netdev - Re: [PATCH v3 net-next 3/4] tcp: make tcp

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Date:   Wed, 28 Nov 2018 15:54:13 +0100
From:   Jean-Louis Dupond <jean-louis@...ond.be>
To:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v3 net-next 3/4] tcp: make tcp_space() aware of socket
 backlog

On 27/11/18 23:42, Eric Dumazet wrote:
> Jean-Louis Dupond reported poor iscsi TCP receive performance
> that we tracked to backlog drops.
>
> Apparently we fail to send window updates reflecting the
> fact that we are under stress.
>
> Note that we might lack a proper window increase when
> backlog is fully processed, since __release_sock() clears
> sk->sk_backlog.len _after_ all skbs have been processed.
>
> This should not matter in practice. If we had a significant
> load through socket backlog, we are in a dangerous
> situation.
>
> Reported-by: Jean-Louis Dupond <jean-louis@...ond.be>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
> ---
>   include/net/tcp.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 0c61bf0a06dac95268c26b6302a2afbaef4c88b3..3b522259da7d5a54d7d3730ddd8d8c9ef24313e1 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1368,7 +1368,7 @@ static inline int tcp_win_from_space(const struct sock *sk, int space)
>   /* Note: caller must be prepared to deal with negative returns */
>   static inline int tcp_space(const struct sock *sk)
>   {
> -	return tcp_win_from_space(sk, sk->sk_rcvbuf -
> +	return tcp_win_from_space(sk, sk->sk_rcvbuf - sk->sk_backlog.len -
>   				  atomic_read(&sk->sk_rmem_alloc));
>   }
>   

Tested-by: Jean-Louis Dupond<jean-louis@...ond.be>

Big difference in performance :)
Thanks a lot!