From: Eric Dumazet <eduma...@google.com>
slow start after idle might reduce cwnd, but we perform this
after first packet was cooked and sent.
With TSO/GSO, it means that we might send a full TSO packet
even if cwnd should have been reduced to IW10.
Moving the SSAI check in skb_entail() makes sense, because
we slightly reduce number of times this check is done,
especially for large send() and TCP Small queue callbacks from
softirq context.
Tested:
Following packetdrill test demonstrates the problem
// Test of slow start after idle
`sysctl -q net.ipv4.tcp_slow_start_after_idle=1`
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
+0 bind(3, ..., ...) = 0
+0 listen(3, 1) = 0
+0 < S 0:0(0) win 65535 <mss 1000,sackOK,nop,nop,nop,wscale 7>
+0 > S. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 6>
+.100 < . 1:1(0) ack 1 win 511
+0 accept(3, ..., ...) = 4
+0 setsockopt(4, SOL_SOCKET, SO_SNDBUF, [200000], 4) = 0
+0 write(4, ..., 26000) = 26000
+0 > . 1:5001(5000) ack 1
+0 > . 5001:10001(5000) ack 1
+0 %{ assert tcpi_snd_cwnd == 10 }%
+.100 < . 1:1(0) ack 10001 win 511
+0 %{ assert tcpi_snd_cwnd == 20, tcpi_snd_cwnd }%
+0 > . 10001:20001(10000) ack 1
+0 > P. 20001:26001(6000) ack 1
+.100 < . 1:1(0) ack 26001 win 511
+0 %{ assert tcpi_snd_cwnd == 36, tcpi_snd_cwnd }%
+4 write(4, ..., 20000) = 20000
// If slow start after idle works properly, we should send 5 MSS here (cwnd/2)
+0 > . 26001:31001(5000) ack 1
+0 %{ assert tcpi_snd_cwnd == 10, tcpi_snd_cwnd }%
+0 > . 31001:36001(5000) ack 1
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Neal Cardwell <ncardw...@google.com>
Cc: Yuchung Cheng <ych...@google.com>
---
include/net/tcp.h | 1 +
net/ipv4/tcp.c | 8 ++++++++
net/ipv4/tcp_output.c | 12 ++++--------
3 files changed, 13 insertions(+), 8 deletions(-)
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 364426a..639f64e 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1165,6 +1165,7 @@ static inline void tcp_sack_reset(struct
tcp_options_received *rx_opt)
}
u32 tcp_default_init_rwnd(u32 mss);
+void tcp_cwnd_restart(struct sock *sk, s32 delta);
/* Determine a window scaling and initial window to offer. */
void tcp_select_initial_window(int __space, __u32 mss, __u32 *rcv_wnd,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 45534a5..e228433 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -627,6 +627,14 @@ static void skb_entail(struct sock *sk, struct sk_buff
*skb)
sk_mem_charge(sk, skb->truesize);
if (tp->nonagle & TCP_NAGLE_PUSH)
tp->nonagle &= ~TCP_NAGLE_PUSH;
+
+ if (sysctl_tcp_slow_start_after_idle &&
+ sk->sk_write_queue.next == skb) {
+ s32 delta = tcp_time_stamp - tp->lsndtime;
+
+ if (delta > inet_csk(sk)->icsk_rto)
+ tcp_cwnd_restart(sk, delta);
+ }
}
static inline void tcp_mark_urg(struct tcp_sock *tp, int flags)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 444ab5b..1188e4f 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -137,12 +137,12 @@ static __u16 tcp_advertise_mss(struct sock *sk)
}
/* RFC2861. Reset CWND after idle period longer RTO to "restart window".
- * This is the first part of cwnd validation mechanism. */
-static void tcp_cwnd_restart(struct sock *sk, const struct dst_entry *dst)
+ * This is the first part of cwnd validation mechanism.
+ */
+void tcp_cwnd_restart(struct sock *sk, s32 delta)
{
struct tcp_sock *tp = tcp_sk(sk);
- s32 delta = tcp_time_stamp - tp->lsndtime;
- u32 restart_cwnd = tcp_init_cwnd(tp, dst);
+ u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
u32 cwnd = tp->snd_cwnd;
tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
@@ -164,10 +164,6 @@ static void tcp_event_data_sent(struct tcp_sock *tp,
struct inet_connection_sock *icsk = inet_csk(sk);
const u32 now = tcp_time_stamp;
- if (sysctl_tcp_slow_start_after_idle &&
- (!tp->packets_out && (s32)(now - tp->lsndtime) > icsk->icsk_rto))
- tcp_cwnd_restart(sk, __sk_dst_get(sk));
-
tp->lsndtime = now;
/* If it is a reply for ato after last received
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