Re: [PATCH 3/3] Check num sacks in SACK fast path

[David Miller <davem@davemloft.net>]

From: Baruch Even <baruch@ev-en.org>
Date: Thu, 1 Feb 2007 09:22:52 +0200

> * David Miller <davem@davemloft.net> [070131 22:52]:
> > We should never see a SACK block from sequence zero to zero,
> > which would be an empty SACK block.
> 
> That would work as well at the cost of extra writing to memory for each
> ack packet. Though I won't guess what is worse, the extra memory used or
> the extra writing.

Good point.  The recv_sack_cache is 32-bytes, so sits in at most
2 cache lines.  The writes are consequetive and in order so would
compress into cpu store buffers.

But it's definitely not free. :-)

> > Something like the following?
> > 
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index c26076f..84cd722 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -999,6 +1001,10 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
> >  				return 0;
> >  		}
> >  	}
> > +	for (; i <= 4; i++) {
> 
> That won't work though, the <= should be <, I've actually used
> ARRAY_SIZE just to be on the safe side.

Thanks for catching that.