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

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Date:   Thu, 22 Sep 2016 16:21:30 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...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 Thu, 2016-09-22 at 19:34 -0300, Marcelo Ricardo Leitner wrote:
> 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")
> 

Hi Marcelo

Yes, it is still needed, some drivers provide linear skbs, so the
skb->truesize of ack packets will likely be the same (skb->head points
to a full size frame allocated by the driver)