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SubjectRe: [PATCH] tcp: do not update snd_una if it is same with ack_seq
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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@gmail.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 ?

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