From f921a4a5bffa8a0005b190fb9421a7fc1fd716b6 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 17 Oct 2023 12:45:26 +0000 Subject: tcp: tsq: relax tcp_small_queue_check() when rtx queue contains a single skb In commit 75eefc6c59fd ("tcp: tsq: add a shortcut in tcp_small_queue_check()") we allowed to send an skb regardless of TSQ limits being hit if rtx queue was empty or had a single skb, in order to better fill the pipe when/if TX completions were slow. Then later, commit 75c119afe14f ("tcp: implement rb-tree based retransmit queue") accidentally removed the special case for one skb in rtx queue. Stefan Wahren reported a regression in single TCP flow throughput using a 100Mbit fec link, starting from commit 65466904b015 ("tcp: adjust TSO packet sizes based on min_rtt"). This last commit only made the regression more visible, because it locked the TCP flow on a particular behavior where TSQ prevented two skbs being pushed downstream, adding silences on the wire between each TSO packet. Many thanks to Stefan for his invaluable help ! Fixes: 75c119afe14f ("tcp: implement rb-tree based retransmit queue") Link: https://lore.kernel.org/netdev/7f31ddc8-9971-495e-a1f6-819df542e0af@gmx.net/ Reported-by: Stefan Wahren Tested-by: Stefan Wahren Signed-off-by: Eric Dumazet Acked-by: Neal Cardwell Link: https://lore.kernel.org/r/20231017124526.4060202-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- net/ipv4/tcp_output.c | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) (limited to 'net') diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index bbd85672fda7..f0723460753c 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2542,6 +2542,18 @@ static bool tcp_pacing_check(struct sock *sk) return true; } +static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk) +{ + const struct rb_node *node = sk->tcp_rtx_queue.rb_node; + + /* No skb in the rtx queue. */ + if (!node) + return true; + + /* Only one skb in rtx queue. */ + return !node->rb_left && !node->rb_right; +} + /* TCP Small Queues : * Control number of packets in qdisc/devices to two packets / or ~1 ms. * (These limits are doubled for retransmits) @@ -2579,12 +2591,12 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb, limit += extra_bytes; } if (refcount_read(&sk->sk_wmem_alloc) > limit) { - /* Always send skb if rtx queue is empty. + /* Always send skb if rtx queue is empty or has one skb. * No need to wait for TX completion to call us back, * after softirq/tasklet schedule. * This helps when TX completions are delayed too much. */ - if (tcp_rtx_queue_empty(sk)) + if (tcp_rtx_queue_empty_or_single_skb(sk)) return false; set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags); -- cgit v1.2.3