netdev - Re: [PATCH net-next] tcp: remove loop to compute wscale

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Date:   Thu, 29 Nov 2018 11:03:36 -0500
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: remove loop to compute wscale

On Thu, Nov 29, 2018 at 10:56 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> We can remove the loop and conditional branches
> and compute wscale efficiently thanks to ilog2()
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Very nice, thank you, Eric!

> ---
>  net/ipv4/tcp_output.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 3ae4a277991a3473f61b1f73acd5ee355cf6a5df..d3b691f3a9e896ce340ae081a0d24092024361c8 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -233,16 +233,14 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
>         if (init_rcv_wnd)
>                 *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
>
> -       (*rcv_wscale) = 0;
> +       *rcv_wscale = 0;
>         if (wscale_ok) {
>                 /* Set window scaling on max possible window */
>                 space = max_t(u32, space, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
>                 space = max_t(u32, space, sysctl_rmem_max);
>                 space = min_t(u32, space, *window_clamp);
> -               while (space > U16_MAX && (*rcv_wscale) < TCP_MAX_WSCALE) {
> -                       space >>= 1;
> -                       (*rcv_wscale)++;
> -               }
> +               *rcv_wscale = clamp_t(int, ilog2(space) - 15,
> +                                     0, TCP_MAX_WSCALE);
>         }
>         /* Set the clamp no higher than max representable value */
>         (*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);
> --
> 2.20.0.rc0.387.gc7a69e6b6c-goog
>