lkml.org 
[lkml]   [2009]   [Jan]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH][TCP]: simplify tcp_mark_lost_retrans()
On Wed, 7 Jan 2009, Arnd Hannemann wrote:

> I noticed

Good that somebody else is looking TCP code besides me... :-)

> that in tcp_mark_lost_retrans the for-loop is only entered
> if tcp_is_fack(tp) evaluates to true:
>
> if (!tcp_is_fack(tp) || !tp->retrans_out ||
> !after(received_upto, tp->lost_retrans_low) ||
> icsk->icsk_ca_state != TCP_CA_Recovery)
> return;
>
> Therefore the following check in the for-loop seems to be redundant,
> because it always evaluates to true:
>
> (tcp_is_fack(tp) ||
> !before(received_upto,
> ack_seq + tp->reordering * tp->mss_cache))
>
> Did I miss something?

It was just a left over from the RFC3517 SACK addition which added that
!tcp_is_fack(tp) there above. ...It would have been nice to have similar
lost rexmit feature without FACK as well but calculating that wasn't
trivial (or I didn't find that too trivial) and could end up being
extremely expensive in case of large holes. (So I also left it there as
sort of reminder).

On the second thought, it would be possible to count skbs we pass while
walking from the beginning and use that a remaining_sacked counter
to get rid of all heurestics too and base the counting only on sacked
stuff which aligns with the spirit of rfc3517 much better than
sacked+holes used by fack.

> Best regards,
> Arnd Hannemann
>
> From: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
>
> Because the for loop is only executed for FACK-enabled flows remove
> redundant checks within the loop.
>
> Signed-off-by: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
> ---
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 99b7ecb..cd8b4bd 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1178,10 +1178,7 @@ static void tcp_mark_lost_retrans(struct sock *sk)
> if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
> continue;
>
> - if (after(received_upto, ack_seq) &&
> - (tcp_is_fack(tp) ||
> - !before(received_upto,
> - ack_seq + tp->reordering * tp->mss_cache))) {
> + if (after(received_upto, ack_seq)) {
> TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
> tp->retrans_out -= tcp_skb_pcount(skb);


--
i.


\
 
 \ /
  Last update: 2009-01-07 11:43    [from the cache]
©2003-2011 Jasper Spaans