netdev - Re: [PATCH] tcp: do not update snd_una if it is same with ack

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Date:   Sat, 3 Nov 2018 17:40:39 -0700
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
Subject: Re: [PATCH] tcp: do not update snd_una if it is same with ack_seq



On 11/03/2018 09:54 AM, Yafang Shao wrote:
> In the slow path, TCP_SKB_SB(skb)->ack_seq may be same with tp->snd_una,
> and under this condition we don't need to update the snd_una.
> 
> Furthermore, tcp_ack_update_window() is only called in slow path,
> so introducing this check won't affect the fast path processing.
> 
> By the way, '&' is a little faster than '-', so I replaced after() with
> "flag & FLAG_SND_UNA_ADVANCED".
> 
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> ---
>  net/ipv4/tcp_input.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 2868ef2..db5a6b7 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -3376,7 +3376,8 @@ static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32
>  		}
>  	}
>  
> -	tcp_snd_una_update(tp, ack);
> +	if (after(ack, tp->snd_una))
> +		tcp_snd_una_update(tp, ack);
>  

Adding this after() here is confusing, how ack could be before snd_una ?
That would be a serious bug.

I do not see why another conditional has any gain here.

You are trying to avoid very cheap operations :

    u32 delta = ack - tp->snd_una;

    tp->bytes_acked += delta;
    tp->snd_una = ack;

Maybe the real reason for this patch is not explained in the changelog ?