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Message-ID: <CAL+tcoBYvV58rz9KcrH5QTdc1jPi=H1VXt1yUj-pYwA1FE9CEg@mail.gmail.com> Date: Tue, 30 May 2023 20:52:30 +0800 From: Jason Xing <kerneljasonxing@...il.com> To: Eric Dumazet <edumazet@...gle.com> Cc: toke@...e.dk, Yuchung Cheng <ycheng@...gle.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>, netdev@...r.kernel.org, zhangweiping <zhangweiping@...iglobal.com>, tiozhang <tiozhang@...iglobal.com>, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, fuyuanli@...iglobal.com Subject: Re: [PATCH net] tcp: introduce a compack timer handler in sack compression On Tue, May 30, 2023 at 7:10 PM Jason Xing <kerneljasonxing@...il.com> wrote: > > On Tue, May 30, 2023 at 6:01 PM Eric Dumazet <edumazet@...gle.com> wrote: > > > > On Tue, May 30, 2023 at 9:45 AM Jason Xing <kerneljasonxing@...il.com> wrote: > > > > > > On Tue, May 30, 2023 at 3:12 PM Eric Dumazet <edumazet@...gle.com> wrote: > > > > > > > > On Mon, May 29, 2023 at 1:38 PM fuyuanli <fuyuanli@...iglobal.com> wrote: > > > > > > > > > > We've got some issues when sending a compressed ack is deferred to > > > > > release phrase due to the socket owned by another user: > > > > > 1. a compressed ack would not be sent because of lack of ICSK_ACK_TIMER > > > > > flag. > > > > > > > > Are you sure ? Just add it then, your patch will be a one-liner > > > > instead of a complex one adding > > > > more code in a fast path. > > > > > > Honestly, at the very beginning, we just added one line[1] to fix > > > this. After I digged more into this part, I started to doubt if we > > > should reuse the delayed ack logic. > > > > > > Because in the sack compression logic there is no need to do more > > > things as delayed ack does in the tcp_delack_timer_handler() function. > > > > > > Besides, here are some things extra to be done if we defer to send an > > > ack in sack compression: > > > 1) decrease tp->compressed_ack. The same as "tp->compressed_ack--;" in > > > tcp_compressed_ack_kick(). > > > 2) initialize icsk->icsk_ack.timeout. Actually we don't need to do > > > this because we don't modify the expiration time in the sack > > > compression hrtimer. > > > > Yes, we do not need this, see my following comment. > > > > > 3) don't need to count the LINUX_MIB_DELAYEDACKS counter. > > > 4) I wonder even if those checks about the ack schedule or ping pong > > > mode in tcp_delack_timer_handler() for sack compression? I'm not sure > > > about it. > > > > > > So one line cannot solve it perfectly. That's the reason why we > > > introduce a new logic which can be clearer. > > > > > > I'm wondering if adding one check in the fast path is really that > > > unacceptable (it may hurt performance?) because a new logic would be > > > clearer for the whole sack compression. > > > > We definitely can solve the minor issue by not polluting the fast path. > > > > We also want simple/localised fixes, not something invasive (and risky). > > Now I got it :) > > > > > > > > > [1] > > > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c > > > index cc072d2cfcd8..d9e76d761cc6 100644 > > > --- a/net/ipv4/tcp_input.c > > > +++ b/net/ipv4/tcp_input.c > > > @@ -5568,6 +5568,7 @@ static void __tcp_ack_snd_check(struct sock *sk, > > > int ofo_possible) > > > > > > READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns), > > > rtt * (NSEC_PER_USEC >> 3)/20); > > > sock_hold(sk); > > > + inet_csk(sk)->icsk_ack.pending |= ICSK_ACK_TIMER; > > > > Why not simply use existing storage/variables (tp->compressed_ack), > > instead of trying > > to reuse something else or add another bit, then complain that this > > does not work well ? > > > > Again, just fix tcp_delack_timer_handler(), it already can fetch existing state. > > > > As a bonus, no need to send one patch for net, and another in net-next, > > trying to 'fix' issues that should have been fixed cleanly in a single patch. > > > > Something like: > > > > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c > > index b839c2f91292f7346f33d6dcbf597594473a5aca..16bc4cedceb8a5e88f61f9abc2c0a8cc9322676a > > 100644 > > --- a/net/ipv4/tcp_timer.c > > +++ b/net/ipv4/tcp_timer.c > > @@ -290,10 +290,20 @@ static int tcp_write_timeout(struct sock *sk) > > void tcp_delack_timer_handler(struct sock *sk) > > { > > struct inet_connection_sock *icsk = inet_csk(sk); > > + struct tcp_sock *tp = tcp_sk(sk); > > > > - if (((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) || > > - !(icsk->icsk_ack.pending & ICSK_ACK_TIMER)) > > + if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)) > > + return; > > + > > + if (!(icsk->icsk_ack.pending & ICSK_ACK_TIMER) && > > + !tp->compressed_ack) > > + return; > > + > > + if (tp->compressed_ack) { > > + tcp_mstamp_refresh(tp); > > + tcp_sack_compress_send_ack(sk); > > I wonder if we could use this combination as below instead since the > above function counts the snmp counter and clears the @compressed_ack, > which is against what we normally do in tcp_compressed_ack_kick() if > the socket is not owned? I take it back. After I considered it for a while, I think it's working because @compressed_ack and snmp counter are a pair which means we can do both of them together. It doesn't have any impact though it looks a bit different from before. Thanks, Jason > > " > if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1) > __sock_put(sk); > tcp_send_ack(sk); > " > > Above is extracted from tcp_sack_compress_send_ack() > > > return; > > + } > > > > if (time_after(icsk->icsk_ack.timeout, jiffies)) { > > sk_reset_timer(sk, &icsk->icsk_delack_timer, > > icsk->icsk_ack.timeout); > > @@ -312,7 +322,7 @@ void tcp_delack_timer_handler(struct sock *sk) > > inet_csk_exit_pingpong_mode(sk); > > icsk->icsk_ack.ato = TCP_ATO_MIN; > > } > > - tcp_mstamp_refresh(tcp_sk(sk)); > > + tcp_mstamp_refresh(tp); > > tcp_send_ack(sk); > > __NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKS); > > } > > Thank you so much, Eric. > > I will let fuyuanli submit a v2 patch including that one without > introducing a new logic/flag. > > Thanks again, > Jason
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