On Tue, 8 Dec 2020 22:57:59 -0500 Neal Cardwell wrote: > From: Neal Cardwell <[email protected]> > > When cwnd is not a multiple of the TSO skb size of N*MSS, we can get > into persistent scenarios where we have the following sequence: > > (1) ACK for full-sized skb of N*MSS arrives > -> tcp_write_xmit() transmit full-sized skb with N*MSS > -> move pacing release time forward > -> exit tcp_write_xmit() because pacing time is in the future > > (2) TSQ callback or TCP internal pacing timer fires > -> try to transmit next skb, but TSO deferral finds remainder of > available cwnd is not big enough to trigger an immediate send > now, so we defer sending until the next ACK. > > (3) repeat... > > So we can get into a case where we never mark ourselves as > cwnd-limited for many seconds at a time, even with > bulk/infinite-backlog senders, because: > > o In case (1) above, every time in tcp_write_xmit() we have enough > cwnd to send a full-sized skb, we are not fully using the cwnd > (because cwnd is not a multiple of the TSO skb size). So every time we > send data, we are not cwnd limited, and so in the cwnd-limited > tracking code in tcp_cwnd_validate() we mark ourselves as not > cwnd-limited. > > o In case (2) above, every time in tcp_write_xmit() that we try to > transmit the "remainder" of the cwnd but defer, we set the local > variable is_cwnd_limited to true, but we do not send any packets, so > sent_pkts is zero, so we don't call the cwnd-limited logic to update > tp->is_cwnd_limited. > > Fixes: ca8a22634381 ("tcp: make cwnd-limited checks measurement-based, and > gentler") > Reported-by: Ingemar Johansson <[email protected]> > Signed-off-by: Neal Cardwell <[email protected]> > Signed-off-by: Yuchung Cheng <[email protected]> > Acked-by: Soheil Hassas Yeganeh <[email protected]> > Signed-off-by: Eric Dumazet <[email protected]>
Applied, thank you!
