Re: [2.6.20.21 review 12/35] TCP: Fix TCP handling of SACK in bidirectional flows.

Willy Tarreau
2007-10-13T17:29:52Z

Hi Ilpo,

On Sat, Oct 13, 2007 at 08:15:52PM +0300, Ilpo Järvinen wrote:
> On Sat, 13 Oct 2007, Willy Tarreau wrote:
>
> > It's possible that new SACK blocks that should trigger new LOST
> > markings arrive with new data (which previously made is_dupack
> > false). In addition, I think this fixes a case where we get
> > a cumulative ACK with enough SACK blocks to trigger the fast
> > recovery (is_dupack would be false there too).
> >
> > I'm not completely pleased with this solution because readability
> > of the code is somewhat questionable as 'is_dupack' in SACK case
> > is no longer about dupacks only but would mean something like
> > 'lost_marker_work_todo' too... But because of Eifel stuff done
> > in CA_Recovery, the FLAG_DATA_SACKED check cannot be placed to
> > the if statement which seems attractive solution. Nevertheless,
> > I didn't like adding another variable just for that either... :-)
> >
> > Signed-off-by: Ilpo JÃ¤rvinen <ilpo.jarvinen@helsinki.fi>
> > Signed-off-by: David S. Miller <davem@davemloft.net>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
> > ---
> > net/ipv4/tcp_input.c | 5 ++++-
> > 1 files changed, 4 insertions(+), 1 deletions(-)
> >
> > Index: 2.6/net/ipv4/tcp_input.c
> > ===================================================================
> > --- 2.6.orig/net/ipv4/tcp_input.c
> > +++ 2.6/net/ipv4/tcp_input.c
> > @@ -1951,7 +1951,10 @@ tcp_fastretrans_alert(struct sock *sk, u
> > {
> > struct inet_connection_sock *icsk = inet_csk(sk);
> > struct tcp_sock *tp = tcp_sk(sk);
> > - int is_dupack = (tp->snd_una == prior_snd_una && !(flag&FLAG_NOT_DUP));
> > + int is_dupack = (tp->snd_una == prior_snd_una &&
> > + (!(flag&FLAG_NOT_DUP) ||
> > + ((flag&FLAG_DATA_SACKED) &&
> > + (tp->fackets_out > tp->reordering))));
> >
> > /* Some technical things:
> > * 1. Reno does not count dupacks (sacked_out) automatically. */
>
> FYI,
>
> This ended up being a non complete fix. Day after these two patches
> (11-12) I submitted two other patches to complete this fix series (got
> bitten by release-early-release-often, fixed day-after-submission
> thoughts in those two later patches). For some reason these two keep
> floating around as separate ones from those two later ones.
>
> To make things even more complicated, eb7bdad82e8 (see stable-2.6.22)
> could have been split more logically to do_lost addition and
> FLAG_SND_UNA_ADVANCED parts (but that didn't occur to me back then).
>
> All of them listed here (from stable-2.6.22 since one of them is
> reduced from mainline version):
>
> 6d742fb6e2b8913457e1282e1be77d6f4e45af00 Fix TCP DSACK cwnd handling
> eb7bdad82e8af48e1ed1b650268dc85ca7e9ff39 Handle snd_una in tcp_cwnd_down()
> 8385cffd22359ad561a173accefeb354bd606ce4 TCP: Fix TCP handling of SACK in
> 783366ad4b212cde069c50903494eb6a6b83958c TCP: Fix TCP rate-halving on

Thanks for your help, I really appreciate it. In fact, I've reviewed them
four, but two of them did not apply and the code looked somewhat different,
so I considered them irrelevant to 2.6.20. I didn't understand that they
were all related, so maybe I checked them in a wrong order.

I'll recheck all that in the right sequence and will merge them four, or
get back to you if something still puzzles me.

Thanks!
Willy