Re: [PATCH net] tcp: fix potential xmit stalls caused by TCP_NOTSENT_LOWAT

[Soheil Hassas Yeganeh <soheil@google.com>]

On Sun, Apr 24, 2022 at 8:34 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>
> From: Eric Dumazet <edumazet@google.com>
>
> I had this bug sitting for too long in my pile, it is time to fix it.
>
> Thanks to Doug Porter for reminding me of it!
>
> We had various attempts in the past, including commit
> 0cbe6a8f089e ("tcp: remove SOCK_QUEUE_SHRUNK"),
> but the issue is that TCP stack currently only generates
> EPOLLOUT from input path, when tp->snd_una has advanced
> and skb(s) cleaned from rtx queue.
>
> If a flow has a big RTT, and/or receives SACKs, it is possible
> that the notsent part (tp->write_seq - tp->snd_nxt) reaches 0
> and no more data can be sent until tp->snd_una finally advances.
>
> What is needed is to also check if POLLOUT needs to be generated
> whenever tp->snd_nxt is advanced, from output path.
>
> This bug triggers more often after an idle period, as
> we do not receive ACK for at least one RTT. tcp_notsent_lowat
> could be a fraction of what CWND and pacing rate would allow to
> send during this RTT.
>
> In a followup patch, I will remove the bogus call
> to tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED)
> from tcp_check_space(). Fact that we have decided to generate
> an EPOLLOUT does not mean the application has immediately
> refilled the transmit queue. This optimistic call
> might have been the reason the bug seemed not too serious.
>
> Tested:
>
> 200 ms rtt, 1% packet loss, 32 MB tcp_rmem[2] and tcp_wmem[2]
>
> $ echo 500000 >/proc/sys/net/ipv4/tcp_notsent_lowat
> $ cat bench_rr.sh
> SUM=0
> for i in {1..10}
> do
>  V=`netperf -H remote_host -l30 -t TCP_RR -- -r 10000000,10000 -o LOCAL_BYTES_SENT | egrep -v "MIGRATED|Bytes"`
>  echo $V
>  SUM=$(($SUM + $V))
> done
> echo SUM=$SUM
>
> Before patch:
> $ bench_rr.sh
> 130000000
> 80000000
> 140000000
> 140000000
> 140000000
> 140000000
> 130000000
> 40000000
> 90000000
> 110000000
> SUM=1140000000
>
> After patch:
> $ bench_rr.sh
> 430000000
> 590000000
> 530000000
> 450000000
> 450000000
> 350000000
> 450000000
> 490000000
> 480000000
> 460000000
> SUM=4680000000  # This is 410 % of the value before patch.
>
> Fixes: c9bee3b7fdec ("tcp: TCP_NOTSENT_LOWAT socket option")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Doug Porter <dsp@fb.com>
> Cc: Soheil Hassas Yeganeh <soheil@google.com>
> Cc: Neal Cardwell <ncardwell@google.com>

Acked-by: Soheil Hassas Yeganeh <soheil@google.com>

Thank you for the fix!

> ---
>  include/net/tcp.h     |  1 +
>  net/ipv4/tcp_input.c  | 12 +++++++++++-
>  net/ipv4/tcp_output.c |  1 +
>  3 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 9987b3fba9f202632916cc439af9d17f1e68bcd3..cc1295037533a7741e454f7c040f77a21deae02b 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -621,6 +621,7 @@ void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req);
>  void tcp_reset(struct sock *sk, struct sk_buff *skb);
>  void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb);
>  void tcp_fin(struct sock *sk);
> +void tcp_check_space(struct sock *sk);
>
>  /* tcp_timer.c */
>  void tcp_init_xmit_timers(struct sock *);
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 2088f93fa37b5fb9110e7933242a27bd4009990e..48f6075228600896daa6507c4cd06acfc851a0fa 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5454,7 +5454,17 @@ static void tcp_new_space(struct sock *sk)
>         INDIRECT_CALL_1(sk->sk_write_space, sk_stream_write_space, sk);
>  }
>
> -static void tcp_check_space(struct sock *sk)
> +/* Caller made space either from:
> + * 1) Freeing skbs in rtx queues (after tp->snd_una has advanced)
> + * 2) Sent skbs from output queue (and thus advancing tp->snd_nxt)
> + *
> + * We might be able to generate EPOLLOUT to the application if:
> + * 1) Space consumed in output/rtx queues is below sk->sk_sndbuf/2
> + * 2) notsent amount (tp->write_seq - tp->snd_nxt) became
> + *    small enough that tcp_stream_memory_free() decides it
> + *    is time to generate EPOLLOUT.
> + */
> +void tcp_check_space(struct sock *sk)
>  {
>         /* pairs with tcp_poll() */
>         smp_mb();
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 9ede847f4199844c5884e3f62ea450562072a0a7..1ca2f28c9981018e6cfaee3435d711467af6048d 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -82,6 +82,7 @@ static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
>
>         NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
>                       tcp_skb_pcount(skb));
> +       tcp_check_space(sk);
>  }
>
>  /* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
> --
> 2.36.0.rc2.479.g8af0fa9b8e-goog
>