netdev - Re: [PATCH net v3] tcp: fix mishandling when the sack compression is deferred

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Date: Wed, 31 May 2023 15:24:26 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: fuyuanli <fuyuanli@...iglobal.com>, "David S. Miller" <davem@...emloft.net>, 
	David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Neal Cardwell <ncardwell@...gle.com>, ycheng <ycheng@...gle.com>, toke <toke@...e.dk>, 
	netdev@...r.kernel.org, Weiping Zhang <zhangweiping@...iglobal.com>, 
	Tio Zhang <tiozhang@...iglobal.com>
Subject: Re: [PATCH net v3] tcp: fix mishandling when the sack compression is deferred

On Wed, May 31, 2023 at 2:48 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, May 30, 2023 at 6:03 PM fuyuanli <fuyuanli@...iglobal.com> wrote:
> >
> > In this patch, we mainly try to handle sending a compressed ack
> > correctly if it's deferred.
> >
> > Here are more details in the old logic:
> > When sack compression is triggered in the tcp_compressed_ack_kick(),
> > if the sock is owned by user, it will set TCP_DELACK_TIMER_DEFERRED
> > and then defer to the release cb phrase. Later once user releases
> > the sock, tcp_delack_timer_handler() should send a ack as expected,
> > which, however, cannot happen due to lack of ICSK_ACK_TIMER flag.
> > Therefore, the receiver would not sent an ack until the sender's
> > retransmission timeout. It definitely increases unnecessary latency.
> >
> > Fixes: 5d9f4262b7ea ("tcp: add SACK compression")
> > Suggested-by: Eric Dumazet <edumazet@...gle.com>
> > Signed-off-by: fuyuanli <fuyuanli@...iglobal.com>
> > Signed-off-by: Jason Xing <kerneljasonxing@...il.com>
> > Link: https://lore.kernel.org/netdev/20230529113804.GA20300@didi-ThinkCentre-M920t-N000/
> > ---
> > v3:
> > 1) remove the flag which is newly added in v2 patch.
> > 2) adjust the commit message.
> >
> > v2:
> > 1) change the commit title and message
> > 2) reuse the delayed ack logic when handling the sack compression
> > as suggested by Eric.
> > 3) "merge" another related patch into this one. See the second link.
> > ---
> >  include/net/tcp.h    |  1 +
> >  net/ipv4/tcp_input.c |  2 +-
> >  net/ipv4/tcp_timer.c | 16 +++++++++++++---
> >  3 files changed, 15 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index 18a038d16434..6e1cd583a899 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -480,6 +480,7 @@ int tcp_disconnect(struct sock *sk, int flags);
> >

[...]
> >  void tcp_finish_connect(struct sock *sk, struct sk_buff *skb);
> >  int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size);
> > +void tcp_sack_compress_send_ack(struct sock *sk);
> >  void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb);
> >
> >  /* From syncookies.c */
>
>
> Minor nit, could you move this in the following section ?

Sure, it's much better.

>
> /* tcp_input.c */
> void tcp_rearm_rto(struct sock *sk);
> 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_sack_compress_send_ack() is in fact in tcp_input.c
>
> Then add
>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
>
> Thanks !

Thanks for your review :)

Jason