| Message ID | 20240406182107.261472-3-jmaloy@redhat.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | tcp: add support for SO_PEEK_OFF socket option | expand |
On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote: > > From: Jon Maloy <jmaloy@redhat.com> > > Testing of the previous commit ("tcp: add support for SO_PEEK_OFF") > in this series along with the pasta protocol splicer revealed a bug in > the way tcp handles window advertising during extreme memory squeeze > situations. > > The excerpt of the below logging session shows what is happeing: > > [5201<->54494]: ==== Activating log @ tcp_select_window()/268 ==== > [5201<->54494]: (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE > [5201<->54494]: tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0 > [5201<->54494]: ADVERTISING WINDOW SIZE 0 > [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(->) > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > [5201<->54494]: NOT calling tcp_send_ack() > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83 > > [...] I would prefer a packetdrill test, it is not clear what is happening... In particular, have you used SO_RCVBUF ? > > [5201<->54494]: tcp_recvmsg_locked(->) > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > [5201<->54494]: NOT calling tcp_send_ack() > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1 > > [5201<->54494]: tcp_recvmsg_locked(->) > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > [5201<->54494]: NOT calling tcp_send_ack() > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0 > > [5201<->54494]: tcp_recvmsg_locked(->) > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: NOT calling tcp_send_ack() > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 > > We can see that although we are adverising a window size of zero, > tp->rcv_wnd is not updated accordingly. This leads to a discrepancy > between this side's and the peer's view of the current window size. > - The peer thinks the window is zero, and stops sending. > - This side ends up in a cycle where it repeatedly caclulates a new > window size it finds too small to advertise. > > Hence no messages are received, and no acknowledges are sent, and > the situation remains locked even after the last queued receive buffer > has been consumed. > > We fix this by setting tp->rcv_wnd to 0 before we return from the > function tcp_select_window() in this particular case. > Further testing shows that the connection recovers neatly from the > squeeze situation, and traffic can continue indefinitely. > > Reviewed-by: Stefano Brivio <sbrivio@redhat.com> > Signed-off-by: Jon Maloy <jmaloy@redhat.com> > --- > net/ipv4/tcp_output.c | 14 +++++++++----- > 1 file changed, 9 insertions(+), 5 deletions(-) > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > index 9282fafc0e61..57ead8f3c334 100644 > --- a/net/ipv4/tcp_output.c > +++ b/net/ipv4/tcp_output.c > @@ -263,11 +263,15 @@ static u16 tcp_select_window(struct sock *sk) > u32 cur_win, new_win; > > /* Make the window 0 if we failed to queue the data because we > - * are out of memory. The window is temporary, so we don't store > - * it on the socket. > + * are out of memory. The window needs to be stored in the socket > + * for the connection to recover. > */ > - if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) > - return 0; > + if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) { > + new_win = 0; > + tp->rcv_wnd = 0; > + tp->rcv_wup = tp->rcv_nxt; > + goto out; > + } > > cur_win = tcp_receive_window(tp); > new_win = __tcp_select_window(sk); > @@ -301,7 +305,7 @@ static u16 tcp_select_window(struct sock *sk) > > /* RFC1323 scaling applied */ > new_win >>= tp->rx_opt.rcv_wscale; > - > +out: > /* If we advertise zero window, disable fast path. */ > if (new_win == 0) { > tp->pred_flags = 0; > -- > 2.42.0 > Any particular reason to not cc Menglong Dong ? (I just did) This code was added in commit e2142825c120d4317abf7160a0fc34b3de532586 Author: Menglong Dong <imagedong@tencent.com> Date: Fri Aug 11 10:55:27 2023 +0800 net: tcp: send zero-window ACK when no memory For now, skb will be dropped when no memory, which makes client keep retrans util timeout and it's not friendly to the users. In this patch, we reply an ACK with zero-window in this case to update the snd_wnd of the sender to 0. Therefore, the sender won't timeout the connection and will probe the zero-window with the retransmits. Signed-off-by: Menglong Dong <imagedong@tencent.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
On Sun, Apr 7, 2024 at 2:38 AM Eric Dumazet <edumazet@google.com> wrote: > > On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote: > > > > From: Jon Maloy <jmaloy@redhat.com> > > > > Testing of the previous commit ("tcp: add support for SO_PEEK_OFF") > > in this series along with the pasta protocol splicer revealed a bug in > > the way tcp handles window advertising during extreme memory squeeze > > situations. > > > > The excerpt of the below logging session shows what is happeing: > > > > [5201<->54494]: ==== Activating log @ tcp_select_window()/268 ==== > > [5201<->54494]: (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE > > [5201<->54494]: tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0 > > [5201<->54494]: ADVERTISING WINDOW SIZE 0 > > [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83 > > > > [...] > > I would prefer a packetdrill test, it is not clear what is happening... > > In particular, have you used SO_RCVBUF ? > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 > > > > We can see that although we are adverising a window size of zero, > > tp->rcv_wnd is not updated accordingly. This leads to a discrepancy > > between this side's and the peer's view of the current window size. > > - The peer thinks the window is zero, and stops sending. > > - This side ends up in a cycle where it repeatedly caclulates a new > > window size it finds too small to advertise. > > > > Hence no messages are received, and no acknowledges are sent, and > > the situation remains locked even after the last queued receive buffer > > has been consumed. > > > > We fix this by setting tp->rcv_wnd to 0 before we return from the > > function tcp_select_window() in this particular case. > > Further testing shows that the connection recovers neatly from the > > squeeze situation, and traffic can continue indefinitely. > > > > Reviewed-by: Stefano Brivio <sbrivio@redhat.com> > > Signed-off-by: Jon Maloy <jmaloy@redhat.com> > > --- > > net/ipv4/tcp_output.c | 14 +++++++++----- > > 1 file changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c > > index 9282fafc0e61..57ead8f3c334 100644 > > --- a/net/ipv4/tcp_output.c > > +++ b/net/ipv4/tcp_output.c > > @@ -263,11 +263,15 @@ static u16 tcp_select_window(struct sock *sk) > > u32 cur_win, new_win; > > > > /* Make the window 0 if we failed to queue the data because we > > - * are out of memory. The window is temporary, so we don't store > > - * it on the socket. > > + * are out of memory. The window needs to be stored in the socket > > + * for the connection to recover. > > */ > > - if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) > > - return 0; > > + if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) { > > + new_win = 0; > > + tp->rcv_wnd = 0; > > + tp->rcv_wup = tp->rcv_nxt; > > + goto out; > > + } > > > > cur_win = tcp_receive_window(tp); > > new_win = __tcp_select_window(sk); > > @@ -301,7 +305,7 @@ static u16 tcp_select_window(struct sock *sk) > > > > /* RFC1323 scaling applied */ > > new_win >>= tp->rx_opt.rcv_wscale; > > - > > +out: > > /* If we advertise zero window, disable fast path. */ > > if (new_win == 0) { > > tp->pred_flags = 0; > > -- > > 2.42.0 > > > > Any particular reason to not cc Menglong Dong ? > (I just did) He is not working at Tencent any more. Let me CC here one more time.
On Sun, Apr 7, 2024 at 2:52 PM Jason Xing <kerneljasonxing@gmail.com> wrote: > > On Sun, Apr 7, 2024 at 2:38 AM Eric Dumazet <edumazet@google.com> wrote: > > > > On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote: > > > > > > From: Jon Maloy <jmaloy@redhat.com> > > > > > > Testing of the previous commit ("tcp: add support for SO_PEEK_OFF") > > > in this series along with the pasta protocol splicer revealed a bug in > > > the way tcp handles window advertising during extreme memory squeeze > > > situations. > > > > > > The excerpt of the below logging session shows what is happeing: > > > > > > [5201<->54494]: ==== Activating log @ tcp_select_window()/268 ==== > > > [5201<->54494]: (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE > > > [5201<->54494]: tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0 > > > [5201<->54494]: ADVERTISING WINDOW SIZE 0 > > > [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > > [5201<->54494]: NOT calling tcp_send_ack() > > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83 > > > > > > [...] > > > > I would prefer a packetdrill test, it is not clear what is happening... > > > > In particular, have you used SO_RCVBUF ? > > > > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > > [5201<->54494]: NOT calling tcp_send_ack() > > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1 > > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > > [5201<->54494]: NOT calling tcp_send_ack() > > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0 > > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: NOT calling tcp_send_ack() > > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 > > > > > > We can see that although we are adverising a window size of zero, > > > tp->rcv_wnd is not updated accordingly. This leads to a discrepancy > > > between this side's and the peer's view of the current window size. > > > - The peer thinks the window is zero, and stops sending. Hi! In my original logic, the client will send a zero-window ack when it drops the skb because it is out of the memory. And the peer SHOULD keep retrans the dropped packet. Does the peer do the transmission in this case? The receive window of the peer SHOULD recover once the retransmission is successful. > > > - This side ends up in a cycle where it repeatedly caclulates a new > > > window size it finds too small to advertise. Yeah, the zero-window suppressed the sending of ack in __tcp_cleanup_rbuf, which I wasn't aware of. The ack will recover the receive window of the peer. Does it make the peer retrans the dropped data immediately? In my opinion, the peer still needs to retrans the dropped packet until the retransmission timer timeout. Isn't it? If it is, maybe we can do the retransmission immediately if we are in zero-window from a window-shrink, which can make the recovery faster. [......] > > Any particular reason to not cc Menglong Dong ? > > (I just did) > > He is not working at Tencent any more. Let me CC here one more time. Thanks for CC the new email of mine, it's very kind of you, xing :/
On Sat, Apr 6, 2024 at 8:37 PM Eric Dumazet <edumazet@google.com> wrote: > > On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote: > > > > From: Jon Maloy <jmaloy@redhat.com> > > > > Testing of the previous commit ("tcp: add support for SO_PEEK_OFF") > > in this series along with the pasta protocol splicer revealed a bug in > > the way tcp handles window advertising during extreme memory squeeze > > situations. > > > > The excerpt of the below logging session shows what is happeing: > > > > [5201<->54494]: ==== Activating log @ tcp_select_window()/268 ==== > > [5201<->54494]: (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM) --> TRUE > > [5201<->54494]: tcp_select_window(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354, returning 0 > > [5201<->54494]: ADVERTISING WINDOW SIZE 0 > > [5201<->54494]: __tcp_transmit_skb(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 83 > > > > [...] > > I would prefer a packetdrill test, it is not clear what is happening... > > In particular, have you used SO_RCVBUF ? > > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 131072 bytes, window now: 250164, qlen: 1 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: (win_now: 250164, new_win: 262144 >= (2 * win_now): 500328))? --> time_to_ack: 0 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0 > > > > [5201<->54494]: tcp_recvmsg_locked(->) > > [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: NOT calling tcp_send_ack() > > [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 > > [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 > > > > We can see that although we are adverising a window size of zero, > > tp->rcv_wnd is not updated accordingly. This leads to a discrepancy > > between this side's and the peer's view of the current window size. > > - The peer thinks the window is zero, and stops sending. > > - This side ends up in a cycle where it repeatedly caclulates a new > > window size it finds too small to advertise. > > > > Hence no messages are received, and no acknowledges are sent, and > > the situation remains locked even after the last queued receive buffer > > has been consumed. > > > > We fix this by setting tp->rcv_wnd to 0 before we return from the > > function tcp_select_window() in this particular case. > > Further testing shows that the connection recovers neatly from the > > squeeze situation, and traffic can continue indefinitely. > > > > Reviewed-by: Stefano Brivio <sbrivio@redhat.com> > > Signed-off-by: Jon Maloy <jmaloy@redhat.com> I do not think this patch is good. If we reach zero window, it is a sign something is wrong. TCP has heuristics to slow down the sender if the receiver does not drain the receive queue fast enough. MSG_PEEK is an obvious reason, and SO_RCVLOWAT too. I suggest you take a look at tcp_set_rcvlowat(), see what is needed for SO_PEEK_OFF (ab)use ? In short, when SO_PEEK_OFF is in action : - TCP needs to not delay ACK when receive queue starts to fill - TCP needs to make sure sk_rcvbuf and tp->window_clamp grow (if autotuning is enabled)
On 2024-04-07 03:51, Menglong Dong wrote: > On Sun, Apr 7, 2024 at 2:52 PM Jason Xing <kerneljasonxing@gmail.com> wrote: >> On Sun, Apr 7, 2024 at 2:38 AM Eric Dumazet <edumazet@google.com> wrote: [...] >>>> [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 >>>> >>>> We can see that although we are adverising a window size of zero, >>>> tp->rcv_wnd is not updated accordingly. This leads to a discrepancy >>>> between this side's and the peer's view of the current window size. >>>> - The peer thinks the window is zero, and stops sending. > Hi! > > In my original logic, the client will send a zero-window > ack when it drops the skb because it is out of the > memory. And the peer SHOULD keep retrans the dropped > packet. > > Does the peer do the transmission in this case? The receive > window of the peer SHOULD recover once the > retransmission is successful. The "peer" is this case is our user-space protocol splicer, emulating the behavior of of the remote end socket. At a first glance, it looks like it is *not* performing any retransmits at all when it sees a zero window at the receiver, so this might indeed be the problem. I will be out of office today, but I will test this later this week. ///jon > >>>> - This side ends up in a cycle where it repeatedly caclulates a new >>>> window size it finds too small to advertise. > Yeah, the zero-window suppressed the sending of ack in > __tcp_cleanup_rbuf, which I wasn't aware of. > > The ack will recover the receive window of the peer. Does > it make the peer retrans the dropped data immediately? > In my opinion, the peer still needs to retrans the dropped > packet until the retransmission timer timeout. Isn't it? > > If it is, maybe we can do the retransmission immediately > if we are in zero-window from a window-shrink, which can > make the recovery faster. > > [......] >>> Any particular reason to not cc Menglong Dong ? >>> (I just did) >> He is not working at Tencent any more. Let me CC here one more time. > Thanks for CC the new email of mine, it's very kind of you, > xing :/ >
On 2024-04-08 06:03, Eric Dumazet wrote: > On Sat, Apr 6, 2024 at 8:37 PM Eric Dumazet <edumazet@google.com> wrote: >> On Sat, Apr 6, 2024 at 8:21 PM <jmaloy@redhat.com> wrote: [...] >>> [5201<->54494]: tcp_recvmsg_locked(<-) returning 57036 bytes, window now: 250164, qlen: 0 >>> >>> [5201<->54494]: tcp_recvmsg_locked(->) >>> [5201<->54494]: __tcp_cleanup_rbuf(->) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 >>> [5201<->54494]: NOT calling tcp_send_ack() >>> [5201<->54494]: __tcp_cleanup_rbuf(<-) tp->rcv_wup: 2812454294, tp->rcv_wnd: 5812224, tp->rcv_nxt 2818016354 >>> [5201<->54494]: tcp_recvmsg_locked(<-) returning -11 bytes, window now: 250164, qlen: 0 >>> >>> We can see that although we are adverising a window size of zero, >>> tp->rcv_wnd is not updated accordingly. This leads to a discrepancy >>> between this side's and the peer's view of the current window size. >>> - The peer thinks the window is zero, and stops sending. >>> - This side ends up in a cycle where it repeatedly caclulates a new >>> window size it finds too small to advertise. >>> >>> Hence no messages are received, and no acknowledges are sent, and >>> the situation remains locked even after the last queued receive buffer >>> has been consumed. >>> >>> We fix this by setting tp->rcv_wnd to 0 before we return from the >>> function tcp_select_window() in this particular case. >>> Further testing shows that the connection recovers neatly from the >>> squeeze situation, and traffic can continue indefinitely. >>> >>> Reviewed-by: Stefano Brivio <sbrivio@redhat.com> >>> Signed-off-by: Jon Maloy <jmaloy@redhat.com> > I do not think this patch is good. If we reach zero window, it is a > sign something is wrong. > > TCP has heuristics to slow down the sender if the receiver does not > drain the receive queue fast enough. > > MSG_PEEK is an obvious reason, and SO_RCVLOWAT too. > > I suggest you take a look at tcp_set_rcvlowat(), see what is needed > for SO_PEEK_OFF (ab)use ? > > In short, when SO_PEEK_OFF is in action : > - TCP needs to not delay ACK when receive queue starts to fill > - TCP needs to make sure sk_rcvbuf and tp->window_clamp grow (if > autotuning is enabled) > We are not talking about the same socket here. The one being overloaded is the terminating socket at the guest side. This is just a regular socket not using MSG_PEEK or SO_PEEK_OFF. SO_PEEK_OFF is used in the intermediate socket terminating the connection towards the remote end. We want to preserve the message in its receive queue until it has been acknowledged by the guest side, so we don't need to keep a copy of it in user space. This seems to work flawlessly. Anyway, I think this is worth taking a closer look at, as you say. I don't think this situation should occur at all. ///jon
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 9282fafc0e61..57ead8f3c334 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -263,11 +263,15 @@ static u16 tcp_select_window(struct sock *sk) u32 cur_win, new_win; /* Make the window 0 if we failed to queue the data because we - * are out of memory. The window is temporary, so we don't store - * it on the socket. + * are out of memory. The window needs to be stored in the socket + * for the connection to recover. */ - if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) - return 0; + if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM)) { + new_win = 0; + tp->rcv_wnd = 0; + tp->rcv_wup = tp->rcv_nxt; + goto out; + } cur_win = tcp_receive_window(tp); new_win = __tcp_select_window(sk); @@ -301,7 +305,7 @@ static u16 tcp_select_window(struct sock *sk) /* RFC1323 scaling applied */ new_win >>= tp->rx_opt.rcv_wscale; - +out: /* If we advertise zero window, disable fast path. */ if (new_win == 0) { tp->pred_flags = 0;