On Fri, 26 Feb 2021 11:41:22 +0100 Eric Dumazet wrote: > > > 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 > > > > The SYNACK does not include the prior payload. > > > > > 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 this rtx is not spurious. > > > > However in your prior email you wrote : > > > > bytes_in: 0 > > bytes_out: 742 > > bytes_acked: 742 > > > > Are you sure that at the time of the retransmit, bytes_acked was 742 ? > > I do not see how this could happen. > > Yes, this packetdrill test confirms TCP INFO stuff seems correct .
Looks like it's TcpExtTCPSpuriousRtxHostQueues - the TFO fails as it might, but at the time the syn is still not kfree_skb()d because of the IRQ coalescing settings, so __tcp_retransmit_skb() returns -EBUSY and we have to wait for a timeout. Credit to Neil Spring @FB for figuring it out.
