On Thu, 25 Feb 2021 15:25:15 -0800 Jakub Kicinski wrote:
> Hi!
>
> We see large (4-8x) increase of what looks like TCP RTOs after rising
> the Tx coalescing above Rx coalescing timeout.
>
> Quick tracing of the events seems to indicate that the data has already
> been acked when we enter tcp:tcp_retransmit_skb:
Seems like I'm pretty lost here and the tcp:tcp_retransmit_skb events
are less spurious than I thought. Looking at some tcpdump traces we see:
0.045277 IP6 A > B: Flags [SEW], seq 2248382925:2248383296, win 61920, options
[mss 1440,sackOK,TS val 658870494 ecr 0,nop,wscale 11], length 371
0.045348 IP6 B > A: Flags [S.E], seq 961169456, ack 2248382926, win 65535,
options [mss 1440,sackOK,TS val 883864022 ecr 658870494,nop,wscale 9], length 0
0.045369 IP6 A > B: Flags [P.], seq 1:372, ack 1, win 31, options [nop,nop,TS
val 658870494 ecr 883864022], length 371
So looks potentially TFO related?
To try to count timeouts I run:
bpftrace --btf -e 'tracepoint:tcp:tcp_retransmit_skb {
$icsk = (struct inet_connection_sock *)args->skaddr;
if ($icsk->icsk_ca_state != 4) { return; }
if ($icsk->icsk_pending) { return; }
printf(...);
}'
At tx-usecs coalescing of 25us I see 0 of those events.
At 100us there is a few.
At 200us there is a lot.
