netdev - Re: [PATCH net v3 3/3] tcp_cubic: fix to use emulated Reno cwnd one RTT in the future

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Message-ID: <CADVnQy=URvcxoU70b0TMJ9gpYVWgKE_CNwQXrP9r2ZJ3EGWgfg@mail.gmail.com>
Date: Fri, 16 Aug 2024 14:32:35 -0400
From: Neal Cardwell <ncardwell@...gle.com>
To: Mingrui Zhang <mrzhang97@...il.com>
Cc: edumazet@...gle.com, davem@...emloft.net, netdev@...r.kernel.org, 
	Lisong Xu <xu@....edu>
Subject: Re: [PATCH net v3 3/3] tcp_cubic: fix to use emulated Reno cwnd one
 RTT in the future

On Thu, Aug 15, 2024 at 5:41 PM Mingrui Zhang <mrzhang97@...il.com> wrote:
>
> The original code estimates RENO snd_cwnd using the estimated
> RENO snd_cwnd at the current time (i.e., tcp_cwnd).
>
> The patched code estimates RENO snd_cwnd using the estimated
> RENO snd_cwnd after one RTT (i.e., tcp_cwnd_next_rtt),
> because ca->cnt is used to increase snd_cwnd for the next RTT.
>
> Fixes: 89b3d9aaf467 ("[TCP] cubic: precompute constants")
> Signed-off-by: Mingrui Zhang <mrzhang97@...il.com>
> Signed-off-by: Lisong Xu <xu@....edu>
> ---
> v2->v3: Corrent the "Fixes:" footer content
> v1->v2: Separate patches
>
>  net/ipv4/tcp_cubic.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
> index 7bc6db82de66..a1467f99a233 100644
> --- a/net/ipv4/tcp_cubic.c
> +++ b/net/ipv4/tcp_cubic.c
> @@ -315,8 +315,11 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
>                         ca->tcp_cwnd++;
>                 }
>
> -               if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
> -                       delta = ca->tcp_cwnd - cwnd;
> +               /* Reno cwnd one RTT in the future */
> +               u32 tcp_cwnd_next_rtt = ca->tcp_cwnd + (ca->ack_cnt + cwnd) / delta;
> +
> +               if (tcp_cwnd_next_rtt > cwnd) {  /* if bic is slower than Reno */
> +                       delta = tcp_cwnd_next_rtt - cwnd;
>                         max_cnt = cwnd / delta;
>                         if (ca->cnt > max_cnt)
>                                 ca->cnt = max_cnt;
> --

I'm getting a compilation error with clang:

net/ipv4/tcp_cubic.c:322:7: error: mixing declarations and code
is incompatible with standards before C99
[-Werror,-Wdeclaration-after-statement]
    u32 tcp_cwnd_next_rtt = ca->tcp_cwnd + (ca->ack_cnt + cwnd) / delta;

Can you please try something like the following:

-               u32 scale = beta_scale;
+               u32 scale = beta_scale, tcp_cwnd_next_rtt;
...
+               tcp_cwnd_next_rtt = ca->tcp_cwnd + (ca->ack_cnt + cwnd) / delta;

Thanks!
neal