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| author | Eric Dumazet <edumazet@google.com> | 2025-10-11 11:57:42 +0000 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2025-10-29 14:01:19 +0100 |
| commit | 7e2f67b26288bf615455940f6a95d1fa5b4e7c84 (patch) | |
| tree | a1372580c10b036ced0f7e207bbf12b7a9482028 /net | |
| parent | 60acb5c3bbe3022087a9a740b0c2defbdac478d1 (diff) | |
| download | linux-7e2f67b26288bf615455940f6a95d1fa5b4e7c84.tar.gz linux-7e2f67b26288bf615455940f6a95d1fa5b4e7c84.tar.bz2 linux-7e2f67b26288bf615455940f6a95d1fa5b4e7c84.zip | |
tcp: fix tcp_tso_should_defer() vs large RTT
[ 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>
Diffstat (limited to 'net')
| -rw-r--r-- | net/ipv4/tcp_output.c | 19 |
1 files changed, 15 insertions, 4 deletions
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 32e38ac5ee2b..88e8d6543948 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2177,7 +2177,8 @@ 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; @@ -2239,9 +2240,19 @@ 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. |