5.4-stable review patch. If anyone has any objections, please let me know.
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From: Eric Dumazet edumazet@google.com
[ Upstream commit 295ce1eb36ae47dc862d6c8a1012618a25516208 ]
Neal reported that using neper tcp_stream with TCP_TX_DELAY set to 50ms would often lead to flows stuck in a small cwnd mode, regardless of the congestion control.
While tcp_stream sets TCP_TX_DELAY too late after the connect(), it highlighted two kernel bugs.
The following heuristic in tcp_tso_should_defer() seems wrong for large RTT:
delta = tp->tcp_clock_cache - head->tstamp; /* If next ACK is likely to come too late (half srtt), do not defer */ if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0) goto send_now;
If next ACK is expected to come in more than 1 ms, we should not defer because we prefer a smooth ACK clocking.
While blamed commit was a step in the good direction, it was not generic enough.
Another patch fixing TCP_TX_DELAY for established flows will be proposed when net-next reopens.
Fixes: 50c8339e9299 ("tcp: tso: restore IW10 after TSO autosizing") Reported-by: Neal Cardwell ncardwell@google.com Signed-off-by: Eric Dumazet edumazet@google.com Reviewed-by: Neal Cardwell ncardwell@google.com Tested-by: Neal Cardwell ncardwell@google.com Link: https://patch.msgid.link/20251011115742.1245771-1-edumazet@google.com [pabeni@redhat.com: fixed whitespace issue] Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_output.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-)
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 4f203cbbc99b5..6492110e0c9b0 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1952,7 +1952,8 @@ static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb, u32 max_segs) { const struct inet_connection_sock *icsk = inet_csk(sk); - u32 send_win, cong_win, limit, in_flight; + u32 send_win, cong_win, limit, in_flight, threshold; + u64 srtt_in_ns, expected_ack, how_far_is_the_ack; struct tcp_sock *tp = tcp_sk(sk); struct sk_buff *head; int win_divisor; @@ -2014,9 +2015,19 @@ static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb, head = tcp_rtx_queue_head(sk); if (!head) goto send_now; - delta = tp->tcp_clock_cache - head->tstamp; - /* If next ACK is likely to come too late (half srtt), do not defer */ - if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0) + + srtt_in_ns = (u64)(NSEC_PER_USEC >> 3) * tp->srtt_us; + /* When is the ACK expected ? */ + expected_ack = head->tstamp + srtt_in_ns; + /* How far from now is the ACK expected ? */ + how_far_is_the_ack = expected_ack - tp->tcp_clock_cache; + + /* If next ACK is likely to come too late, + * ie in more than min(1ms, half srtt), do not defer. + */ + threshold = min(srtt_in_ns >> 1, NSEC_PER_MSEC); + + if ((s64)(how_far_is_the_ack - threshold) > 0) goto send_now;
/* Ok, it looks like it is advisable to defer.