6.5-stable review patch. If anyone has any objections, please let me know.
------------------
From: Eric Dumazet <edumazet@xxxxxxxxxx>
commit f921a4a5bffa8a0005b190fb9421a7fc1fd716b6 upstream.
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@xxxxxxx/
Reported-by: Stefan Wahren <wahrenst@xxxxxxx>
Tested-by: Stefan Wahren <wahrenst@xxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231017124526.4060202-1-edumazet@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
net/ipv4/tcp_output.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2527,6 +2527,18 @@ static bool tcp_pacing_check(struct sock
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)
@@ -2564,12 +2576,12 @@ static bool tcp_small_queue_check(struct
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);
