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:

quoted

+bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
+{
+	u32 limit = sk->sk_rcvbuf + sk->sk_sndbuf;

                                 ^^^
...

quoted

+	if (!skb->data_len)
+		skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
+
+	if (unlikely(sk_add_backlog(sk, skb, limit))) {

...

quoted

-	} else if (unlikely(sk_add_backlog(sk, skb,
-					   sk->sk_rcvbuf + sk->sk_sndbuf))) {

	                                                 ^---- [1]

quoted

-		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)