RFC net-next] tcp: make cwnd-limited not affected by tcp internal pacing | Date | Thu, 19 Mar 2020 14:32:29 +0800 |
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The current cwnd-limited is set when cwnd is fully used (inflight >= cwnd), which allows the congestion algorithm to accurately determine whether cwnd needs to be added.
However, there may be a problem when using tcp internal pacing: In congestion avoidance phase, when a burst of packets are acked by a stretched ACK or a burst of ACKs, this makes a large reduction in inflight in a short time. At this time, the sender sends data according to the pacing rate cannot fill CWND and cwnd-limited is not set. The worst case is that cwnd-limited is set only after the last packet in a window is sent. This causes the congestion algorithm to be too conservative to increase CWND.
The idea is that once cwnd-limited is set, it maintains a window period. In this period, it is considered that the CWND is limited. This makes the congestion algorithm unaffected by tcp internal pacing.
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com> --- include/linux/tcp.h | 2 +- net/ipv4/tcp_output.c | 14 ++++++++------ 2 files changed, 9 insertions(+), 7 deletions(-)
diff --git a/include/linux/tcp.h b/include/linux/tcp.h index 3dc9640..3b3329f 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -286,7 +286,7 @@ struct tcp_sock { u32 packets_out; /* Packets which are "in flight" */ u32 retrans_out; /* Retransmitted packets out */ u32 max_packets_out; /* max packets_out in last window */ - u32 max_packets_seq; /* right edge of max_packets_out flight */ + u32 cwnd_limited_seq; /* snd_nxt at cwnd limited */ u16 urg_data; /* Saved octet of OOB data and control flags */ u8 ecn_flags; /* ECN status bits. */ diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 306e25d..31dd6dc 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1705,14 +1705,16 @@ static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited) const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops; struct tcp_sock *tp = tcp_sk(sk); - /* Track the maximum number of outstanding packets in each - * window, and remember whether we were cwnd-limited then. + /* Remember whether we were cwnd-limited in last window, + * and track the maximum number of outstanding packets in each window. */ - if (!before(tp->snd_una, tp->max_packets_seq) || - tp->packets_out > tp->max_packets_out) { - tp->max_packets_out = tp->packets_out; - tp->max_packets_seq = tp->snd_nxt; + if (is_cwnd_limited || + !before(tp->snd_una, tp->cwnd_limited_seq)) { tp->is_cwnd_limited = is_cwnd_limited; + tp->cwnd_limited_seq = tp->snd_nxt; + tp->max_packets_out = tp->packets_out; + } else if (tp->packets_out > tp->max_packets_out) { + tp->max_packets_out = tp->packets_out; } if (tcp_is_cwnd_limited(sk)) { -- 1.8.3.1
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