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

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Date:   Fri, 23 Sep 2016 11:09:33 -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 Fri, Sep 23, 2016 at 06:42:51AM -0700, Eric Dumazet wrote:
> On Fri, 2016-09-23 at 09:45 -0300, Marcelo Ricardo Leitner wrote:
> 
> > Aye. In that case, what about using tail instead of end?
> 
> 
> What do you mean exactly ?

Something like:
-skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
+skb->truesize = SKB_TRUESIZE(skb_tail_offset(skb));

And define skb_tail_offset() to something similar skb_end_offset(), so
that it would account only for the data and not unused space in the
buffer.

> 
> >  Because
> > accounting for something that we have to tweak the limits to accept is
> > like adding a constant to both sides of the equation.
> > But perhaps that would cut out too much of the fat which could be used
> > later by the stack.
> 
> Are you facing a particular problem with current code ?
> 

For TCP, no, just wondering. :-)

I'm having similar issues with SCTP: if the socket gets backlogged, the
buffer accounting gets pretty messy. SCTP calculates the rwnd to be just
rcvbuf/2, but this ratio is often different in backlog and it causes the
advertized window to be too big, resulting in packet drops in the
backlog.

SCTP has some way to identify and compensate this "extra" rwnd, via
rwnd_press, and will shrink it if it detects that the window is bigger
than the buffer available. But as the socket is backlogged, it's doesn't
kick in soon enough to prevent such drops.

It's not just a matter of adjusting the overhead ratio (rcvbuf/2)
because with SCTP the packets may have different sizes, so a packet with
a chunk of 100 bytes will have a ratio and another with 1000 bytes will
have another, within the same association.

So I'm leaning towards on updating truesize before adding to the
backlog, but to account just for the actual packet, regardless of the
buffer that was used for it. It still has the overhead ratio issue with
different packet sizes, though, but smaller.

Note that SCTP doesn't have buffer auto-tuning yet.

> I am working to reduce the SACK at their source (the receiver), instead
> of trying to filter them when they had to travel all the way back to TCP
> sender.
>

Cool.