netdev - Re: [PATCH net-next] tcp: add tcp_add

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Message-ID: <20160922223411.GA17222@localhost.localdomain>
Date:   Thu, 22 Sep 2016 19:34:11 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] tcp: add tcp_add_backlog()

On Sat, Aug 27, 2016 at 07:37:54AM -0700, Eric Dumazet wrote:
> +bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
> +{
> +	u32 limit = sk->sk_rcvbuf + sk->sk_sndbuf;
                                 ^^^
...
> +	if (!skb->data_len)
> +		skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
> +
> +	if (unlikely(sk_add_backlog(sk, skb, limit))) {
...
> -	} else if (unlikely(sk_add_backlog(sk, skb,
> -					   sk->sk_rcvbuf + sk->sk_sndbuf))) {
	                                                 ^---- [1]
> -		bh_unlock_sock(sk);
> -		__NET_INC_STATS(net, LINUX_MIB_TCPBACKLOGDROP);
> +	} else if (tcp_add_backlog(sk, skb)) {

Hi Eric, after this patch, do you think we still need to add sk_sndbuf
as a stretching factor to the backlog here?

It was added by [1] and it was justified that the (s)ack packets were
just too big for the rx buf size. Maybe this new patch alone is enough
already, as such packets will have a very small truesize then.

  Marcelo

[1] da882c1f2eca ("tcp: sk_add_backlog() is too agressive for TCP")