From: Eric Dumazet edumazet@google.com
[ Upstream commit 4bfe744ff1644fbc0a991a2677dc874475dd6776 ]
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 Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Sasha Levin sashal@kernel.org --- 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 4602959b58a1..181db7dab176 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -585,6 +585,7 @@ void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req); void tcp_reset(struct sock *sk); 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 9382caeb721a..f5cc025003cd 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -5114,7 +5114,17 @@ static void tcp_new_space(struct sock *sk) sk->sk_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) { if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) { sock_reset_flag(sk, SOCK_QUEUE_SHRUNK); diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 83c0e859bb33..1a5c42c67d42 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -81,6 +81,7 @@ static void tcp_event_new_data_sent(struct sock *sk, const 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