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tcp: fix sk_rcvbuf overshoot

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Current autosizing in tcp_rcv_space_adjust() is too aggressive.

Instead of betting on possible losses and over estimate BDP,
it is better to only account for slow start.

The following patch is then adding a more precise tuning
in the events of packet losses.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250513193919.1089692-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2025-05-13 19:39:10 +00:00 committed by Jakub Kicinski
parent c1269d3d12
commit 65c5287892
1 changed files with 25 additions and 34 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

59
net/ipv4/tcp_input.c
View file

@ -747,6 +747,29 @@ static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
}
}
static void tcp_rcvbuf_grow(struct sock *sk)
{
const struct net *net = sock_net(sk);
struct tcp_sock *tp = tcp_sk(sk);
int rcvwin, rcvbuf, cap;
if (!READ_ONCE(net->ipv4.sysctl_tcp_moderate_rcvbuf) ||
(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
return;
/* slow start: allow the sender to double its rate. */
rcvwin = tp->rcvq_space.space << 1;
cap = READ_ONCE(net->ipv4.sysctl_tcp_rmem[2]);
rcvbuf = min_t(u32, tcp_space_from_win(sk, rcvwin), cap);
if (rcvbuf > sk->sk_rcvbuf) {
WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);
/* Make the window clamp follow along. */
WRITE_ONCE(tp->window_clamp,
tcp_win_from_space(sk, rcvbuf));
}
}
/*
* This function should be called every time data is copied to user space.
* It calculates the appropriate TCP receive buffer space.
@ -771,42 +794,10 @@ void tcp_rcv_space_adjust(struct sock *sk)
trace_tcp_rcvbuf_grow(sk, time);
/* A bit of theory :
* copied = bytes received in previous RTT, our base window
* To cope with packet losses, we need a 2x factor
* To cope with slow start, and sender growing its cwin by 100 %
* every RTT, we need a 4x factor, because the ACK we are sending
* now is for the next RTT, not the current one :
* <prev RTT . ><current RTT .. ><next RTT .... >
*/
if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf) &&
!(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
u64 rcvwin, grow;
int rcvbuf;
/* minimal window to cope with packet losses, assuming
* steady state. Add some cushion because of small variations.
*/
rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
/* Accommodate for sender rate increase (eg. slow start) */
grow = rcvwin * (copied - tp->rcvq_space.space);
do_div(grow, tp->rcvq_space.space);
rcvwin += (grow << 1);
rcvbuf = min_t(u64, tcp_space_from_win(sk, rcvwin),
READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
if (rcvbuf > sk->sk_rcvbuf) {
WRITE_ONCE(sk->sk_rcvbuf, rcvbuf);
/* Make the window clamp follow along. */
WRITE_ONCE(tp->window_clamp,
tcp_win_from_space(sk, rcvbuf));
}
}
tp->rcvq_space.space = copied;
tcp_rcvbuf_grow(sk);
new_measure:
tp->rcvq_space.seq = tp->copied_seq;
tp->rcvq_space.time = tp->tcp_mstamp;