linux-kernel - Re: [PATCH net-next] tcp: minor optimization in tcp ack fast path processing

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Date:   Wed, 7 Nov 2018 07:16:18 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yafang Shao <laoar.shao@...il.com>, davem@...emloft.net,
        edumazet@...gle.com
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH net-next] tcp: minor optimization in tcp ack fast path
 processing



On 11/07/2018 03:20 AM, Yafang Shao wrote:
> Bitwise operation is a little faster.


> So I replace after() with (flag & FLAG_SND_UNA_ADVANCED) as this flag is
> already set before.
> 
> Cc: Joe Perches <joe@...ches.com>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> ---
>  net/ipv4/tcp_input.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 2868ef2..0167015 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3610,7 +3610,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
>  	if (flag & FLAG_UPDATE_TS_RECENT)
>  		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
>  
> -	if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
> +	if (!(flag & FLAG_SLOWPATH) && (flag & FLAG_SND_UNA_ADVANCED)) {
>  		/* Window is constant, pure forward advance.
>  		 * No more checks are required.
>  		 * Note, we use the fact that SND.UNA>=SND.WL2.
> 

What about reducing this to a single conditional jump ?

if ((flag & (FLAG_SLOWPATH | FLAG_SND_UNA_ADVANCED)) == FLAG_SND_UNA_ADVANCED)  {